Merge remote-tracking branch 'upstream/master' into dbus-codegen

.editorconfig

@@ -55,16 +55,17 @@ dotnet_naming_symbols.constant_fields.required_modifiers = const
 
 dotnet_naming_style.pascal_case_style.capitalization = pascal_case
 
-# static fields should have s_ prefix
-dotnet_naming_rule.static_fields_should_have_prefix.severity = suggestion
-dotnet_naming_rule.static_fields_should_have_prefix.symbols  = static_fields
-dotnet_naming_rule.static_fields_should_have_prefix.style    = static_prefix_style
+# private static fields should have s_ prefix
+dotnet_naming_rule.private_static_fields_should_have_prefix.severity = suggestion
+dotnet_naming_rule.private_static_fields_should_have_prefix.symbols  = private_static_fields
+dotnet_naming_rule.private_static_fields_should_have_prefix.style    = private_static_prefix_style
 
-dotnet_naming_symbols.static_fields.applicable_kinds   = field
-dotnet_naming_symbols.static_fields.required_modifiers = static
+dotnet_naming_symbols.private_static_fields.applicable_kinds   = field
+dotnet_naming_symbols.private_static_fields.required_modifiers = static
+dotnet_naming_symbols.private_static_fields.applicable_accessibilities = private
 
-dotnet_naming_style.static_prefix_style.required_prefix = s_
-dotnet_naming_style.static_prefix_style.capitalization = camel_case
+dotnet_naming_style.private_static_prefix_style.required_prefix = s_
+dotnet_naming_style.private_static_prefix_style.capitalization = camel_case
 
 # internal and private fields should be _camelCase
 dotnet_naming_rule.camel_case_for_private_internal_fields.severity = suggestion
@@ -117,7 +118,7 @@ csharp_space_after_dot = false
 csharp_space_after_keywords_in_control_flow_statements = true
 csharp_space_after_semicolon_in_for_statement = true
 csharp_space_around_binary_operators = before_and_after
-csharp_space_around_declaration_statements = do_not_ignore
+csharp_space_around_declaration_statements = false
 csharp_space_before_colon_in_inheritance_clause = true
 csharp_space_before_comma = false
 csharp_space_before_dot = false
@@ -211,5 +212,5 @@ indent_size = 2
 # Shell scripts
 [*.sh]
 end_of_line = lf
-[*.{cmd, bat}]
+[*.{cmd,bat}]
 end_of_line = crlf

.ncrunch/Avalonia.UnitTests.v3.ncrunchproject

@@ -0,0 +1,5 @@
+<ProjectConfiguration>
+  <Settings>
+    <XUnit2Enabled>False</XUnit2Enabled>
+  </Settings>
+</ProjectConfiguration>

.ncrunch/GpuInterop.v3.ncrunchproject

@@ -0,0 +1,5 @@
+<ProjectConfiguration>
+  <Settings>
+    <IgnoreThisComponentCompletely>True</IgnoreThisComponentCompletely>
+  </Settings>
+</ProjectConfiguration>

Avalonia.Desktop.slnf

@@ -47,6 +47,7 @@
       "tests\\Avalonia.Base.UnitTests\\Avalonia.Base.UnitTests.csproj",
       "tests\\Avalonia.Benchmarks\\Avalonia.Benchmarks.csproj",
       "tests\\Avalonia.Controls.DataGrid.UnitTests\\Avalonia.Controls.DataGrid.UnitTests.csproj",
+      "tests\\Avalonia.Controls.ItemsRepeater.UnitTests\\Avalonia.Controls.ItemsRepeater.UnitTests.csproj",
       "tests\\Avalonia.Controls.UnitTests\\Avalonia.Controls.UnitTests.csproj",
       "tests\\Avalonia.DesignerSupport.TestApp\\Avalonia.DesignerSupport.TestApp.csproj",
       "tests\\Avalonia.DesignerSupport.Tests\\Avalonia.DesignerSupport.Tests.csproj",

build/HarfBuzzSharp.props

@@ -1,7 +1,7 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="HarfBuzzSharp" Version="2.8.2.1-preview.108" />
-    <PackageReference Condition="'$(IncludeLinuxSkia)' == 'true'" Include="HarfBuzzSharp.NativeAssets.Linux" Version="2.8.2.1-preview.108" />
-    <PackageReference Condition="'$(IncludeWasmSkia)' == 'true'" Include="HarfBuzzSharp.NativeAssets.WebAssembly" Version="2.8.2.1-preview.108" />
+    <PackageReference Include="HarfBuzzSharp" Version="2.8.2.3" />
+    <PackageReference Condition="'$(IncludeLinuxSkia)' == 'true'" Include="HarfBuzzSharp.NativeAssets.Linux" Version="2.8.2.3" />
+    <PackageReference Condition="'$(IncludeWasmSkia)' == 'true'" Include="HarfBuzzSharp.NativeAssets.WebAssembly" Version="2.8.2.3" />
   </ItemGroup>
 </Project>

build/ImageSharp.props

@@ -1,5 +1,5 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="SixLabors.ImageSharp" Version="2.1.1" />
+    <PackageReference Include="SixLabors.ImageSharp" Version="2.1.3" />
   </ItemGroup>
 </Project>

build/Moq.props

@@ -1,5 +1,5 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="Moq" Version="4.14.1" />
+    <PackageReference Include="Moq" Version="4.18.4" />
   </ItemGroup>
 </Project>

build/SharedVersion.props

@@ -3,6 +3,7 @@
   <PropertyGroup>
     <Product>Avalonia</Product>
     <Version>11.0.999</Version>
+    <Authors>Avalonia Team</Authors>
     <Copyright>Copyright 2022 &#169; The AvaloniaUI Project</Copyright>
     <PackageProjectUrl>https://avaloniaui.net</PackageProjectUrl>
     <RepositoryUrl>https://github.com/AvaloniaUI/Avalonia/</RepositoryUrl>

build/SkiaSharp.props

@@ -1,7 +1,7 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="SkiaSharp" Version="2.88.1" />
-    <PackageReference Condition="'$(IncludeLinuxSkia)' == 'true'" Include="SkiaSharp.NativeAssets.Linux" Version="2.88.1" />
-    <PackageReference Condition="'$(IncludeWasmSkia)' == 'true'" Include="SkiaSharp.NativeAssets.WebAssembly" Version="2.88.1" />
+    <PackageReference Include="SkiaSharp" Version="2.88.3" />
+    <PackageReference Condition="'$(IncludeLinuxSkia)' == 'true'" Include="SkiaSharp.NativeAssets.Linux" Version="2.88.3" />
+    <PackageReference Condition="'$(IncludeWasmSkia)' == 'true'" Include="SkiaSharp.NativeAssets.WebAssembly" Version="2.88.3" />
   </ItemGroup>
 </Project>

build/XUnit.props

@@ -1,13 +1,12 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="xunit" Version="2.4.1" />
-    <PackageReference Include="xunit.abstractions" Version="2.0.3" />
-    <PackageReference Include="xunit.assert" Version="2.4.1" />
-    <PackageReference Include="xunit.core" Version="2.4.1" />
-    <PackageReference Include="xunit.extensibility.core" Version="2.4.1" />
-    <PackageReference Include="xunit.extensibility.execution" Version="2.4.1" />
-    <PackageReference Include="xunit.runner.console" Version="2.4.1" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
+    <PackageReference Include="xunit" Version="2.4.2" />
+    <PackageReference Include="xunit.assert" Version="2.4.2" />
+    <PackageReference Include="xunit.core" Version="2.4.2" />
+    <PackageReference Include="xunit.extensibility.core" Version="2.4.2" />
+    <PackageReference Include="xunit.extensibility.execution" Version="2.4.2" />
+    <PackageReference Include="xunit.runner.console" Version="2.4.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" />
     <PackageReference Include="Xunit.SkippableFact" Version="1.4.13" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.7.0" />
   </ItemGroup>

samples/IntegrationTestApp/MainWindow.axaml

@@ -70,6 +70,7 @@
             <ComboBoxItem>Item 0</ComboBoxItem>
             <ComboBoxItem>Item 1</ComboBoxItem>
           </ComboBox>
+          <CheckBox Name="ComboBoxWrapSelection" IsChecked="{Binding #BasicComboBox.WrapSelection}">Wrap Selection</CheckBox>
           <Button Name="ComboBoxSelectionClear">Clear Selection</Button>
           <Button Name="ComboBoxSelectFirst">Select First</Button>
         </StackPanel>

src/Avalonia.Base/Animation/KeySpline.cs

@@ -79,15 +79,12 @@ namespace Avalonia.Animation
         /// <param name="culture">culture of the string</param>
         /// <exception cref="FormatException">Thrown if the string does not have 4 values</exception>
         /// <returns>A <see cref="KeySpline"/> with the appropriate values set</returns>
-        public static KeySpline Parse(string value, CultureInfo culture)
+        public static KeySpline Parse(string value, CultureInfo? culture)
         {
-            if (culture is null)
-                culture = CultureInfo.InvariantCulture;
+            culture ??= CultureInfo.InvariantCulture;
 
-            using (var tokenizer = new StringTokenizer((string)value, culture, exceptionMessage: $"Invalid KeySpline string: \"{value}\"."))
-            {
-                return new KeySpline(tokenizer.ReadDouble(), tokenizer.ReadDouble(), tokenizer.ReadDouble(), tokenizer.ReadDouble());
-            }
+            using var tokenizer = new StringTokenizer(value, culture, exceptionMessage: $"Invalid KeySpline string: \"{value}\".");
+            return new KeySpline(tokenizer.ReadDouble(), tokenizer.ReadDouble(), tokenizer.ReadDouble(), tokenizer.ReadDouble());
         }
 
         /// <summary>

src/Avalonia.Base/AvaloniaObject.cs

@@ -152,7 +152,7 @@ namespace Avalonia
             property = property ?? throw new ArgumentNullException(nameof(property));
             VerifyAccess();
 
-            _values?.ClearLocalValue(property);
+            _values.ClearLocalValue(property);
         }
 
         /// <summary>
@@ -242,7 +242,14 @@ namespace Avalonia
             return registered.InvokeGetter(this);
         }
 
-        /// <inheritdoc/>
+        /// <summary>
+        /// Gets an <see cref="AvaloniaProperty"/> base value.
+        /// </summary>
+        /// <param name="property">The property.</param>
+        /// <remarks>
+        /// Gets the value of the property excluding animated values, otherwise <see cref="Optional{T}.Empty"/>.
+        /// Note that this method does not return property values that come from inherited or default values.
+        /// </remarks>
         public Optional<T> GetBaseValue<T>(StyledProperty<T> property)
         {
             _ = property ?? throw new ArgumentNullException(nameof(property));
@@ -261,7 +268,7 @@ namespace Avalonia
 
             VerifyAccess();
 
-            return _values?.IsAnimating(property) ?? false;
+            return _values.IsAnimating(property);
         }
 
         /// <summary>
@@ -279,7 +286,7 @@ namespace Avalonia
 
             VerifyAccess();
 
-            return _values?.IsSet(property) ?? false;
+            return _values.IsSet(property);
         }
 
         /// <summary>
@@ -515,14 +522,12 @@ namespace Avalonia
         /// <param name="property">The property.</param>
         public void CoerceValue(AvaloniaProperty property) => _values.CoerceValue(property);
 
-        /// <inheritdoc/>
         internal void AddInheritanceChild(AvaloniaObject child)
         {
             _inheritanceChildren ??= new List<AvaloniaObject>();
             _inheritanceChildren.Add(child);
         }
-        
-        /// <inheritdoc/>
+
         internal void RemoveInheritanceChild(AvaloniaObject child)
         {
             _inheritanceChildren?.Remove(child);
@@ -541,24 +546,11 @@ namespace Avalonia
                 return new AvaloniaPropertyValue(
                     property,
                     GetValue(property),
-                    BindingPriority.Unset,
-                    "Local Value");
-            }
-            else if (_values != null)
-            {
-                var result = _values.GetDiagnostic(property);
-
-                if (result != null)
-                {
-                    return result;
-                }
+                    BindingPriority.LocalValue,
+                    null);
             }
 
-            return new AvaloniaPropertyValue(
-                property,
-                GetValue(property),
-                BindingPriority.Unset,
-                "Unset");
+            return _values.GetDiagnostic(property);
         }
 
         internal ValueStore GetValueStore() => _values;

src/Avalonia.Base/Data/Converters/DefaultValueConverter.cs

@@ -30,7 +30,7 @@ namespace Avalonia.Data.Converters
         {
             if (value == null)
             {
-                return targetType.IsValueType ? AvaloniaProperty.UnsetValue : null;
+                return null;
             }
 
             if (typeof(ICommand).IsAssignableFrom(targetType) && value is Delegate d && d.Method.GetParameters().Length <= 1)

src/Avalonia.Base/Diagnostics/AvaloniaObjectExtensions.cs

@@ -1,6 +1,3 @@
-using System;
-using Avalonia.Data;
-
 namespace Avalonia.Diagnostics
 {
     /// <summary>

src/Avalonia.Base/Input/DragEventArgs.cs

@@ -1,36 +1,28 @@
 using System;
 using Avalonia.Interactivity;
 using Avalonia.Metadata;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Input
 {
     public class DragEventArgs : RoutedEventArgs
     {
-        private Interactive _target;
-        private Point _targetLocation;
+        private readonly Interactive _target;
+        private readonly Point _targetLocation;
 
         public DragDropEffects DragEffects { get; set; }
 
-        public IDataObject Data { get; private set; }
+        public IDataObject Data { get; }
 
-        public KeyModifiers KeyModifiers { get; private set; }
+        public KeyModifiers KeyModifiers { get; }
 
         public Point GetPosition(Visual relativeTo)
         {
-            var point = new Point(0, 0);
-
             if (relativeTo == null)
             {
                 throw new ArgumentNullException(nameof(relativeTo));
             }
 
-            if (_target != null)
-            {
-                point = _target.TranslatePoint(_targetLocation, relativeTo) ?? point;
-            }
-
-            return point;
+            return _target.TranslatePoint(_targetLocation, relativeTo) ?? new Point(0, 0);
         }
 
         [Unstable]

src/Avalonia.Base/Input/KeyGesture.cs

@@ -136,7 +136,7 @@ namespace Avalonia.Input
             return StringBuilderCache.GetStringAndRelease(s);
         }
 
-        public bool Matches(KeyEventArgs keyEvent) =>
+        public bool Matches(KeyEventArgs? keyEvent) =>
             keyEvent != null &&
             keyEvent.KeyModifiers == KeyModifiers &&
             ResolveNumPadOperationKey(keyEvent.Key) == ResolveNumPadOperationKey(Key);

src/Avalonia.Base/Input/KeyboardNavigationHandler.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Diagnostics.CodeAnalysis;
-using System.Linq;
 using Avalonia.Input.Navigation;
 using Avalonia.VisualTree;
 
@@ -51,7 +50,7 @@ namespace Avalonia.Input
 
             // If there's a custom keyboard navigation handler as an ancestor, use that.
             var custom = (element as Visual)?.FindAncestorOfType<ICustomKeyboardNavigation>(true);
-            if (custom is object && HandlePreCustomNavigation(custom, element, direction, out var ce))
+            if (custom is not null && HandlePreCustomNavigation(custom, element, direction, out var ce))
                 return ce;
 
             var result = direction switch
@@ -117,32 +116,27 @@ namespace Avalonia.Input
             NavigationDirection direction,
             [NotNullWhen(true)] out IInputElement? result)
         {
-            if (customHandler != null)
+            var (handled, next) = customHandler.GetNext(element, direction);
+
+            if (handled)
             {
-                var (handled, next) = customHandler.GetNext(element, direction);
-
-                if (handled)
+                if (next is not null)
                 {
-                    if (next != null)
-                    {
-                        result = next;
-                        return true;
-                    }
-                    else if (direction == NavigationDirection.Next || direction == NavigationDirection.Previous)
-                    {
-                        var r = direction switch
-                        {
-                            NavigationDirection.Next => TabNavigation.GetNextTabOutside(customHandler),
-                            NavigationDirection.Previous => TabNavigation.GetPrevTabOutside(customHandler),
-                            _ => throw new NotSupportedException(),
-                        };
+                    result = next;
+                    return true;
+                }
 
-                        if (r is object)
-                        {
-                            result = r;
-                            return true;
-                        }
-                    }
+                var r = direction switch
+                {
+                    NavigationDirection.Next => TabNavigation.GetNextTabOutside(customHandler),
+                    NavigationDirection.Previous => TabNavigation.GetPrevTabOutside(customHandler),
+                    _ => null
+                };
+
+                if (r is not null)
+                {
+                    result = r;
+                    return true;
                 }
             }
 

src/Avalonia.Base/Input/Navigation/TabNavigation.cs

@@ -1,6 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Input.Navigation
@@ -54,8 +52,7 @@ namespace Avalonia.Input.Navigation
                 // Avoid the endless loop here for Cycle groups
                 if (loopStartElement == nextTabElement)
                     break;
-                if (loopStartElement == null)
-                    loopStartElement = nextTabElement;
+                loopStartElement ??= nextTabElement;
 
                 var firstTabElementInside = GetNextTab(null, nextTabElement, true);
                 if (firstTabElementInside != null)
@@ -80,12 +77,9 @@ namespace Avalonia.Input.Navigation
 
         public static IInputElement? GetNextTabOutside(ICustomKeyboardNavigation e)
         {
-            if (e is IInputElement container)
+            if (e is IInputElement container && GetLastInTree(container) is { } last)
             {
-                var last = GetLastInTree(container);
-
-                if (last is object)
-                    return GetNextTab(last, false);
+                return GetNextTab(last, false);
             }
 
             return null;
@@ -93,11 +87,8 @@ namespace Avalonia.Input.Navigation
 
         public static IInputElement? GetPrevTab(IInputElement? e, IInputElement? container, bool goDownOnly)
         {
-            if (e is null && container is null)
-                throw new InvalidOperationException("Either 'e' or 'container' must be non-null.");
-
-            if (container is null)
-                container = GetGroupParent(e!);
+            container ??=
+                GetGroupParent(e ?? throw new InvalidOperationException("Either 'e' or 'container' must be non-null."));
 
             KeyboardNavigationMode tabbingType = GetKeyNavigationMode(container);
 
@@ -163,8 +154,7 @@ namespace Avalonia.Input.Navigation
                 // Avoid the endless loop here
                 if (loopStartElement == nextTabElement)
                     break;
-                if (loopStartElement == null)
-                    loopStartElement = nextTabElement;
+                loopStartElement ??= nextTabElement;
 
                 // At this point nextTabElement is TabGroup
                 var lastTabElementInside = GetPrevTab(null, nextTabElement, true);
@@ -189,22 +179,18 @@ namespace Avalonia.Input.Navigation
 
         public static IInputElement? GetPrevTabOutside(ICustomKeyboardNavigation e)
         {
-            if (e is IInputElement container)
+            if (e is IInputElement container && GetFirstChild(container) is { } first)
             {
-                var first = GetFirstChild(container);
-
-                if (first is object)
-                    return GetPrevTab(first, null, false);
+                return GetPrevTab(first, null, false);
             }
 
             return null;
         }
 
-        private static IInputElement? FocusedElement(IInputElement e)
+        private static IInputElement? FocusedElement(IInputElement? e)
         {
-            var iie = e;
             // Focus delegation is enabled only if keyboard focus is outside the container
-            if (iie != null && !iie.IsKeyboardFocusWithin)
+            if (e != null && !e.IsKeyboardFocusWithin)
             {
                 var focusedElement = (FocusManager.Instance as FocusManager)?.GetFocusedElement(e);
                 if (focusedElement != null)
@@ -229,13 +215,11 @@ namespace Avalonia.Input.Navigation
         private static IInputElement? GetFirstChild(IInputElement e)
         {
             // If the element has a FocusedElement it should be its first child
-            if (FocusedElement(e) is IInputElement focusedElement)
+            if (FocusedElement(e) is { } focusedElement)
                 return focusedElement;
 
             // Return the first visible element.
-            var uiElement = e as InputElement;
-
-            if (uiElement is null || IsVisibleAndEnabled(uiElement))
+            if (e is not InputElement uiElement || IsVisibleAndEnabled(uiElement))
             {
                 if (e is Visual elementAsVisual)
                 {
@@ -265,7 +249,7 @@ namespace Avalonia.Input.Navigation
         private static IInputElement? GetLastChild(IInputElement e)
         {
             // If the element has a FocusedElement it should be its last child
-            if (FocusedElement(e) is IInputElement focusedElement)
+            if (FocusedElement(e) is { } focusedElement)
                 return focusedElement;
 
             // Return the last visible element.
@@ -273,9 +257,7 @@ namespace Avalonia.Input.Navigation
 
             if (uiElement == null || IsVisibleAndEnabled(uiElement))
             {
-                var elementAsVisual = e as Visual;
-
-                if (elementAsVisual != null)
+                if (e is Visual elementAsVisual)
                 {
                     var children = elementAsVisual.VisualChildren;
                     var count = children.Count;
@@ -322,7 +304,7 @@ namespace Avalonia.Input.Navigation
             return firstTabElement;
         }
 
-        private static IInputElement? GetLastInTree(IInputElement container)
+        private static IInputElement GetLastInTree(IInputElement container)
         {
             IInputElement? result;
             IInputElement? c = container;

src/Avalonia.Base/Input/Platform/IClipboard.cs

@@ -6,9 +6,9 @@ namespace Avalonia.Input.Platform
     [NotClientImplementable]
     public interface IClipboard
     {
-        Task<string> GetTextAsync();
+        Task<string?> GetTextAsync();
 
-        Task SetTextAsync(string text);
+        Task SetTextAsync(string? text);
 
         Task ClearAsync();
 
@@ -16,6 +16,6 @@ namespace Avalonia.Input.Platform
         
         Task<string[]> GetFormatsAsync();
         
-        Task<object> GetDataAsync(string format);
+        Task<object?> GetDataAsync(string format);
     }
 }

src/Avalonia.Base/LogicalTree/LogicalExtensions.cs

@@ -48,7 +48,7 @@ namespace Avalonia.LogicalTree
         /// <param name="logical">The logical.</param>
         /// <param name="includeSelf">If given logical should be included in search.</param>
         /// <returns>First ancestor of given type.</returns>
-        public static T? FindLogicalAncestorOfType<T>(this ILogical logical, bool includeSelf = false) where T : class
+        public static T? FindLogicalAncestorOfType<T>(this ILogical? logical, bool includeSelf = false) where T : class
         {
             if (logical is null)
             {
@@ -120,7 +120,7 @@ namespace Avalonia.LogicalTree
         /// <param name="logical">The logical.</param>
         /// <param name="includeSelf">If given logical should be included in search.</param>
         /// <returns>First descendant of given type.</returns>
-        public static T? FindLogicalDescendantOfType<T>(this ILogical logical, bool includeSelf = false) where T : class
+        public static T? FindLogicalDescendantOfType<T>(this ILogical? logical, bool includeSelf = false) where T : class
         {
             if (logical is null)
             {
@@ -185,7 +185,7 @@ namespace Avalonia.LogicalTree
         /// True if <paramref name="logical"/> is an ancestor of <paramref name="target"/>;
         /// otherwise false.
         /// </returns>
-        public static bool IsLogicalAncestorOf(this ILogical logical, ILogical target)
+        public static bool IsLogicalAncestorOf(this ILogical? logical, ILogical? target)
         {
             var current = target?.LogicalParent;
 

src/Avalonia.Base/Media/Color.cs

@@ -147,16 +147,11 @@ namespace Avalonia.Media
         /// <param name="s">The color string.</param>
         /// <param name="color">The parsed color</param>
         /// <returns>The status of the operation.</returns>
-        public static bool TryParse(string s, out Color color)
+        public static bool TryParse(string? s, out Color color)
         {
             color = default;
 
-            if (s is null)
-            {
-                return false;
-            }
-
-            if (s.Length == 0)
+            if (string.IsNullOrEmpty(s))
             {
                 return false;
             }
@@ -336,7 +331,7 @@ namespace Avalonia.Media
         /// <summary>
         /// Parses the given string representing a CSS color value into a new <see cref="Color"/>.
         /// </summary>
-        private static bool TryParseCssFormat(string s, out Color color)
+        private static bool TryParseCssFormat(string? s, out Color color)
         {
             bool prefixMatched = false;
 

src/Avalonia.Base/Media/DrawingContext.cs

@@ -240,7 +240,7 @@ namespace Avalonia.Media
         /// </summary>
         /// <param name="foreground">The foreground brush.</param>
         /// <param name="glyphRun">The glyph run.</param>
-        public void DrawGlyphRun(IBrush foreground, GlyphRun glyphRun)
+        public void DrawGlyphRun(IBrush? foreground, GlyphRun glyphRun)
         {
             _ = glyphRun ?? throw new ArgumentNullException(nameof(glyphRun));
 

src/Avalonia.Base/Media/DrawingGroup.cs

@@ -13,14 +13,14 @@ namespace Avalonia.Media
         public static readonly StyledProperty<double> OpacityProperty =
             AvaloniaProperty.Register<DrawingGroup, double>(nameof(Opacity), 1);
 
-        public static readonly StyledProperty<Transform> TransformProperty =
-            AvaloniaProperty.Register<DrawingGroup, Transform>(nameof(Transform));
+        public static readonly StyledProperty<Transform?> TransformProperty =
+            AvaloniaProperty.Register<DrawingGroup, Transform?>(nameof(Transform));
 
-        public static readonly StyledProperty<Geometry> ClipGeometryProperty =
-            AvaloniaProperty.Register<DrawingGroup, Geometry>(nameof(ClipGeometry));
+        public static readonly StyledProperty<Geometry?> ClipGeometryProperty =
+            AvaloniaProperty.Register<DrawingGroup, Geometry?>(nameof(ClipGeometry));
 
-        public static readonly StyledProperty<IBrush> OpacityMaskProperty =
-            AvaloniaProperty.Register<DrawingGroup, IBrush>(nameof(OpacityMask));
+        public static readonly StyledProperty<IBrush?> OpacityMaskProperty =
+            AvaloniaProperty.Register<DrawingGroup, IBrush?>(nameof(OpacityMask));
 
         public static readonly DirectProperty<DrawingGroup, DrawingCollection> ChildrenProperty =
             AvaloniaProperty.RegisterDirect<DrawingGroup, DrawingCollection>(
@@ -36,19 +36,19 @@ namespace Avalonia.Media
             set => SetValue(OpacityProperty, value);
         }
 
-        public Transform Transform
+        public Transform? Transform
         {
             get => GetValue(TransformProperty);
             set => SetValue(TransformProperty, value);
         }
 
-        public Geometry ClipGeometry
+        public Geometry? ClipGeometry
         {
             get => GetValue(ClipGeometryProperty);
             set => SetValue(ClipGeometryProperty, value);
         }
 
-        public IBrush OpacityMask
+        public IBrush? OpacityMask
         {
             get => GetValue(OpacityMaskProperty);
             set => SetValue(OpacityMaskProperty, value);
@@ -159,7 +159,7 @@ namespace Avalonia.Media
 
             public void DrawGeometry(IBrush? brush, IPen? pen, IGeometryImpl geometry)
             {
-                if (((brush == null) && (pen == null)) || (geometry == null))
+                if ((brush == null) && (pen == null))
                 {
                     return;
                 }
@@ -167,9 +167,9 @@ namespace Avalonia.Media
                 AddNewGeometryDrawing(brush, pen, new PlatformGeometry(geometry));
             }
 
-            public void DrawGlyphRun(IBrush foreground, IRef<IGlyphRunImpl> glyphRun)
+            public void DrawGlyphRun(IBrush? foreground, IRef<IGlyphRunImpl> glyphRun)
             {
-                if (foreground == null || glyphRun == null)
+                if (foreground == null)
                 {
                     return;
                 }
@@ -184,7 +184,7 @@ namespace Avalonia.Media
                 AddDrawing(glyphRunDrawing);
             }
 
-            public void DrawLine(IPen pen, Point p1, Point p2)
+            public void DrawLine(IPen? pen, Point p1, Point p2)
             {
                 if (pen == null)
                 {

src/Avalonia.Base/Media/DrawingImage.cs

@@ -20,8 +20,8 @@ namespace Avalonia.Media
         /// <summary>
         /// Defines the <see cref="Drawing"/> property.
         /// </summary>
-        public static readonly StyledProperty<Drawing> DrawingProperty =
-            AvaloniaProperty.Register<DrawingImage, Drawing>(nameof(Drawing));
+        public static readonly StyledProperty<Drawing?> DrawingProperty =
+            AvaloniaProperty.Register<DrawingImage, Drawing?>(nameof(Drawing));
 
         /// <inheritdoc/>
         public event EventHandler? Invalidated;
@@ -30,7 +30,7 @@ namespace Avalonia.Media
         /// Gets or sets the drawing content.
         /// </summary>
         [Content]
-        public Drawing Drawing
+        public Drawing? Drawing
         {
             get => GetValue(DrawingProperty);
             set => SetValue(DrawingProperty, value);

src/Avalonia.Base/Media/FontFamily.cs

@@ -119,7 +119,7 @@ namespace Avalonia.Media
 
                 case 2:
                     {
-                        var source = segments[0].StartsWith("/")
+                        var source = segments[0].StartsWith("/", StringComparison.Ordinal)
                             ? new Uri(segments[0], UriKind.Relative)
                             : new Uri(segments[0], UriKind.RelativeOrAbsolute);
 
@@ -188,7 +188,7 @@ namespace Avalonia.Media
         {
             unchecked
             {
-                return ((FamilyNames != null ? FamilyNames.GetHashCode() : 0) * 397) ^ (Key != null ? Key.GetHashCode() : 0);
+                return (FamilyNames.GetHashCode() * 397) ^ (Key is not null ? Key.GetHashCode() : 0);
             }
         }
 

src/Avalonia.Base/Media/Fonts/FontFamilyKey.cs

@@ -41,10 +41,7 @@ namespace Avalonia.Media.Fonts
             {
                 var hash = (int)2166136261;
 
-                if (Source != null)
-                {
-                    hash = (hash * 16777619) ^ Source.GetHashCode();
-                }
+                hash = (hash * 16777619) ^ Source.GetHashCode();
 
                 if (BaseUri != null)
                 {

src/Avalonia.Base/Media/FormattedText.cs

@@ -1354,7 +1354,7 @@ namespace Avalonia.Media
                     {
                         var highlightBounds = currentLine.GetTextBounds(x0,x1 - x0);
 
-                        if (highlightBounds != null)
+                        if (highlightBounds.Count > 0)
                         {
                             foreach (var bound in highlightBounds)
                             {
@@ -1365,7 +1365,7 @@ namespace Avalonia.Media
                                     // Convert logical units (which extend leftward from the right edge
                                     // of the paragraph) to physical units.
                                     //
-                                    // Note that since rect is in logical units, rect.Right corresponds to 
+                                    // Note that since rect is in logical units, rect.Right corresponds to
                                     // the visual *left* edge of the rectangle in the RTL case. Specifically,
                                     // is the distance leftward from the right edge of the formatting rectangle
                                     // whose width is the paragraph width passed to FormatLine.
@@ -1384,7 +1384,7 @@ namespace Avalonia.Media
                                 else
                                 {
                                     accumulatedBounds = Geometry.Combine(accumulatedBounds, rectangleGeometry, GeometryCombineMode.Union);
-                                }                                  
+                                }
                             }
                         }
                     }

src/Avalonia.Base/Media/GeometryDrawing.cs

@@ -15,8 +15,8 @@ namespace Avalonia.Media
         /// <summary>
         /// Defines the <see cref="Geometry"/> property.
         /// </summary>
-        public static readonly StyledProperty<Geometry> GeometryProperty =
-            AvaloniaProperty.Register<GeometryDrawing, Geometry>(nameof(Geometry));
+        public static readonly StyledProperty<Geometry?> GeometryProperty =
+            AvaloniaProperty.Register<GeometryDrawing, Geometry?>(nameof(Geometry));
 
         /// <summary>
         /// Defines the <see cref="Brush"/> property.
@@ -34,7 +34,7 @@ namespace Avalonia.Media
         /// Gets or sets the <see cref="Avalonia.Media.Geometry"/> that describes the shape of this <see cref="GeometryDrawing"/>.
         /// </summary>
         [Content]
-        public Geometry Geometry
+        public Geometry? Geometry
         {
             get => GetValue(GeometryProperty);
             set => SetValue(GeometryProperty, value);

src/Avalonia.Base/Media/GlyphRunDrawing.cs

@@ -2,19 +2,19 @@
 {
     public class GlyphRunDrawing : Drawing
     {
-        public static readonly StyledProperty<IBrush> ForegroundProperty =
-            AvaloniaProperty.Register<GlyphRunDrawing, IBrush>(nameof(Foreground));
+        public static readonly StyledProperty<IBrush?> ForegroundProperty =
+            AvaloniaProperty.Register<GlyphRunDrawing, IBrush?>(nameof(Foreground));
 
-        public static readonly StyledProperty<GlyphRun> GlyphRunProperty =
-            AvaloniaProperty.Register<GlyphRunDrawing, GlyphRun>(nameof(GlyphRun));
+        public static readonly StyledProperty<GlyphRun?> GlyphRunProperty =
+            AvaloniaProperty.Register<GlyphRunDrawing, GlyphRun?>(nameof(GlyphRun));
 
-        public IBrush Foreground
+        public IBrush? Foreground
         {
             get => GetValue(ForegroundProperty);
             set => SetValue(ForegroundProperty, value);
         }
 
-        public GlyphRun GlyphRun
+        public GlyphRun? GlyphRun
         {
             get => GetValue(GlyphRunProperty);
             set => SetValue(GlyphRunProperty, value);

src/Avalonia.Base/Media/HslColor.cs

@@ -254,7 +254,7 @@ namespace Avalonia.Media
         /// <param name="s">The HSL color string to parse.</param>
         /// <param name="hslColor">The parsed <see cref="HslColor"/>.</param>
         /// <returns>True if parsing was successful; otherwise, false.</returns>
-        public static bool TryParse(string s, out HslColor hslColor)
+        public static bool TryParse(string? s, out HslColor hslColor)
         {
             bool prefixMatched = false;
 

src/Avalonia.Base/Media/HsvColor.cs

@@ -254,7 +254,7 @@ namespace Avalonia.Media
         /// <param name="s">The HSV color string to parse.</param>
         /// <param name="hsvColor">The parsed <see cref="HsvColor"/>.</param>
         /// <returns>True if parsing was successful; otherwise, false.</returns>
-        public static bool TryParse(string s, out HsvColor hsvColor)
+        public static bool TryParse(string? s, out HsvColor hsvColor)
         {
             bool prefixMatched = false;
 

src/Avalonia.Base/Media/IVisualBrush.cs

@@ -1,5 +1,4 @@
 using Avalonia.Metadata;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Media
 {
@@ -12,6 +11,6 @@ namespace Avalonia.Media
         /// <summary>
         /// Gets the visual to draw.
         /// </summary>
-        Visual Visual { get; }
+        Visual? Visual { get; }
     }
 }

src/Avalonia.Base/Media/Immutable/ImmutableDashStyle.cs

@@ -39,17 +39,8 @@ namespace Avalonia.Media.Immutable
             {
                 return true;
             }
-            else if (other is null)
-            {
-                return false;
-            }
 
-            if (Offset != other.Offset)
-            {
-                return false;
-            }
-
-            return SequenceEqual(Dashes, other.Dashes);
+            return other is not null && Offset == other.Offset && SequenceEqual(_dashes, other.Dashes);
         }
 
         /// <inheritdoc/>
@@ -58,30 +49,27 @@ namespace Avalonia.Media.Immutable
             var hashCode = 717868523;
             hashCode = hashCode * -1521134295 + Offset.GetHashCode();
 
-            if (_dashes != null)
+            foreach (var i in _dashes)
             {
-                foreach (var i in _dashes)
-                {
-                    hashCode = hashCode * -1521134295 + i.GetHashCode();
-                }
+                hashCode = hashCode * -1521134295 + i.GetHashCode();
             }
 
             return hashCode;
         }
 
-        private static bool SequenceEqual(IReadOnlyList<double> left, IReadOnlyList<double>? right)
+        private static bool SequenceEqual(double[] left, IReadOnlyList<double>? right)
         {
             if (ReferenceEquals(left, right))
             {
                 return true;
             }
 
-            if (left == null || right == null || left.Count != right.Count)
+            if (right is null || left.Length != right.Count)
             {
                 return false;
             }
 
-            for (var c = 0; c < left.Count; c++)
+            for (var c = 0; c < left.Length; c++)
             {
                 if (left[c] != right[c])
                 {

src/Avalonia.Base/Media/Immutable/ImmutableVisualBrush.cs

@@ -1,5 +1,4 @@
 using Avalonia.Media.Imaging;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Media.Immutable
 {
@@ -31,11 +30,11 @@ namespace Avalonia.Media.Immutable
             RelativeRect? destinationRect = null,
             double opacity = 1,
             ImmutableTransform? transform = null,
-            RelativePoint transformOrigin = new RelativePoint(),
+            RelativePoint transformOrigin = default,
             RelativeRect? sourceRect = null,
             Stretch stretch = Stretch.Uniform,
             TileMode tileMode = TileMode.None,
-            Imaging.BitmapInterpolationMode bitmapInterpolationMode = Imaging.BitmapInterpolationMode.Default)
+            BitmapInterpolationMode bitmapInterpolationMode = BitmapInterpolationMode.Default)
             : base(
                   alignmentX,
                   alignmentY,
@@ -62,6 +61,6 @@ namespace Avalonia.Media.Immutable
         }
 
         /// <inheritdoc/>
-        public Visual Visual { get; }
+        public Visual? Visual { get; }
     }
 }

src/Avalonia.Base/Media/TextDecoration.cs

@@ -22,8 +22,8 @@ namespace Avalonia.Media
         /// <summary>
         /// Defines the <see cref="Stroke"/> property.
         /// </summary>
-        public static readonly StyledProperty<IBrush> StrokeProperty =
-            AvaloniaProperty.Register<TextDecoration, IBrush>(nameof(Stroke));
+        public static readonly StyledProperty<IBrush?> StrokeProperty =
+            AvaloniaProperty.Register<TextDecoration, IBrush?>(nameof(Stroke));
 
         /// <summary>
         /// Defines the <see cref="StrokeThicknessUnit"/> property.
@@ -34,8 +34,8 @@ namespace Avalonia.Media
         /// <summary>
         /// Defines the <see cref="StrokeDashArray"/> property.
         /// </summary>
-        public static readonly StyledProperty<AvaloniaList<double>> StrokeDashArrayProperty =
-            AvaloniaProperty.Register<TextDecoration, AvaloniaList<double>>(nameof(StrokeDashArray));
+        public static readonly StyledProperty<AvaloniaList<double>?> StrokeDashArrayProperty =
+            AvaloniaProperty.Register<TextDecoration, AvaloniaList<double>?>(nameof(StrokeDashArray));
 
         /// <summary>
         /// Defines the <see cref="StrokeDashOffset"/> property.
@@ -82,7 +82,7 @@ namespace Avalonia.Media
         /// <summary>
         /// Gets or sets the <see cref="IBrush"/> that specifies how the <see cref="TextDecoration"/> is painted.
         /// </summary>
-        public IBrush Stroke
+        public IBrush? Stroke
         {
             get { return GetValue(StrokeProperty); }
             set { SetValue(StrokeProperty, value); }
@@ -101,7 +101,7 @@ namespace Avalonia.Media
         /// Gets or sets a collection of <see cref="double"/> values that indicate the pattern of dashes and gaps
         /// that is used to draw the <see cref="TextDecoration"/>.
         /// </summary>
-        public AvaloniaList<double> StrokeDashArray
+        public AvaloniaList<double>? StrokeDashArray
         {
             get { return GetValue(StrokeDashArrayProperty); }
             set { SetValue(StrokeDashArrayProperty, value); }
@@ -220,7 +220,7 @@ namespace Avalonia.Media
 
                 var intersections = glyphRun.PlatformImpl.Item.GetIntersections((float)(thickness * 0.5d - offsetY), (float)(thickness * 1.5d - offsetY));
 
-                if (intersections != null && intersections.Count > 0)
+                if (intersections.Count > 0)
                 {
                     var last = baselineOrigin.X;
                     var finalPos = last + glyphRun.Size.Width;

src/Avalonia.Base/Media/VisualBrush.cs

@@ -1,5 +1,4 @@
 using Avalonia.Media.Immutable;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Media
 {
@@ -11,8 +10,8 @@ namespace Avalonia.Media
         /// <summary>
         /// Defines the <see cref="Visual"/> property.
         /// </summary>
-        public static readonly StyledProperty<Visual> VisualProperty =
-            AvaloniaProperty.Register<VisualBrush, Visual>(nameof(Visual));
+        public static readonly StyledProperty<Visual?> VisualProperty =
+            AvaloniaProperty.Register<VisualBrush, Visual?>(nameof(Visual));
 
         static VisualBrush()
         {
@@ -38,7 +37,7 @@ namespace Avalonia.Media
         /// <summary>
         /// Gets or sets the visual to draw.
         /// </summary>
-        public Visual Visual
+        public Visual? Visual
         {
             get { return GetValue(VisualProperty); }
             set { SetValue(VisualProperty, value); }

src/Avalonia.Base/Platform/IDrawingContextImpl.cs

@@ -49,7 +49,7 @@ namespace Avalonia.Platform
         /// <param name="pen">The stroke pen.</param>
         /// <param name="p1">The first point of the line.</param>
         /// <param name="p2">The second point of the line.</param>
-        void DrawLine(IPen pen, Point p1, Point p2);
+        void DrawLine(IPen? pen, Point p1, Point p2);
 
         /// <summary>
         /// Draws a geometry.
@@ -91,7 +91,7 @@ namespace Avalonia.Platform
         /// </summary>
         /// <param name="foreground">The foreground.</param>
         /// <param name="glyphRun">The glyph run.</param>
-        void DrawGlyphRun(IBrush foreground, IRef<IGlyphRunImpl> glyphRun);
+        void DrawGlyphRun(IBrush? foreground, IRef<IGlyphRunImpl> glyphRun);
 
         /// <summary>
         /// Creates a new <see cref="IRenderTargetBitmapImpl"/> that can be used as a render layer

src/Avalonia.Base/Platform/Internal/AssemblyDescriptor.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Reflection;
@@ -18,26 +19,21 @@ internal class AssemblyDescriptor : IAssemblyDescriptor
 {
     public AssemblyDescriptor(Assembly assembly)
     {
-        Assembly = assembly;
+        Assembly = assembly ?? throw new ArgumentNullException(nameof(assembly));
+        Resources = assembly.GetManifestResourceNames()
+            .ToDictionary(n => n, n => (IAssetDescriptor)new AssemblyResourceDescriptor(assembly, n));
+        Name = assembly.GetName().Name;
 
-        if (assembly != null)
+        using var resources = assembly.GetManifestResourceStream(Constants.AvaloniaResourceName);
+        if (resources != null)
         {
-            Resources = assembly.GetManifestResourceNames()
-                .ToDictionary(n => n, n => (IAssetDescriptor)new AssemblyResourceDescriptor(assembly, n));
-            Name = assembly.GetName().Name;
-            using (var resources = assembly.GetManifestResourceStream(Constants.AvaloniaResourceName))
-            {
-                if (resources != null)
-                {
-                    Resources.Remove(Constants.AvaloniaResourceName);
+            Resources.Remove(Constants.AvaloniaResourceName);
 
-                    var indexLength = new BinaryReader(resources).ReadInt32();
-                    var index = AvaloniaResourcesIndexReaderWriter.ReadIndex(new SlicedStream(resources, 4, indexLength));
-                    var baseOffset = indexLength + 4;
-                    AvaloniaResources = index.ToDictionary(r => GetPathRooted(r), r => (IAssetDescriptor)
-                        new AvaloniaResourceDescriptor(assembly, baseOffset + r.Offset, r.Size));
-                }
-            }
+            var indexLength = new BinaryReader(resources).ReadInt32();
+            var index = AvaloniaResourcesIndexReaderWriter.ReadIndex(new SlicedStream(resources, 4, indexLength));
+            var baseOffset = indexLength + 4;
+            AvaloniaResources = index.ToDictionary(GetPathRooted, r => (IAssetDescriptor)
+                new AvaloniaResourceDescriptor(assembly, baseOffset + r.Offset, r.Size));
         }
     }
 
@@ -45,6 +41,7 @@ internal class AssemblyDescriptor : IAssemblyDescriptor
     public Dictionary<string, IAssetDescriptor>? Resources { get; }
     public Dictionary<string, IAssetDescriptor>? AvaloniaResources { get; }
     public string? Name { get; }
+
     private static string GetPathRooted(AvaloniaResourcesIndexEntry r) =>
         r.Path![0] == '/' ? r.Path : '/' + r.Path;
 }

src/Avalonia.Base/Rendering/Composition/Animations/CompositionAnimation.cs

@@ -23,7 +23,7 @@ namespace Avalonia.Rendering.Composition.Animations
     public abstract class CompositionAnimation : CompositionObject,  ICompositionAnimationBase
     {
         private readonly CompositionPropertySet _propertySet;
-        internal CompositionAnimation(Compositor compositor) : base(compositor, null!)
+        internal CompositionAnimation(Compositor compositor) : base(compositor, null)
         {
             _propertySet = new CompositionPropertySet(compositor);
         }

src/Avalonia.Base/Rendering/Composition/Animations/CompositionAnimationGroup.cs

@@ -19,7 +19,7 @@ namespace Avalonia.Rendering.Composition.Animations
         public void Remove(CompositionAnimation value) => Animations.Remove(value);
         public void RemoveAll() => Animations.Clear();
 
-        public CompositionAnimationGroup(Compositor compositor) : base(compositor, null!)
+        public CompositionAnimationGroup(Compositor compositor) : base(compositor, null)
         {
         }
     }

src/Avalonia.Base/Rendering/Composition/Animations/ImplicitAnimationCollection.cs

@@ -23,7 +23,7 @@ namespace Avalonia.Rendering.Composition.Animations
     {
         private Dictionary<string, ICompositionAnimationBase> _inner = new Dictionary<string, ICompositionAnimationBase>();
         private IDictionary<string, ICompositionAnimationBase> _innerface;
-        internal ImplicitAnimationCollection(Compositor compositor) : base(compositor, null!)
+        internal ImplicitAnimationCollection(Compositor compositor) : base(compositor, null)
         {
             _innerface = _inner;
         }

src/Avalonia.Base/Rendering/Composition/CompositingRenderer.cs

@@ -20,15 +20,17 @@ public class CompositingRenderer : IRendererWithCompositor
 {
     private readonly IRenderRoot _root;
     private readonly Compositor _compositor;
-    CompositionDrawingContext _recorder = new();
-    DrawingContext _recordingContext;
-    private HashSet<Visual> _dirty = new();
-    private HashSet<Visual> _recalculateChildren = new();
-    private bool _queuedUpdate;
-    private Action _update;
-    private bool _updating;
+    private readonly CompositionDrawingContext _recorder = new();
+    private readonly DrawingContext _recordingContext;
+    private readonly HashSet<Visual> _dirty = new();
+    private readonly HashSet<Visual> _recalculateChildren = new();
+    private readonly Action _update;
 
-    internal CompositionTarget CompositionTarget;
+    private bool _queuedUpdate;
+    private bool _updating;
+    private bool _isDisposed;
+
+    internal CompositionTarget CompositionTarget { get; }
     
     /// <summary>
     /// Asks the renderer to only draw frames on the render thread. Makes Paint to wait until frame is rendered.
@@ -38,6 +40,17 @@ public class CompositingRenderer : IRendererWithCompositor
     /// <inheritdoc/>
     public RendererDiagnostics Diagnostics { get; }
 
+    /// <inheritdoc />
+    public Compositor Compositor => _compositor;
+
+    /// <summary>
+    /// Initializes a new instance of <see cref="CompositingRenderer"/>
+    /// </summary>
+    /// <param name="root">The render root using this renderer.</param>
+    /// <param name="compositor">The associated compositors.</param>
+    /// <param name="surfaces">
+    /// A function returning the list of native platform's surfaces that can be consumed by rendering subsystems.
+    /// </param>
     public CompositingRenderer(IRenderRoot root, Compositor compositor, Func<IEnumerable<object>> surfaces)
     {
         _root = root;
@@ -66,7 +79,7 @@ public class CompositingRenderer : IRendererWithCompositor
     /// <inheritdoc/>
     public event EventHandler<SceneInvalidatedEventArgs>? SceneInvalidated;
 
-    void QueueUpdate()
+    private void QueueUpdate()
     {
         if(_queuedUpdate)
             return;
@@ -77,9 +90,11 @@ public class CompositingRenderer : IRendererWithCompositor
     /// <inheritdoc/>
     public void AddDirty(Visual visual)
     {
+        if (_isDisposed)
+            return;
         if (_updating)
             throw new InvalidOperationException("Visual was invalidated during the render pass");
-        _dirty.Add((Visual)visual);
+        _dirty.Add(visual);
         QueueUpdate();
     }
 
@@ -126,9 +141,11 @@ public class CompositingRenderer : IRendererWithCompositor
     /// <inheritdoc/>
     public void RecalculateChildren(Visual visual)
     {
+        if (_isDisposed)
+            return;
         if (_updating)
             throw new InvalidOperationException("Visual was invalidated during the render pass");
-        _recalculateChildren.Add((Visual)visual);
+        _recalculateChildren.Add(visual);
         QueueUpdate();
     }
 
@@ -171,7 +188,7 @@ public class CompositingRenderer : IRendererWithCompositor
             if (sortedChildren != null)
                 for (var c = 0; c < visualChildren.Count; c++)
                 {
-                    if (!ReferenceEquals(compositionChildren[c], ((Visual)sortedChildren[c].visual).CompositionVisual))
+                    if (!ReferenceEquals(compositionChildren[c], sortedChildren[c].visual.CompositionVisual))
                     {
                         mismatch = true;
                         break;
@@ -179,7 +196,7 @@ public class CompositingRenderer : IRendererWithCompositor
                 }
             else
                 for (var c = 0; c < visualChildren.Count; c++)
-                    if (!ReferenceEquals(compositionChildren[c], ((Visual)visualChildren[c]).CompositionVisual))
+                    if (!ReferenceEquals(compositionChildren[c], visualChildren[c].CompositionVisual))
                     {
                         mismatch = true;
                         break;
@@ -201,7 +218,7 @@ public class CompositingRenderer : IRendererWithCompositor
         {
             foreach (var ch in sortedChildren)
             {
-                var compositionChild = ((Visual)ch.visual).CompositionVisual;
+                var compositionChild = ch.visual.CompositionVisual;
                 if (compositionChild != null)
                     compositionChildren.Add(compositionChild);
             }
@@ -210,7 +227,7 @@ public class CompositingRenderer : IRendererWithCompositor
         else
             foreach (var ch in v.GetVisualChildren())
             {
-                var compositionChild = ((Visual)ch).CompositionVisual;
+                var compositionChild = ch.CompositionVisual;
                 if (compositionChild != null)
                     compositionChildren.Add(compositionChild);
             }
@@ -289,13 +306,18 @@ public class CompositingRenderer : IRendererWithCompositor
             _updating = false;
         }
     }
-    
+
+    /// <inheritdoc />
     public void Resized(Size size)
     {
     }
 
+    /// <inheritdoc />
     public void Paint(Rect rect)
     {
+        if (_isDisposed)
+            return;
+
         QueueUpdate();
         CompositionTarget.RequestRedraw();
         if(RenderOnlyOnRenderThread && Compositor.Loop.RunsInBackground)
@@ -304,17 +326,34 @@ public class CompositingRenderer : IRendererWithCompositor
             CompositionTarget.ImmediateUIThreadRender();
     }
 
-    public void Start() => CompositionTarget.IsEnabled = true;
-
-    public void Stop()
+    /// <inheritdoc />
+    public void Start()
     {
-        CompositionTarget.IsEnabled = false;
+        if (_isDisposed)
+            return;
+
+        CompositionTarget.IsEnabled = true;
     }
 
-    public ValueTask<object?> TryGetRenderInterfaceFeature(Type featureType) => Compositor.TryGetRenderInterfaceFeature(featureType);
+    /// <inheritdoc />
+    public void Stop()
+        => CompositionTarget.IsEnabled = false;
 
+    /// <inheritdoc />
+    public ValueTask<object?> TryGetRenderInterfaceFeature(Type featureType)
+        => Compositor.TryGetRenderInterfaceFeature(featureType);
+
+    /// <inheritdoc />
     public void Dispose()
     {
+        if (_isDisposed)
+            return;
+
+        _isDisposed = true;
+        _dirty.Clear();
+        _recalculateChildren.Clear();
+        SceneInvalidated = null;
+
         Stop();
         CompositionTarget.Dispose();
         
@@ -323,9 +362,4 @@ public class CompositingRenderer : IRendererWithCompositor
         if (Compositor.Loop.RunsInBackground)
             _compositor.Commit().Wait();
     }
-
-    /// <summary>
-    /// The associated <see cref="Avalonia.Rendering.Composition.Compositor"/> object
-    /// </summary>
-    public Compositor Compositor => _compositor;
 }

src/Avalonia.Base/Rendering/Composition/CompositionDrawingSurface.cs

@@ -1,4 +1,3 @@
-using System;
 using System.Threading.Tasks;
 using Avalonia.Rendering.Composition.Server;
 using Avalonia.Threading;
@@ -7,7 +6,7 @@ namespace Avalonia.Rendering.Composition;
 
 public class CompositionDrawingSurface : CompositionSurface
 {
-    internal new ServerCompositionDrawingSurface Server => (ServerCompositionDrawingSurface)base.Server;
+    internal new ServerCompositionDrawingSurface Server => (ServerCompositionDrawingSurface)base.Server!;
     internal CompositionDrawingSurface(Compositor compositor) : base(compositor, new ServerCompositionDrawingSurface(compositor.Server))
     {
     }

src/Avalonia.Base/Rendering/Composition/CompositionObject.cs

@@ -22,7 +22,7 @@ namespace Avalonia.Rendering.Composition
         public ImplicitAnimationCollection? ImplicitAnimations { get; set; }
 
         private protected InlineDictionary<CompositionProperty, IAnimationInstance> PendingAnimations;
-        internal CompositionObject(Compositor compositor, ServerObject server)
+        internal CompositionObject(Compositor compositor, ServerObject? server)
         {
             Compositor = compositor;
             Server = server;
@@ -32,7 +32,7 @@ namespace Avalonia.Rendering.Composition
         /// The associated Compositor
         /// </summary>
         public Compositor Compositor { get; }
-        internal ServerObject Server { get; }
+        internal ServerObject? Server { get; }
         public bool IsDisposed { get; private set; }
         private bool _registeredForSerialization;
 

src/Avalonia.Base/Rendering/Composition/CompositionPropertySet.cs

@@ -23,7 +23,7 @@ namespace Avalonia.Rendering.Composition
         private readonly Dictionary<string, ExpressionVariant> _variants = new Dictionary<string, ExpressionVariant>();
         private readonly Dictionary<string, CompositionObject> _objects = new Dictionary<string, CompositionObject>();
         
-        internal CompositionPropertySet(Compositor compositor) : base(compositor, null!)
+        internal CompositionPropertySet(Compositor compositor) : base(compositor, null)
         {
         }
 

src/Avalonia.Base/Rendering/Composition/Drawing/CompositionDrawingContext.cs

@@ -88,8 +88,13 @@ internal class CompositionDrawingContext : IDrawingContextImpl, IDrawingContextW
     }
 
     /// <inheritdoc/>
-    public void DrawLine(IPen pen, Point p1, Point p2)
+    public void DrawLine(IPen? pen, Point p1, Point p2)
     {
+        if (pen is null)
+        {
+            return;
+        }
+
         var next = NextDrawAs<LineNode>();
 
         if (next == null || !next.Item.Equals(Transform, pen, p1, p2))
@@ -159,8 +164,13 @@ internal class CompositionDrawingContext : IDrawingContextImpl, IDrawingContextW
     public object? GetFeature(Type t) => null;
 
     /// <inheritdoc/>
-    public void DrawGlyphRun(IBrush foreground, IRef<IGlyphRunImpl> glyphRun)
+    public void DrawGlyphRun(IBrush? foreground, IRef<IGlyphRunImpl> glyphRun)
     {
+        if (foreground is null)
+        {
+            return;
+        }
+
         var next = NextDrawAs<GlyphRunNode>();
 
         if (next == null || !next.Item.Equals(Transform, foreground, glyphRun))

src/Avalonia.Base/Rendering/Composition/Expressions/Expression.cs

@@ -165,8 +165,6 @@ namespace Avalonia.Rendering.Composition.Expressions
         
         public override ExpressionVariant Evaluate(ref ExpressionEvaluationContext context)
         {
-            if (context.ForeignFunctionInterface == null)
-                return default;
             var args = new List<ExpressionVariant>();
             foreach (var expr in Parameters)
                 args.Add(expr.Evaluate(ref context));

src/Avalonia.Base/Rendering/Composition/Expressions/ExpressionEvaluationContext.cs

@@ -1,5 +1,4 @@
 using System.Collections.Generic;
-using Avalonia.Rendering.Composition.Server;
 
 // Special license applies <see href="https://raw.githubusercontent.com/AvaloniaUI/Avalonia/master/src/Avalonia.Base/Rendering/Composition/License.md">License.md</see>
 

src/Avalonia.Base/Rendering/Composition/Server/DrawingContextProxy.cs

@@ -66,7 +66,7 @@ internal class CompositorDrawingContextProxy : IDrawingContextImpl, IDrawingCont
         _impl.DrawBitmap(source, opacityMask, opacityMaskRect, destRect);
     }
 
-    public void DrawLine(IPen pen, Point p1, Point p2)
+    public void DrawLine(IPen? pen, Point p1, Point p2)
     {
         _impl.DrawLine(pen, p1, p2);
     }
@@ -86,7 +86,7 @@ internal class CompositorDrawingContextProxy : IDrawingContextImpl, IDrawingCont
         _impl.DrawEllipse(brush, pen, rect);
     }
 
-    public void DrawGlyphRun(IBrush foreground, IRef<IGlyphRunImpl> glyphRun)
+    public void DrawGlyphRun(IBrush? foreground, IRef<IGlyphRunImpl> glyphRun)
     {
         _impl.DrawGlyphRun(foreground, glyphRun);
     }

src/Avalonia.Base/Rendering/IRenderRoot.cs

@@ -1,6 +1,4 @@
 using Avalonia.Metadata;
-using Avalonia.Platform;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Rendering
 {

src/Avalonia.Base/Rendering/IRenderer.cs

@@ -90,6 +90,9 @@ namespace Avalonia.Rendering
     
     public interface IRendererWithCompositor : IRenderer
     {
+        /// <summary>
+        /// The associated <see cref="Avalonia.Rendering.Composition.Compositor"/> object
+        /// </summary>
         Compositor Compositor { get; }
     }
 }

src/Avalonia.Base/Rendering/ImmediateRenderer.cs

@@ -48,8 +48,10 @@ namespace Avalonia.Rendering
         /// <inheritdoc/>
         void IVisualBrushRenderer.RenderVisualBrush(IDrawingContextImpl context, IVisualBrush brush)
         {
-            var visual = brush.Visual;
-            Render(new DrawingContext(context), visual, visual.Bounds);
+            if (brush.Visual is { } visual)
+            {
+                Render(new DrawingContext(context), visual, visual.Bounds);
+            }
         }
 
         internal static void Render(Visual visual, DrawingContext context, bool updateTransformedBounds)

src/Avalonia.Base/Rendering/SceneGraph/ExperimentalAcrylicNode.cs

@@ -80,11 +80,8 @@ namespace Avalonia.Rendering.SceneGraph
             {
                 p *= Transform.Invert();
 
-                if (Material != null)
-                {
-                    var rect = Rect.Rect;
-                    return rect.ContainsExclusive(p);
-                }
+                var rect = Rect.Rect;
+                return rect.ContainsExclusive(p);
             }
 
             return false;

src/Avalonia.Base/Utilities/TypeUtilities.cs

@@ -212,7 +212,7 @@ namespace Avalonia.Utilities
 
             var toTypeConverter = TypeDescriptor.GetConverter(toUnderl);
 
-            if (toTypeConverter.CanConvertFrom(from) == true)
+            if (toTypeConverter.CanConvertFrom(from))
             {
                 result = toTypeConverter.ConvertFrom(null, culture, value);
                 return true;
@@ -220,7 +220,7 @@ namespace Avalonia.Utilities
 
             var fromTypeConverter = TypeDescriptor.GetConverter(from);
 
-            if (fromTypeConverter.CanConvertTo(toUnderl) == true)
+            if (fromTypeConverter.CanConvertTo(toUnderl))
             {
                 result = fromTypeConverter.ConvertTo(null, culture, value, toUnderl);
                 return true;
@@ -329,7 +329,7 @@ namespace Avalonia.Utilities
         }
 
         [RequiresUnreferencedCode(TrimmingMessages.ImplicitTypeConvertionRequiresUnreferencedCodeMessage)]
-        public static T ConvertImplicit<T>(object value)
+        public static T ConvertImplicit<T>(object? value)
         {
             if (TryConvertImplicit(typeof(T), value, out var result))
             {
@@ -369,11 +369,6 @@ namespace Avalonia.Utilities
         /// </remarks>
         public static bool IsNumeric(Type type)
         {
-            if (type == null)
-            {
-                return false;
-            }
-
             var underlyingType = Nullable.GetUnderlyingType(type);
 
             if (underlyingType != null)

src/Avalonia.Base/Utilities/WeakEvent.cs

@@ -1,8 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
-using System.Linq;
-using System.Reflection;
 using System.Runtime.CompilerServices;
 using Avalonia.Threading;
 
@@ -15,7 +11,7 @@ public class WeakEvent<TSender, TEventArgs> : WeakEvent where TEventArgs : Event
 {
     private readonly Func<TSender, EventHandler<TEventArgs>, Action> _subscribe;
 
-    readonly ConditionalWeakTable<object, Subscription> _subscriptions = new();
+    private readonly ConditionalWeakTable<object, Subscription> _subscriptions = new();
 
     internal WeakEvent(
         Action<TSender, EventHandler<TEventArgs>> subscribe,
@@ -51,56 +47,6 @@ public class WeakEvent<TSender, TEventArgs> : WeakEvent where TEventArgs : Event
         private readonly WeakEvent<TSender, TEventArgs> _ev;
         private readonly TSender _target;
         private readonly Action _compact;
-
-        struct Entry
-        {
-            WeakReference<IWeakEventSubscriber<TEventArgs>>? _reference;
-            int _hashCode;
-
-            public Entry(IWeakEventSubscriber<TEventArgs> r)
-            {
-                if (r == null)
-                {
-                    _reference = null;
-                    _hashCode = 0;
-                    return;
-                }
-
-                _hashCode = r.GetHashCode();
-                _reference = new WeakReference<IWeakEventSubscriber<TEventArgs>>(r);
-            }
-
-            public bool IsEmpty
-            {
-                get
-                {
-                    if (_reference == null)
-                        return true;
-                    if (_reference.TryGetTarget(out _))
-                        return false;
-                    _reference = null;
-                    return true;
-                }
-            }
-
-            public bool TryGetTarget([MaybeNullWhen(false)]out IWeakEventSubscriber<TEventArgs> target)
-            {
-                if (_reference == null)
-                {
-                    target = null!;
-                    return false;
-                }
-                return _reference.TryGetTarget(out target);
-            }
-
-            public bool Equals(IWeakEventSubscriber<TEventArgs> r)
-            {
-                if (_reference == null || r.GetHashCode() != _hashCode)
-                    return false;
-                return _reference.TryGetTarget(out var target) && target == r;
-            }
-        }
-
         private readonly Action _unsubscribe;
         private readonly WeakHashList<IWeakEventSubscriber<TEventArgs>> _list = new();
         private bool _compactScheduled;
@@ -114,7 +60,7 @@ public class WeakEvent<TSender, TEventArgs> : WeakEvent where TEventArgs : Event
             _unsubscribe = ev._subscribe(target, OnEvent);
         }
 
-        void Destroy()
+        private void Destroy()
         {
             if(_destroyed)
                 return;
@@ -134,15 +80,15 @@ public class WeakEvent<TSender, TEventArgs> : WeakEvent where TEventArgs : Event
                 ScheduleCompact();
         }
 
-        void ScheduleCompact()
+        private void ScheduleCompact()
         {
             if(_compactScheduled || _destroyed)
                 return;
             _compactScheduled = true;
             Dispatcher.UIThread.Post(_compact, DispatcherPriority.Background);
         }
-        
-        void Compact()
+
+        private void Compact()
         {
             if(!_compactScheduled)
                 return;
@@ -152,7 +98,7 @@ public class WeakEvent<TSender, TEventArgs> : WeakEvent where TEventArgs : Event
                 Destroy();
         }
 
-        void OnEvent(object? sender, TEventArgs eventArgs)
+        private void OnEvent(object? sender, TEventArgs eventArgs)
         {
             var alive = _list.GetAlive();
             if(alive == null)
@@ -196,4 +142,4 @@ public class WeakEvent
             return () => unsubscribe(s, handler);
         });
     }
-}
+}

src/Avalonia.Base/Utilities/WeakEventHandlerManager.cs

@@ -60,8 +60,7 @@ namespace Avalonia.Utilities
         private static class SubscriptionTypeStorage<TArgs, TSubscriber>
             where TArgs : EventArgs where TSubscriber : class
         {
-            public static readonly ConditionalWeakTable<object, SubscriptionDic<TArgs, TSubscriber>> Subscribers
-                = new ConditionalWeakTable<object, SubscriptionDic<TArgs, TSubscriber>>();
+            public static readonly ConditionalWeakTable<object, SubscriptionDic<TArgs, TSubscriber>> Subscribers = new();
         }
 
         private class SubscriptionDic<T, TSubscriber> : Dictionary<string, Subscription<T, TSubscriber>>
@@ -69,8 +68,7 @@ namespace Avalonia.Utilities
         {
         }
 
-        private static readonly Dictionary<Type, Dictionary<string, EventInfo>> Accessors
-            = new Dictionary<Type, Dictionary<string, EventInfo>>();
+        private static readonly Dictionary<Type, Dictionary<string, EventInfo>> s_accessors = new();
 
         private class Subscription<T, TSubscriber> where T : EventArgs where TSubscriber : class
         {
@@ -81,18 +79,17 @@ namespace Avalonia.Utilities
             private readonly Delegate _delegate;
 
             private Descriptor[] _data = new Descriptor[2];
-            private int _count = 0;
+            private int _count;
 
-            delegate void CallerDelegate(TSubscriber s, object sender, T args);
-            
-            struct Descriptor
+            private delegate void CallerDelegate(TSubscriber s, object? sender, T args);
+
+            private struct Descriptor
             {
-                public WeakReference<TSubscriber> Subscriber;
-                public CallerDelegate Caller;
+                public WeakReference<TSubscriber>? Subscriber;
+                public CallerDelegate? Caller;
             }
 
-            private static Dictionary<MethodInfo, CallerDelegate> s_Callers =
-                new Dictionary<MethodInfo, CallerDelegate>();
+            private static readonly Dictionary<MethodInfo, CallerDelegate> s_callers = new();
             
             public Subscription(SubscriptionDic<T, TSubscriber> sdic,
                 [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicEvents | DynamicallyAccessedMemberTypes.NonPublicEvents)] Type targetType,
@@ -101,8 +98,8 @@ namespace Avalonia.Utilities
                 _sdic = sdic;
                 _target = target;
                 _eventName = eventName;
-                if (!Accessors.TryGetValue(targetType, out var evDic))
-                    Accessors[targetType] = evDic = new Dictionary<string, EventInfo>();
+                if (!s_accessors.TryGetValue(targetType, out var evDic))
+                    s_accessors[targetType] = evDic = new Dictionary<string, EventInfo>();
 
                 if (evDic.TryGetValue(eventName, out var info))
                 {
@@ -123,12 +120,12 @@ namespace Avalonia.Utilities
 
                 var del = new Action<object, T>(OnEvent);
                 _delegate = del.GetMethodInfo().CreateDelegate(_info.EventHandlerType!, del.Target);
-                _info.AddMethod!.Invoke(target, new[] { _delegate });
+                _info.AddMethod!.Invoke(target, new object?[] { _delegate });
             }
 
-            void Destroy()
+            private void Destroy()
             {
-                _info.RemoveMethod!.Invoke(_target, new[] { _delegate });
+                _info.RemoveMethod!.Invoke(_target, new object?[] { _delegate });
                 _sdic.Remove(_eventName);
             }
 
@@ -146,8 +143,8 @@ namespace Avalonia.Utilities
                 MethodInfo method = s.Method;
 
                 var subscriber = (TSubscriber)s.Target!;
-                if (!s_Callers.TryGetValue(method, out var caller))
-                    s_Callers[method] = caller =
+                if (!s_callers.TryGetValue(method, out var caller))
+                    s_callers[method] = caller =
                         (CallerDelegate)Delegate.CreateDelegate(typeof(CallerDelegate), null, method);
                 _data[_count] = new Descriptor
                 {
@@ -178,7 +175,7 @@ namespace Avalonia.Utilities
                 }
             }
 
-            void Compact(bool preventDestroy = false)
+            private void Compact(bool preventDestroy = false)
             {
                 int empty = -1;
                 for (int c = 0; c < _count; c++)
@@ -206,15 +203,15 @@ namespace Avalonia.Utilities
                     Destroy();
             }
 
-            void OnEvent(object sender, T eventArgs)
+            private void OnEvent(object? sender, T eventArgs)
             {
                 var needCompact = false;
-                for(var c=0; c<_count; c++)
+                for (var c = 0; c < _count; c++)
                 {
-                    var r = _data[c].Subscriber;
+                    var r = _data[c].Subscriber!;
                     if (r.TryGetTarget(out var sub))
                     {
-                        _data[c].Caller(sub, sender, eventArgs);
+                        _data[c].Caller!(sub, sender, eventArgs);
                     }
                     else
                         needCompact = true;

src/Avalonia.Base/Visual.cs

@@ -348,7 +348,7 @@ namespace Avalonia
         /// </summary>
         public void InvalidateVisual()
         {
-            VisualRoot?.Renderer?.AddDirty(this);
+            VisualRoot?.Renderer.AddDirty(this);
         }
 
         /// <summary>
@@ -449,7 +449,7 @@ namespace Avalonia
         protected override void LogicalChildrenCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             base.LogicalChildrenCollectionChanged(sender, e);
-            VisualRoot?.Renderer?.RecalculateChildren(this);
+            VisualRoot?.Renderer.RecalculateChildren(this);
         }
 
         /// <summary>
@@ -477,23 +477,19 @@ namespace Avalonia
             OnAttachedToVisualTree(e);
             AttachedToVisualTree?.Invoke(this, e);
             InvalidateVisual();
-            _visualRoot.Renderer?.RecalculateChildren(_visualParent!);
+            _visualRoot.Renderer.RecalculateChildren(_visualParent!);
 
             if (ZIndex != 0 && VisualParent is Visual parent)
                 parent.HasNonUniformZIndexChildren = true;
 
             var visualChildren = VisualChildren;
+            var visualChildrenCount = visualChildren.Count;
 
-            if (visualChildren != null)
+            for (var i = 0; i < visualChildrenCount; i++)
             {
-                var visualChildrenCount = visualChildren.Count;
-
-                for (var i = 0; i < visualChildrenCount; i++)
+                if (visualChildren[i] is { } child)
                 {
-                    if (visualChildren[i] is Visual child)
-                    {
-                        child.OnAttachedToVisualTreeCore(e);
-                    }
+                    child.OnAttachedToVisualTreeCore(e);
                 }
             }
         }
@@ -540,20 +536,16 @@ namespace Avalonia
             }
 
             DetachedFromVisualTree?.Invoke(this, e);
-            e.Root?.Renderer?.AddDirty(this);
+            e.Root.Renderer.AddDirty(this);
 
             var visualChildren = VisualChildren;
+            var visualChildrenCount = visualChildren.Count;
 
-            if (visualChildren != null)
+            for (var i = 0; i < visualChildrenCount; i++)
             {
-                var visualChildrenCount = visualChildren.Count;
-
-                for (var i = 0; i < visualChildrenCount; i++)
+                if (visualChildren[i] is { } child)
                 {
-                    if (visualChildren[i] is Visual child)
-                    {
-                        child.OnDetachedFromVisualTreeCore(e);
-                    }
+                    child.OnDetachedFromVisualTreeCore(e);
                 }
             }
         }
@@ -659,7 +651,7 @@ namespace Avalonia
                 parentVisual.HasNonUniformZIndexChildren = true;
             
             sender?.InvalidateVisual();
-            parent?.VisualRoot?.Renderer?.RecalculateChildren(parent);
+            parent?.VisualRoot?.Renderer.RecalculateChildren(parent);
         }
 
         /// <summary>

src/Avalonia.Base/VisualTree/VisualExtensions.cs

@@ -46,7 +46,7 @@ namespace Avalonia.VisualTree
             Visual? v = visual ?? throw new ArgumentNullException(nameof(visual));
             var result = 0;
 
-            v = v?.VisualParent;
+            v = v.VisualParent;
 
             while (v != null)
             {
@@ -64,17 +64,13 @@ namespace Avalonia.VisualTree
         /// <param name="visual">The first visual.</param>
         /// <param name="target">The second visual.</param>
         /// <returns>The common ancestor, or null if not found.</returns>
-        public static Visual? FindCommonVisualAncestor(this Visual visual, Visual target)
+        public static Visual? FindCommonVisualAncestor(this Visual? visual, Visual? target)
         {
-            Visual? v = visual ?? throw new ArgumentNullException(nameof(visual));
-
-            if (target is null)
+            if (visual is null || target is null)
             {
                 return null;
             }
 
-            Visual? t = target;
-
             void GoUpwards(ref Visual? node, int count)
             {
                 for (int i = 0; i < count; ++i)
@@ -83,6 +79,9 @@ namespace Avalonia.VisualTree
                 }
             }
 
+            Visual? v = visual;
+            Visual? t = target;
+
             // We want to find lowest node first, then make sure that both nodes are at the same height.
             // By doing that we can sometimes find out that other node is our lowest common ancestor.
             var firstHeight = CalculateDistanceFromRoot(v);
@@ -144,7 +143,7 @@ namespace Avalonia.VisualTree
         /// <param name="visual">The visual.</param>
         /// <param name="includeSelf">If given visual should be included in search.</param>
         /// <returns>First ancestor of given type.</returns>
-        public static T? FindAncestorOfType<T>(this Visual visual, bool includeSelf = false) where T : class
+        public static T? FindAncestorOfType<T>(this Visual? visual, bool includeSelf = false) where T : class
         {
             if (visual is null)
             {
@@ -173,7 +172,7 @@ namespace Avalonia.VisualTree
         /// <param name="visual">The visual.</param>
         /// <param name="includeSelf">If given visual should be included in search.</param>
         /// <returns>First descendant of given type.</returns>
-        public static T? FindDescendantOfType<T>(this Visual visual, bool includeSelf = false) where T : class
+        public static T? FindDescendantOfType<T>(this Visual? visual, bool includeSelf = false) where T : class
         {
             if (visual is null)
             {
@@ -392,7 +391,7 @@ namespace Avalonia.VisualTree
         /// True if <paramref name="visual"/> is an ancestor of <paramref name="target"/>;
         /// otherwise false.
         /// </returns>
-        public static bool IsVisualAncestorOf(this Visual visual, Visual target)
+        public static bool IsVisualAncestorOf(this Visual? visual, Visual? target)
         {
             Visual? current = target?.VisualParent;
 

src/Avalonia.Controls.ColorPicker/ColorSlider/ColorSlider.cs

@@ -41,18 +41,6 @@ namespace Avalonia.Controls.Primitives
         {
         }
 
-        /// <inheritdoc/>
-        protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
-        {
-            base.OnAttachedToVisualTree(e);
-        }
-
-        /// <inheritdoc/>
-        protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
-        {
-            base.OnDetachedFromVisualTree(e);
-        }
-
         /// <summary>
         /// Updates the visual state of the control by applying latest PseudoClasses.
         /// </summary>
@@ -123,28 +111,25 @@ namespace Avalonia.Controls.Primitives
                     IsAlphaMaxForced,
                     IsSaturationValueMaxForced);
 
-                if (bgraPixelData != null)
+                if (_backgroundBitmap != null)
                 {
-                    if (_backgroundBitmap != null)
-                    {
-                        // TODO: CURRENTLY DISABLED DUE TO INTERMITTENT CRASHES IN SKIA/RENDERER
-                        //
-                        // Re-use the existing WriteableBitmap
-                        // This assumes the height, width and byte counts are the same and must be set to null
-                        // elsewhere if that assumption is ever not true.
-                        // ColorPickerHelpers.UpdateBitmapFromPixelData(_backgroundBitmap, bgraPixelData);
+                    // TODO: CURRENTLY DISABLED DUE TO INTERMITTENT CRASHES IN SKIA/RENDERER
+                    //
+                    // Re-use the existing WriteableBitmap
+                    // This assumes the height, width and byte counts are the same and must be set to null
+                    // elsewhere if that assumption is ever not true.
+                    // ColorPickerHelpers.UpdateBitmapFromPixelData(_backgroundBitmap, bgraPixelData);
 
-                        // TODO: ALSO DISABLED DISPOSE DUE TO INTERMITTENT CRASHES
-                        //_backgroundBitmap?.Dispose();
-                        _backgroundBitmap = ColorPickerHelpers.CreateBitmapFromPixelData(bgraPixelData, pixelWidth, pixelHeight);
-                    }
-                    else
-                    {
-                        _backgroundBitmap = ColorPickerHelpers.CreateBitmapFromPixelData(bgraPixelData, pixelWidth, pixelHeight);
-                    }
-
-                    Background = new ImageBrush(_backgroundBitmap);
+                    // TODO: ALSO DISABLED DISPOSE DUE TO INTERMITTENT CRASHES
+                    //_backgroundBitmap?.Dispose();
+                    _backgroundBitmap = ColorPickerHelpers.CreateBitmapFromPixelData(bgraPixelData, pixelWidth, pixelHeight);
                 }
+                else
+                {
+                    _backgroundBitmap = ColorPickerHelpers.CreateBitmapFromPixelData(bgraPixelData, pixelWidth, pixelHeight);
+                }
+
+                Background = new ImageBrush(_backgroundBitmap);
             }
         }
 

src/Avalonia.Controls.DataGrid/DataGrid.cs

@@ -3979,7 +3979,7 @@ namespace Avalonia.Controls
                     {
                         if (focusedObject is Control element)
                         {
-                            parent = element.Parent;
+                            parent = element.VisualParent;
                             if (parent != null)
                             {
                                 dataGridWillReceiveRoutedEvent = false;

src/Avalonia.Controls.DataGrid/Utils/TreeHelper.cs

@@ -36,7 +36,7 @@ namespace Avalonia.Controls.Utils
                     {
                         if (child is Control childElement)
                         {
-                            parent = childElement.Parent;
+                            parent = childElement.VisualParent;
                         }
                     }
                     child = parent;

src/Avalonia.Controls.ItemsRepeater/Controls/ItemsRepeater.cs

@@ -44,8 +44,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Layout"/> property.
         /// </summary>
-        public static readonly StyledProperty<AttachedLayout> LayoutProperty =
-            AvaloniaProperty.Register<ItemsRepeater, AttachedLayout>(nameof(Layout), new StackLayout());
+        public static readonly StyledProperty<AttachedLayout?> LayoutProperty =
+            AvaloniaProperty.Register<ItemsRepeater, AttachedLayout?>(nameof(Layout), new StackLayout());
 
         /// <summary>
         /// Defines the <see cref="VerticalCacheLength"/> property.
@@ -53,8 +53,8 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<double> VerticalCacheLengthProperty =
             AvaloniaProperty.Register<ItemsRepeater, double>(nameof(VerticalCacheLength), 2.0);
 
-        private static readonly StyledProperty<VirtualizationInfo> VirtualizationInfoProperty =
-            AvaloniaProperty.RegisterAttached<ItemsRepeater, Control, VirtualizationInfo>("VirtualizationInfo");
+        private static readonly StyledProperty<VirtualizationInfo?> VirtualizationInfoProperty =
+            AvaloniaProperty.RegisterAttached<ItemsRepeater, Control, VirtualizationInfo?>("VirtualizationInfo");
 
         internal static readonly Rect InvalidRect = new Rect(-1, -1, -1, -1);
         internal static readonly Point ClearedElementsArrangePosition = new Point(-10000.0, -10000.0);
@@ -63,7 +63,7 @@ namespace Avalonia.Controls
         private readonly ViewportManager _viewportManager;
         private readonly TargetWeakEventSubscriber<ItemsRepeater, EventArgs> _layoutWeakSubscriber;
         private IEnumerable? _items;
-        private VirtualizingLayoutContext? _layoutContext;
+        private RepeaterLayoutContext? _layoutContext;
         private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
         private bool _isLayoutInProgress;
         private NotifyCollectionChangedEventArgs? _processingItemsSourceChange;
@@ -104,7 +104,7 @@ namespace Avalonia.Controls
         /// The layout used to size and position elements. The default is a StackLayout with
         /// vertical orientation.
         /// </value>
-        public AttachedLayout Layout
+        public AttachedLayout? Layout
         {
             get => GetValue(LayoutProperty);
             set => SetValue(LayoutProperty, value);
@@ -164,18 +164,7 @@ namespace Avalonia.Controls
 
         private bool IsProcessingCollectionChange => _processingItemsSourceChange != null;
 
-        private LayoutContext LayoutContext
-        {
-            get
-            {
-                if (_layoutContext == null)
-                {
-                    _layoutContext = new RepeaterLayoutContext(this);
-                }
-
-                return _layoutContext;
-            }
-        }
+        private RepeaterLayoutContext LayoutContext => _layoutContext ??= new RepeaterLayoutContext(this);
 
         event EventHandler<ChildIndexChangedEventArgs>? IChildIndexProvider.ChildIndexChanged
         {
@@ -269,39 +258,22 @@ namespace Avalonia.Controls
 
         internal void UnpinElement(Control element) => _viewManager.UpdatePin(element, false);
 
-        internal static VirtualizationInfo? TryGetVirtualizationInfo(Control element)
+        internal static VirtualizationInfo? TryGetVirtualizationInfo(Control? element)
         {
-            return (element as AvaloniaObject)?.GetValue(VirtualizationInfoProperty);
-        }
-
-        internal static VirtualizationInfo CreateAndInitializeVirtualizationInfo(Control element)
-        {
-            if (TryGetVirtualizationInfo(element) != null)
-            {
-                throw new InvalidOperationException("VirtualizationInfo already created.");
-            }
-
-            var result = new VirtualizationInfo();
-            element.SetValue(VirtualizationInfoProperty, result);
-            return result;
+            return element?.GetValue(VirtualizationInfoProperty);
         }
 
         internal static VirtualizationInfo GetVirtualizationInfo(Control element)
         {
-            if (element is AvaloniaObject ao)
+            var result = element.GetValue(VirtualizationInfoProperty);
+
+            if (result == null)
             {
-                var result = ao.GetValue(VirtualizationInfoProperty);
-
-                if (result == null)
-                {
-                    result = new VirtualizationInfo();
-                    ao.SetValue(VirtualizationInfoProperty, result);
-                }
-
-                return result;
+                result = new VirtualizationInfo();
+                element.SetValue(VirtualizationInfoProperty, result);
             }
 
-            throw new NotSupportedException("Custom implementations of AvaloniaObject not supported.");
+            return result;
         }
 
         private protected override void InvalidateMeasureOnChildrenChanged()
@@ -309,6 +281,7 @@ namespace Avalonia.Controls
             // Don't invalidate measure when children change.
         }
 
+        /// <inheritdoc />
         protected override Size MeasureOverride(Size availableSize)
         {
             if (_isLayoutInProgress)
@@ -334,7 +307,7 @@ namespace Avalonia.Controls
 
                 if (layout != null)
                 {
-                    var layoutContext = GetLayoutContext();
+                    var layoutContext = LayoutContext;
 
                     desiredSize = layout.Measure(layoutContext, availableSize);
                     extent = new Rect(LayoutOrigin.X, LayoutOrigin.Y, desiredSize.Width, desiredSize.Height);
@@ -364,6 +337,7 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <inheritdoc />
         protected override Size ArrangeOverride(Size finalSize)
         {
             if (_isLayoutInProgress)
@@ -380,7 +354,7 @@ namespace Avalonia.Controls
 
             try
             {
-                var arrangeSize = Layout?.Arrange(GetLayoutContext(), finalSize) ?? default;
+                var arrangeSize = Layout?.Arrange(LayoutContext, finalSize) ?? default;
 
                 // The view manager might clear elements during this call.
                 // That's why we call it before arranging cleared elements
@@ -421,6 +395,7 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <inheritdoc />
         protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
         {
             base.OnAttachedToVisualTree(e);
@@ -428,11 +403,13 @@ namespace Avalonia.Controls
             _viewportManager.ResetScrollers();
         }
 
+        /// <inheritdoc />
         protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
         {
             _viewportManager.ResetScrollers();
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             if (change.Property == ItemsProperty)
@@ -501,7 +478,7 @@ namespace Avalonia.Controls
             if (parent == this)
             {
                 var virtInfo = TryGetVirtualizationInfo(element);
-                return _viewManager.GetElementIndex(virtInfo!);
+                return _viewManager.GetElementIndex(virtInfo);
             }
 
             return -1;
@@ -529,7 +506,7 @@ namespace Avalonia.Controls
         {
             if (index >= 0 && index >= (ItemsSourceView?.Count ?? 0))
             {
-                throw new ArgumentException("Argument index is invalid.", "index");
+                throw new ArgumentException("Argument index is invalid.", nameof(index));
             }
 
             if (_isLayoutInProgress)
@@ -547,7 +524,7 @@ namespace Avalonia.Controls
                     throw new InvalidOperationException("Cannot make an Anchor when there is no attached layout.");
                 }
 
-                element = (Control)GetLayoutContext().GetOrCreateElementAt(index);
+                element = (Control)LayoutContext.GetOrCreateElementAt(index);
                 element.Measure(Size.Infinity);
             }
 
@@ -647,9 +624,9 @@ namespace Avalonia.Controls
 
                     if (Layout is VirtualizingLayout virtualLayout)
                     {
-                        virtualLayout.OnItemsChanged(GetLayoutContext(), newValue, args);
+                        virtualLayout.OnItemsChanged(LayoutContext, newValue, args);
                     }
-                    else if (Layout is NonVirtualizingLayout nonVirtualLayout)
+                    else if (Layout is NonVirtualizingLayout)
                     {
                         // Walk through all the elements and make sure they are cleared for
                         // non-virtualizing layouts.
@@ -693,7 +670,7 @@ namespace Avalonia.Controls
 
                     try
                     {
-                        virtualLayout.OnItemsChanged(GetLayoutContext(), newValue, args);
+                        virtualLayout.OnItemsChanged(LayoutContext, newValue, args);
                     }
                     finally
                     {
@@ -760,7 +737,7 @@ namespace Avalonia.Controls
                 AttachedLayout.ArrangeInvalidatedWeakEvent.Subscribe(newValue, _layoutWeakSubscriber);
             }
 
-            bool isVirtualizingLayout = newValue != null && newValue is VirtualizingLayout;
+            bool isVirtualizingLayout = newValue is VirtualizingLayout;
             _viewportManager.OnLayoutChanged(isVirtualizingLayout);
             InvalidateMeasure();
         }
@@ -788,7 +765,7 @@ namespace Avalonia.Controls
                 {
                     if (Layout is VirtualizingLayout virtualLayout)
                     {
-                        virtualLayout.OnItemsChanged(GetLayoutContext(), sender, args);
+                        virtualLayout.OnItemsChanged(LayoutContext, sender, args);
                     }
                     else
                     {
@@ -807,15 +784,5 @@ namespace Avalonia.Controls
         {
             _viewportManager.OnBringIntoViewRequested(e);
         }
-        
-        private VirtualizingLayoutContext GetLayoutContext()
-        {
-            if (_layoutContext == null)
-            {
-                _layoutContext = new RepeaterLayoutContext(this);
-            }
-
-            return _layoutContext;
-        }
     }
 }

src/Avalonia.Controls.ItemsRepeater/Controls/ViewManager.cs

@@ -53,7 +53,7 @@ namespace Avalonia.Controls
                     }
                 }
             }
-            if (element == null) { element = GetElementFromUniqueIdResetPool(index); };
+            if (element == null) { element = GetElementFromUniqueIdResetPool(index); }
             if (element == null) { element = GetElementFromPinnedElements(index); }
             if (element == null) { element = GetElementFromElementFactory(index); }
 
@@ -221,7 +221,7 @@ namespace Avalonia.Controls
             return nextElement;
         }
 
-        public int GetElementIndex(VirtualizationInfo virtInfo)
+        public int GetElementIndex(VirtualizationInfo? virtInfo)
         {
             if (virtInfo == null)
             {
@@ -627,11 +627,7 @@ namespace Avalonia.Controls
 
             var element = GetElement();
 
-            var virtInfo = ItemsRepeater.TryGetVirtualizationInfo(element);
-            if (virtInfo == null)
-            {
-                virtInfo = ItemsRepeater.CreateAndInitializeVirtualizationInfo(element);
-            }
+            var virtInfo = ItemsRepeater.GetVirtualizationInfo(element);
             // Clear flag
             virtInfo.MustClearDataContext = false;
 
@@ -710,9 +706,8 @@ namespace Avalonia.Controls
                 {
                     if (parent is ItemsRepeater repeater)
                     {
-                        var element = child as Control;
                         if (repeater == owner &&
-                            element is not null &&
+                            child is Control element &&
                             ItemsRepeater.GetVirtualizationInfo(element).IsRealized)
                         {
                             focusedElement = element;
@@ -722,7 +717,7 @@ namespace Avalonia.Controls
                     }
 
                     child = parent;
-                    parent = child?.GetVisualParent();
+                    parent = child.GetVisualParent();
                 }
             }
 

src/Avalonia.Controls.ItemsRepeater/Controls/ViewportManager.cs

@@ -166,7 +166,7 @@ namespace Avalonia.Controls
             if (Math.Abs(_expectedViewportShift.X) > 1 || Math.Abs(_expectedViewportShift.Y) > 1)
             {
                 Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Expecting viewport shift of ({Shift})",
-                    _owner.Layout.LayoutId, _expectedViewportShift);
+                    _owner.Layout?.LayoutId, _expectedViewportShift);
 
                 // There are cases where we might be expecting a shift but not get it. We will
                 // be waiting for the effective viewport event but if the scroll viewer is not able
@@ -287,7 +287,7 @@ namespace Avalonia.Controls
             if (_pendingViewportShift.X != 0 || _pendingViewportShift.Y != 0)
             {
                 Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Layout Updated with pending shift {Shift}- invalidating measure",
-                    _owner.Layout.LayoutId,
+                    _owner.Layout?.LayoutId,
                     _pendingViewportShift);
 
                 // Assume this is never going to come.
@@ -436,7 +436,7 @@ namespace Avalonia.Controls
 
         private void OnEffectiveViewportChanged(object? sender, EffectiveViewportChangedEventArgs e)
         {
-            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: EffectiveViewportChanged event callback", _owner.Layout.LayoutId);
+            Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: EffectiveViewportChanged event callback", _owner.Layout?.LayoutId);
             UpdateViewport(e.EffectiveViewport);
 
             _pendingViewportShift = default;
@@ -490,14 +490,14 @@ namespace Avalonia.Controls
             var previousVisibleWindow = _visibleWindow;
 
             Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Effective Viewport: ({Before})->({After})",
-                _owner.Layout.LayoutId,
+                _owner.Layout?.LayoutId,
                 previousVisibleWindow,
                 viewport);
 
             if (-currentVisibleWindow.X <= ItemsRepeater.ClearedElementsArrangePosition.X &&
                 -currentVisibleWindow.Y <= ItemsRepeater.ClearedElementsArrangePosition.Y)
             {
-                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Viewport is invalid. visible window cleared", _owner.Layout.LayoutId);
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Viewport is invalid. visible window cleared", _owner.Layout?.LayoutId);
                 // We got cleared.
                 _visibleWindow = default;
             }
@@ -509,7 +509,7 @@ namespace Avalonia.Controls
             if (_visibleWindow != previousVisibleWindow)
             {
                 Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Used Viewport: ({Before})->({After})",
-                    _owner.Layout.LayoutId,
+                    _owner.Layout?.LayoutId,
                     previousVisibleWindow,
                     currentVisibleWindow);
                 TryInvalidateMeasure();
@@ -532,7 +532,7 @@ namespace Avalonia.Controls
                 // We invalidate measure instead of just invalidating arrange because
                 // we don't invalidate measure in UpdateViewport if the view is changing to
                 // avoid layout cycles.
-                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Invalidating measure due to viewport change", _owner.Layout.LayoutId);
+                Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: Invalidating measure due to viewport change", _owner.Layout?.LayoutId);
                 _owner.InvalidateMeasure();
             }
         }

src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.ComponentModel;
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Threading;
@@ -17,32 +16,34 @@ namespace Avalonia.Controls.ApplicationLifetimes
         private int _exitCode;
         private CancellationTokenSource? _cts;
         private bool _isShuttingDown;
-        private HashSet<Window> _windows = new HashSet<Window>();
+        private readonly HashSet<Window> _windows = new();
+
+        private static ClassicDesktopStyleApplicationLifetime? s_activeLifetime;
 
-        private static ClassicDesktopStyleApplicationLifetime? _activeLifetime;
         static ClassicDesktopStyleApplicationLifetime()
         {
             Window.WindowOpenedEvent.AddClassHandler(typeof(Window), OnWindowOpened);
-            Window.WindowClosedEvent.AddClassHandler(typeof(Window), WindowClosedEvent);
+            Window.WindowClosedEvent.AddClassHandler(typeof(Window), OnWindowClosed);
         }
 
-        private static void WindowClosedEvent(object? sender, RoutedEventArgs e)
+        private static void OnWindowClosed(object? sender, RoutedEventArgs e)
         {
-            _activeLifetime?._windows.Remove((Window)sender!);
-            _activeLifetime?.HandleWindowClosed((Window)sender!);
+            var window = (Window)sender!;
+            s_activeLifetime?._windows.Remove(window);
+            s_activeLifetime?.HandleWindowClosed(window);
         }
 
         private static void OnWindowOpened(object? sender, RoutedEventArgs e)
         {
-            _activeLifetime?._windows.Add((Window)sender!);
+            s_activeLifetime?._windows.Add((Window)sender!);
         }
 
         public ClassicDesktopStyleApplicationLifetime()
         {
-            if (_activeLifetime != null)
+            if (s_activeLifetime != null)
                 throw new InvalidOperationException(
                     "Can not have multiple active ClassicDesktopStyleApplicationLifetime instances and the previously created one was not disposed");
-            _activeLifetime = this;
+            s_activeLifetime = this;
         }
 
         /// <inheritdoc/>
@@ -65,9 +66,10 @@ namespace Avalonia.Controls.ApplicationLifetimes
         /// <inheritdoc/>
         public Window? MainWindow { get; set; }
 
+        /// <inheritdoc />
         public IReadOnlyList<Window> Windows => _windows.ToArray();
 
-        private void HandleWindowClosed(Window window)
+        private void HandleWindowClosed(Window? window)
         {
             if (window == null)
                 return;
@@ -130,8 +132,8 @@ namespace Avalonia.Controls.ApplicationLifetimes
 
         public void Dispose()
         {
-            if (_activeLifetime == this)
-                _activeLifetime = null;
+            if (s_activeLifetime == this)
+                s_activeLifetime = null;
         }
 
         private bool DoShutdown(

src/Avalonia.Controls/ApplicationLifetimes/IClassicDesktopStyleApplicationLifetime.cs

@@ -40,7 +40,10 @@ namespace Avalonia.Controls.ApplicationLifetimes
         /// The main window.
         /// </value>
         Window? MainWindow { get; set; }
-        
+
+        /// <summary>
+        /// Gets the list of all open windows in the application.
+        /// </summary>
         IReadOnlyList<Window> Windows { get; }
 
         /// <summary>

src/Avalonia.Controls/AutoCompleteBox/AutoCompleteBox.cs

@@ -792,7 +792,7 @@ namespace Avalonia.Controls
                     Control? element = focused as Control;
                     if (element != null)
                     {
-                        parent = element.Parent;
+                        parent = element.VisualParent;
                     }
                 }
                 focused = parent;
@@ -1711,7 +1711,7 @@ namespace Avalonia.Controls
         /// <param name="predicate">The predicate to use for the partial or
         /// exact match.</param>
         /// <returns>Returns the object or null.</returns>
-        private object? TryGetMatch(string? searchText, AvaloniaList<object> view, AutoCompleteFilterPredicate<string?>? predicate)
+        private object? TryGetMatch(string? searchText, AvaloniaList<object>? view, AutoCompleteFilterPredicate<string?>? predicate)
         {
             if (predicate is null)
                 return null;

src/Avalonia.Controls/Automation/AutomationProperties.cs

@@ -38,8 +38,8 @@ namespace Avalonia.Automation
         /// <summary>
         /// Defines the AutomationProperties.AcceleratorKey attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> AcceleratorKeyProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> AcceleratorKeyProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "AcceleratorKey",
                 typeof(AutomationProperties));
 
@@ -54,16 +54,16 @@ namespace Avalonia.Automation
         /// <summary>
         /// Defines the AutomationProperties.AccessKey attached property
         /// </summary>
-        public static readonly AttachedProperty<string> AccessKeyProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> AccessKeyProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "AccessKey",
                 typeof(AutomationProperties));
 
         /// <summary>
         /// Defines the AutomationProperties.AutomationId attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> AutomationIdProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> AutomationIdProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "AutomationId",
                 typeof(AutomationProperties));
 
@@ -78,8 +78,8 @@ namespace Avalonia.Automation
         /// <summary>
         /// Defines the AutomationProperties.HelpText attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> HelpTextProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> HelpTextProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "HelpText",
                 typeof(AutomationProperties));
 
@@ -122,16 +122,16 @@ namespace Avalonia.Automation
         /// <summary>
         /// Defines the AutomationProperties.ItemStatus attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> ItemStatusProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> ItemStatusProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "ItemStatus",
                 typeof(AutomationProperties));
 
         /// <summary>
         /// Defines the AutomationProperties.ItemType attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> ItemTypeProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> ItemTypeProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "ItemType",
                 typeof(AutomationProperties));
 
@@ -155,8 +155,8 @@ namespace Avalonia.Automation
         /// <summary>
         /// Defines the AutomationProperties.Name attached attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> NameProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> NameProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "Name",
                 typeof(AutomationProperties));
 
@@ -193,25 +193,17 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetAcceleratorKey(StyledElement element, string value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(AcceleratorKeyProperty, value);
         }
 
         /// <summary>
         /// Helper for reading AcceleratorKey property from a StyledElement.
         /// </summary>
-        public static string GetAcceleratorKey(StyledElement element)
+        public static string? GetAcceleratorKey(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(AcceleratorKeyProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(AcceleratorKeyProperty);
         }
 
         /// <summary>
@@ -219,11 +211,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetAccessibilityView(StyledElement element, AccessibilityView value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(AccessibilityViewProperty, value);
         }
 
@@ -232,11 +220,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static AccessibilityView GetAccessibilityView(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(AccessibilityViewProperty);
         }
 
@@ -245,50 +229,34 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetAccessKey(StyledElement element, string value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(AccessKeyProperty, value);
         }
 
         /// <summary>
         /// Helper for reading AccessKey property from a StyledElement.
         /// </summary>
-        public static string GetAccessKey(StyledElement element)
+        public static string? GetAccessKey(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(AccessKeyProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(AccessKeyProperty);
         }
 
         /// <summary>
         /// Helper for setting AutomationId property on a StyledElement. 
         /// </summary>
-        public static void SetAutomationId(StyledElement element, string value)
+        public static void SetAutomationId(StyledElement element, string? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(AutomationIdProperty, value);
         }
 
         /// <summary>
         /// Helper for reading AutomationId property from a StyledElement.
         /// </summary>
-        public static string GetAutomationId(StyledElement element)
+        public static string? GetAutomationId(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(AutomationIdProperty);
         }
 
@@ -297,11 +265,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetControlTypeOverride(StyledElement element, AutomationControlType? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(ControlTypeOverrideProperty, value);
         }
 
@@ -310,38 +274,26 @@ namespace Avalonia.Automation
         /// </summary>
         public static AutomationControlType? GetControlTypeOverride(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(ControlTypeOverrideProperty);
         }
 
         /// <summary>
         /// Helper for setting HelpText property on a StyledElement. 
         /// </summary>
-        public static void SetHelpText(StyledElement element, string value)
+        public static void SetHelpText(StyledElement element, string? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(HelpTextProperty, value);
         }
 
         /// <summary>
         /// Helper for reading HelpText property from a StyledElement.
         /// </summary>
-        public static string GetHelpText(StyledElement element)
+        public static string? GetHelpText(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(HelpTextProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(HelpTextProperty);
         }
 
         /// <summary>
@@ -349,11 +301,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetIsColumnHeader(StyledElement element, bool value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(IsColumnHeaderProperty, value);
         }
 
@@ -362,12 +310,8 @@ namespace Avalonia.Automation
         /// </summary>
         public static bool GetIsColumnHeader(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((bool)element.GetValue(IsColumnHeaderProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(IsColumnHeaderProperty);
         }
 
         /// <summary>
@@ -375,11 +319,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetIsRequiredForForm(StyledElement element, bool value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(IsRequiredForFormProperty, value);
         }
 
@@ -388,12 +328,8 @@ namespace Avalonia.Automation
         /// </summary>
         public static bool GetIsRequiredForForm(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((bool)element.GetValue(IsRequiredForFormProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(IsRequiredForFormProperty);
         }
 
         /// <summary>
@@ -401,12 +337,8 @@ namespace Avalonia.Automation
         /// </summary>
         public static bool GetIsRowHeader(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((bool)element.GetValue(IsRowHeaderProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(IsRowHeaderProperty);
         }
 
         /// <summary>
@@ -414,11 +346,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetIsRowHeader(StyledElement element, bool value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(IsRowHeaderProperty, value);
         }
 
@@ -427,11 +355,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetIsOffscreenBehavior(StyledElement element, IsOffscreenBehavior value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(IsOffscreenBehaviorProperty, value);
         }
 
@@ -440,64 +364,44 @@ namespace Avalonia.Automation
         /// </summary>
         public static IsOffscreenBehavior GetIsOffscreenBehavior(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((IsOffscreenBehavior)element.GetValue(IsOffscreenBehaviorProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(IsOffscreenBehaviorProperty);
         }
 
         /// <summary>
         /// Helper for setting ItemStatus property on a StyledElement. 
         /// </summary>
-        public static void SetItemStatus(StyledElement element, string value)
+        public static void SetItemStatus(StyledElement element, string? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(ItemStatusProperty, value);
         }
 
         /// <summary>
         /// Helper for reading ItemStatus property from a StyledElement.
         /// </summary>
-        public static string GetItemStatus(StyledElement element)
+        public static string? GetItemStatus(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(ItemStatusProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(ItemStatusProperty);
         }
 
         /// <summary>
         /// Helper for setting ItemType property on a StyledElement. 
         /// </summary>
-        public static void SetItemType(StyledElement element, string value)
+        public static void SetItemType(StyledElement element, string? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(ItemTypeProperty, value);
         }
 
         /// <summary>
         /// Helper for reading ItemType property from a StyledElement.
         /// </summary>
-        public static string GetItemType(StyledElement element)
+        public static string? GetItemType(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(ItemTypeProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(ItemTypeProperty);
         }
 
         /// <summary>
@@ -505,11 +409,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetLabeledBy(StyledElement element, Control value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(LabeledByProperty, value);
         }
 
@@ -518,11 +418,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static Control GetLabeledBy(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(LabeledByProperty);
         }
 
@@ -531,11 +427,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetLiveSetting(StyledElement element, AutomationLiveSetting value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(LiveSettingProperty, value);
         }
 
@@ -544,38 +436,26 @@ namespace Avalonia.Automation
         /// </summary>
         public static AutomationLiveSetting GetLiveSetting(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((AutomationLiveSetting)element.GetValue(LiveSettingProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(LiveSettingProperty);
         }
 
         /// <summary>
         /// Helper for setting Name property on a StyledElement. 
         /// </summary>
-        public static void SetName(StyledElement element, string value)
+        public static void SetName(StyledElement element, string? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(NameProperty, value);
         }
 
         /// <summary>
         /// Helper for reading Name property from a StyledElement.
         /// </summary>
-        public static string GetName(StyledElement element)
+        public static string? GetName(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(NameProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(NameProperty);
         }
 
         /// <summary>
@@ -583,11 +463,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetPositionInSet(StyledElement element, int value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(PositionInSetProperty, value);
         }
 
@@ -596,12 +472,8 @@ namespace Avalonia.Automation
         /// </summary>
         public static int GetPositionInSet(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((int)element.GetValue(PositionInSetProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(PositionInSetProperty);
         }
 
         /// <summary>
@@ -609,11 +481,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetSizeOfSet(StyledElement element, int value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(SizeOfSetProperty, value);
         }
 
@@ -622,12 +490,8 @@ namespace Avalonia.Automation
         /// </summary>
         public static int GetSizeOfSet(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((int)element.GetValue(SizeOfSetProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(SizeOfSetProperty);
         }
     }
 }

src/Avalonia.Controls/Automation/Peers/ProgressBarAutomationPeer.cs

@@ -0,0 +1,62 @@
+using System;
+using Avalonia.Automation.Peers;
+using Avalonia.Automation.Provider;
+using Avalonia.Controls.Primitives;
+
+namespace Avalonia.Controls.Automation.Peers
+{
+    public class ProgressBarAutomationPeer : RangeBaseAutomationPeer, IRangeValueProvider
+    {
+        public ProgressBarAutomationPeer(RangeBase owner) : base(owner)
+        {
+        }
+
+        protected override string GetClassNameCore()
+        {
+            return "ProgressBar";
+        }
+
+        protected override AutomationControlType GetAutomationControlTypeCore()
+        {
+            return AutomationControlType.ProgressBar;
+        }
+
+        /// <summary>
+        /// Request to set the value that this UI element is representing
+        /// </summary>
+        /// <param name="val">Value to set the UI to, as an object</param>
+        /// <returns>true if the UI element was successfully set to the specified value</returns>
+        void IRangeValueProvider.SetValue(double val)
+        {
+            throw new InvalidOperationException("ProgressBar is ReadOnly, value can't be set.");
+        }
+
+        ///<summary>Indicates that the value can only be read, not modified.
+        ///returns True if the control is read-only</summary>
+        bool IRangeValueProvider.IsReadOnly
+        {
+            get
+            {
+                return true;
+            }
+        }
+
+        ///<summary>Value of a Large Change</summary>
+        double IRangeValueProvider.LargeChange
+        {
+            get
+            {
+                return double.NaN;
+            }
+        }
+
+        ///<summary>Value of a Small Change</summary>
+        double IRangeValueProvider.SmallChange
+        {
+            get
+            {
+                return double.NaN;
+            }
+        }
+    }
+}

src/Avalonia.Controls/Button.cs

@@ -394,10 +394,10 @@ namespace Avalonia.Controls
             if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
             {
                 IsPressed = true;
+                e.Handled = true;
 
                 if (ClickMode == ClickMode.Press)
                 {
-                    e.Handled = true;
                     OnClick();
                 }
             }
@@ -411,11 +411,11 @@ namespace Avalonia.Controls
             if (IsPressed && e.InitialPressMouseButton == MouseButton.Left)
             {
                 IsPressed = false;
+                e.Handled = true;
 
                 if (ClickMode == ClickMode.Release &&
                     this.GetVisualsAt(e.GetPosition(this)).Any(c => this == c || this.IsVisualAncestorOf(c)))
                 {
-                    e.Handled = true;
                     OnClick();
                 }
             }

src/Avalonia.Controls/Calendar/Calendar.cs

@@ -237,11 +237,11 @@ namespace Avalonia.Controls
         private DateTime _selectedYear;
 
         private DateTime _displayDate = DateTime.Today;
-        private DateTime? _displayDateStart = null;
-        private DateTime? _displayDateEnd = null;
+        private DateTime? _displayDateStart;
+        private DateTime? _displayDateEnd;
 
         private bool _isShiftPressed;
-        private bool _displayDateIsChanging = false;
+        private bool _displayDateIsChanging;
 
         internal CalendarDayButton? FocusButton { get; set; }
         internal CalendarButton? FocusCalendarButton { get; set; }
@@ -291,7 +291,7 @@ namespace Avalonia.Controls
             }
             else
             {
-                throw new ArgumentOutOfRangeException("d", "Invalid DayOfWeek");
+                throw new ArgumentOutOfRangeException(nameof(e), "Invalid DayOfWeek");
             }
         }
 
@@ -346,10 +346,10 @@ namespace Avalonia.Controls
             }
         }
 
-        public static readonly StyledProperty<IBrush> HeaderBackgroundProperty =
-            AvaloniaProperty.Register<Calendar, IBrush>(nameof(HeaderBackground));
+        public static readonly StyledProperty<IBrush?> HeaderBackgroundProperty =
+            AvaloniaProperty.Register<Calendar, IBrush?>(nameof(HeaderBackground));
 
-        public IBrush HeaderBackground
+        public IBrush? HeaderBackground
         {
             get { return GetValue(HeaderBackgroundProperty); }
             set { SetValue(HeaderBackgroundProperty, value); }
@@ -478,7 +478,7 @@ namespace Avalonia.Controls
             }
             else
             {
-                throw new ArgumentOutOfRangeException("d", "Invalid SelectionMode");
+                throw new ArgumentOutOfRangeException(nameof(e), "Invalid SelectionMode");
             }
         }
 
@@ -574,7 +574,7 @@ namespace Avalonia.Controls
                     }
                     else
                     {
-                        throw new ArgumentOutOfRangeException("d", "SelectedDate value is not valid.");
+                        throw new ArgumentOutOfRangeException(nameof(e), "SelectedDate value is not valid.");
                     }
                 }
                 else

src/Avalonia.Controls/Calendar/CalendarBlackoutDatesCollection.cs

@@ -15,7 +15,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// The Calendar whose dates this object represents.
         /// </summary>
-        private Calendar _owner;
+        private readonly Calendar _owner;
         
         /// <summary>
         /// Initializes a new instance of the
@@ -79,13 +79,13 @@ namespace Avalonia.Controls.Primitives
 
             if (DateTime.Compare(end, start) > -1)
             {
-                rangeStart = DateTimeHelper.DiscardTime(start).Value;
-                rangeEnd = DateTimeHelper.DiscardTime(end).Value;
+                rangeStart = DateTimeHelper.DiscardTime(start);
+                rangeEnd = DateTimeHelper.DiscardTime(end);
             }
             else
             {
-                rangeStart = DateTimeHelper.DiscardTime(end).Value;
-                rangeEnd = DateTimeHelper.DiscardTime(start).Value;
+                rangeStart = DateTimeHelper.DiscardTime(end);
+                rangeEnd = DateTimeHelper.DiscardTime(start);
             }
 
             int count = Count;
@@ -144,7 +144,7 @@ namespace Avalonia.Controls.Primitives
 
             if (!IsValid(item))
             {
-                throw new ArgumentOutOfRangeException("Value is not valid.");
+                throw new ArgumentOutOfRangeException(nameof(item), "Value is not valid.");
             }
 
             base.InsertItem(index, item);
@@ -186,7 +186,7 @@ namespace Avalonia.Controls.Primitives
 
             if (!IsValid(item))
             {
-                throw new ArgumentOutOfRangeException("Value is not valid.");
+                throw new ArgumentOutOfRangeException(nameof(item), "Value is not valid.");
             }
 
             base.SetItem(index, item);

src/Avalonia.Controls/Calendar/CalendarItem.cs

@@ -44,30 +44,30 @@ namespace Avalonia.Controls.Primitives
         private ITemplate<Control>? _dayTitleTemplate;
         
         private DateTime _currentMonth;
-        private bool _isMouseLeftButtonDown = false;
-        private bool _isMouseLeftButtonDownYearView = false;
-        private bool _isControlPressed = false;
+        private bool _isMouseLeftButtonDown;
+        private bool _isMouseLeftButtonDownYearView;
+        private bool _isControlPressed;
 
-        private System.Globalization.Calendar _calendar = new System.Globalization.GregorianCalendar();
-
-        private PointerPressedEventArgs? _downEventArg;
-        private PointerPressedEventArgs? _downEventArgYearView;
+        private readonly System.Globalization.Calendar _calendar = new GregorianCalendar();
 
         internal Calendar? Owner { get; set; }
         internal CalendarDayButton? CurrentButton { get; set; }
 
-        public static readonly StyledProperty<IBrush> HeaderBackgroundProperty = Calendar.HeaderBackgroundProperty.AddOwner<CalendarItem>();
-        public IBrush HeaderBackground
+        public static readonly StyledProperty<IBrush?> HeaderBackgroundProperty = Calendar.HeaderBackgroundProperty.AddOwner<CalendarItem>();
+
+        public IBrush? HeaderBackground
         {
             get { return GetValue(HeaderBackgroundProperty); }
             set { SetValue(HeaderBackgroundProperty, value); }
         }
+
         public static readonly DirectProperty<CalendarItem, ITemplate<Control>?> DayTitleTemplateProperty =
                 AvaloniaProperty.RegisterDirect<CalendarItem, ITemplate<Control>?>(
                     nameof(DayTitleTemplate),
                     o => o.DayTitleTemplate,
                     (o,v) => o.DayTitleTemplate = v,
                     defaultBindingMode: BindingMode.OneTime);
+
         public ITemplate<Control>? DayTitleTemplate
         {
             get { return _dayTitleTemplate; }
@@ -178,7 +178,7 @@ namespace Avalonia.Controls.Primitives
                 {
                     if (_dayTitleTemplate != null)
                     {
-                        var cell = (Control) _dayTitleTemplate.Build();
+                        var cell = _dayTitleTemplate.Build();
                         cell.DataContext = string.Empty;
                         cell.SetValue(Grid.RowProperty, 0);
                         cell.SetValue(Grid.ColumnProperty, i);
@@ -308,16 +308,13 @@ namespace Avalonia.Controls.Primitives
             for (int childIndex = 0; childIndex < Calendar.ColumnsPerMonth; childIndex++)
             {
                 var daytitle = MonthView!.Children[childIndex];
-                if (daytitle != null)
+                if (Owner != null)
                 {
-                    if (Owner != null)
-                    {
-                        daytitle.DataContext = DateTimeHelper.GetCurrentDateFormat().ShortestDayNames[(childIndex + (int)Owner.FirstDayOfWeek) % NumberOfDaysPerWeek];
-                    }
-                    else
-                    {
-                        daytitle.DataContext = DateTimeHelper.GetCurrentDateFormat().ShortestDayNames[(childIndex + (int)DateTimeHelper.GetCurrentDateFormat().FirstDayOfWeek) % NumberOfDaysPerWeek];
-                    }
+                    daytitle.DataContext = DateTimeHelper.GetCurrentDateFormat().ShortestDayNames[(childIndex + (int)Owner.FirstDayOfWeek) % NumberOfDaysPerWeek];
+                }
+                else
+                {
+                    daytitle.DataContext = DateTimeHelper.GetCurrentDateFormat().ShortestDayNames[(childIndex + (int)DateTimeHelper.GetCurrentDateFormat().FirstDayOfWeek) % NumberOfDaysPerWeek];
                 }
             }
         }
@@ -527,7 +524,7 @@ namespace Avalonia.Controls.Primitives
                 childButton.Content = dateToAdd.Day.ToString(DateTimeHelper.GetCurrentDateFormat());
                 childButton.DataContext = dateToAdd;
 
-                if (DateTime.Compare((DateTime)DateTimeHelper.DiscardTime(DateTime.MaxValue), dateToAdd) > 0)
+                if (DateTime.Compare(DateTimeHelper.DiscardTime(DateTime.MaxValue), dateToAdd) > 0)
                 {
                     // Since we are sure DisplayDate is not equal to
                     // DateTime.MaxValue, it is safe to use AddDays 
@@ -587,7 +584,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (Owner != null)
             {
-                _currentMonth = (DateTime)Owner.SelectedMonth;
+                _currentMonth = Owner.SelectedMonth;
             }
             else
             {
@@ -676,7 +673,7 @@ namespace Avalonia.Controls.Primitives
             if (Owner != null)
             {
                 selectedYear = Owner.SelectedYear;
-                _currentMonth = (DateTime)Owner.SelectedMonth;
+                _currentMonth = Owner.SelectedMonth;
             }
             else
             {
@@ -696,9 +693,9 @@ namespace Avalonia.Controls.Primitives
                 SetYearButtons(decade, decadeEnd);
             }
         }
-        internal void UpdateYearViewSelection(CalendarButton calendarButton)
+        internal void UpdateYearViewSelection(CalendarButton? calendarButton)
         {
-            if (Owner != null && calendarButton != null && calendarButton.DataContext != null)
+            if (Owner != null && calendarButton?.DataContext is DateTime selectedDate)
             {
                 Owner.FocusCalendarButton!.IsCalendarButtonFocused = false;
                 Owner.FocusCalendarButton = calendarButton;
@@ -706,11 +703,11 @@ namespace Avalonia.Controls.Primitives
 
                 if (Owner.DisplayMode == CalendarMode.Year)
                 {
-                    Owner.SelectedMonth = (DateTime)calendarButton.DataContext;
+                    Owner.SelectedMonth = selectedDate;
                 }
                 else
                 {
-                    Owner.SelectedYear = (DateTime)calendarButton.DataContext;
+                    Owner.SelectedYear = selectedDate;
                 }
             }
         }
@@ -719,7 +716,7 @@ namespace Avalonia.Controls.Primitives
         {
             int year;
             int count = -1;
-            foreach (object child in YearView!.Children)
+            foreach (var child in YearView!.Children)
             {
                 CalendarButton childButton = (CalendarButton)child;
                 year = decade + count;
@@ -859,7 +856,8 @@ namespace Avalonia.Controls.Primitives
         {
             if (Owner != null)
             {
-                if (_isMouseLeftButtonDown && sender is CalendarDayButton b && b.IsEnabled && !b.IsBlackout)
+                if (_isMouseLeftButtonDown
+                    && sender is CalendarDayButton { IsEnabled: true, IsBlackout: false, DataContext: DateTime selectedDate } b)
                 {
                     // Update the states of all buttons to be selected starting
                     // from HoverStart to b
@@ -867,7 +865,6 @@ namespace Avalonia.Controls.Primitives
                     {
                         case CalendarSelectionMode.SingleDate:
                             {
-                                DateTime selectedDate = (DateTime)b.DataContext!;
                                 Owner.CalendarDatePickerDisplayDateFlag = true;
                                 if (Owner.SelectedDates.Count == 0)
                                 {
@@ -882,10 +879,9 @@ namespace Avalonia.Controls.Primitives
                         case CalendarSelectionMode.SingleRange:
                         case CalendarSelectionMode.MultipleRange:
                             {
-                                Debug.Assert(b.DataContext != null, "The DataContext should not be null!");
                                 Owner.UnHighlightDays();
                                 Owner.HoverEndIndex = b.Index;
-                                Owner.HoverEnd = (DateTime?)b.DataContext;
+                                Owner.HoverEnd = selectedDate;
                                 // Update the States of the buttons
                                 Owner.HighlightDays();
                                 return;
@@ -904,22 +900,14 @@ namespace Avalonia.Controls.Primitives
                     Owner.Focus();
                 }
 
-                bool ctrl, shift;
-                CalendarExtensions.GetMetaKeyState(e.KeyModifiers, out ctrl, out shift);
-                CalendarDayButton b = (CalendarDayButton)sender!;
+                CalendarExtensions.GetMetaKeyState(e.KeyModifiers, out var ctrl, out var shift);
 
-                if (b != null)
+                if (sender is CalendarDayButton b)
                 {
                     _isControlPressed = ctrl;
-                    if (b.IsEnabled && !b.IsBlackout)
+                    if (b.IsEnabled && !b.IsBlackout && b.DataContext is DateTime selectedDate)
                     {
-                        DateTime selectedDate = (DateTime)b.DataContext!;
                         _isMouseLeftButtonDown = true;
-                        // null check is added for unit tests
-                        if (e != null)
-                        {
-                            _downEventArg = e;
-                        }
 
                         switch (Owner.SelectionMode)
                         {
@@ -1010,12 +998,12 @@ namespace Avalonia.Controls.Primitives
                 }
             }
         }
-        private void AddSelection(CalendarDayButton b)
+        private void AddSelection(CalendarDayButton b, DateTime selectedDate)
         {
             if (Owner != null)
             {
                 Owner.HoverEndIndex = b.Index;
-                Owner.HoverEnd = (DateTime)b.DataContext!;
+                Owner.HoverEnd = selectedDate;
 
                 if (Owner.HoverEnd != null && Owner.HoverStart != null)
                 {
@@ -1025,7 +1013,7 @@ namespace Avalonia.Controls.Primitives
                     // SelectionMode
                     Owner.IsMouseSelection = true;
                     Owner.SelectedDates.AddRange(Owner.HoverStart.Value, Owner.HoverEnd.Value);
-                    Owner.OnDayClick((DateTime)b.DataContext);
+                    Owner.OnDayClick(selectedDate);
                 }
             }
         }
@@ -1039,11 +1027,11 @@ namespace Avalonia.Controls.Primitives
                     Owner.OnDayButtonMouseUp(e);
                 }
                 _isMouseLeftButtonDown = false;
-                if (b != null && b.DataContext != null)
+                if (b != null && b.DataContext is DateTime selectedDate)
                 {
                     if (Owner.SelectionMode == CalendarSelectionMode.None || Owner.SelectionMode == CalendarSelectionMode.SingleDate)
                     {
-                        Owner.OnDayClick((DateTime)b.DataContext);
+                        Owner.OnDayClick(selectedDate);
                         return;
                     }
                     if (Owner.HoverStart.HasValue)
@@ -1058,14 +1046,14 @@ namespace Avalonia.Controls.Primitives
                                         Owner.RemovedItems.Add(item);
                                     }
                                     Owner.SelectedDates.ClearInternal();
-                                    AddSelection(b);
+                                    AddSelection(b, selectedDate);
                                     return;
                                 }
                             case CalendarSelectionMode.MultipleRange:
                                 {
                                     // add the selection (either single day or
                                     // SingleRange day)
-                                    AddSelection(b);
+                                    AddSelection(b, selectedDate);
                                     return;
                                 }
                         }
@@ -1076,7 +1064,7 @@ namespace Avalonia.Controls.Primitives
                         // be able to switch months
                         if (b.IsInactive && b.IsBlackout)
                         {
-                            Owner.OnDayClick((DateTime)b.DataContext);
+                            Owner.OnDayClick(selectedDate);
                         }
                     }
                 }
@@ -1095,9 +1083,9 @@ namespace Avalonia.Controls.Primitives
                         Owner.HoverStart = null;
                         _isMouseLeftButtonDown = false;
                         b.IsSelected = false;
-                        if (b.DataContext != null)
+                        if (b.DataContext is DateTime selectedDate)
                         {
-                            Owner.SelectedDates.Remove((DateTime)b.DataContext);
+                            Owner.SelectedDates.Remove(selectedDate);
                         }
                     }
                 }
@@ -1107,35 +1095,26 @@ namespace Avalonia.Controls.Primitives
 
         private void Month_CalendarButtonMouseDown(object? sender, PointerPressedEventArgs e)
         {
-            CalendarButton b = (CalendarButton)sender!;
-
             _isMouseLeftButtonDownYearView = true;
 
-            if (e != null)
-            {
-                _downEventArgYearView = e;
-            }
-
-            UpdateYearViewSelection(b);
+            UpdateYearViewSelection(sender as CalendarButton);
         }
 
         internal void Month_CalendarButtonMouseUp(object? sender, PointerReleasedEventArgs e)
         {
             _isMouseLeftButtonDownYearView = false;
 
-            if (Owner != null)
+            if (Owner != null && (sender as CalendarButton)?.DataContext is DateTime newMonth)
             {
-                DateTime newmonth = (DateTime)((CalendarButton)sender!).DataContext!;
-
                 if (Owner.DisplayMode == CalendarMode.Year)
                 {
-                    Owner.DisplayDate = newmonth;
+                    Owner.DisplayDate = newMonth;
                     Owner.DisplayMode = CalendarMode.Month;
                 }
                 else
                 {
                     Debug.Assert(Owner.DisplayMode == CalendarMode.Decade, "The owning Calendar should be in decade mode!");
-                    Owner.SelectedMonth = newmonth;
+                    Owner.SelectedMonth = newMonth;
                     Owner.DisplayMode = CalendarMode.Year;
                 }
             }
@@ -1145,8 +1124,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (_isMouseLeftButtonDownYearView)
             {
-                CalendarButton b = (CalendarButton)sender!;
-                UpdateYearViewSelection(b);
+                UpdateYearViewSelection(sender as CalendarButton);
             }
         }
 

src/Avalonia.Controls/Calendar/DateTimeHelper.cs

@@ -53,7 +53,7 @@ namespace Avalonia.Controls
 
         public static int CompareDays(DateTime dt1, DateTime dt2)
         {
-            return DateTime.Compare(DiscardTime(dt1).Value, DiscardTime(dt2).Value);
+            return DateTime.Compare(DiscardTime(dt1), DiscardTime(dt2));
         }
 
         public static int CompareYearMonth(DateTime dt1, DateTime dt2)
@@ -71,14 +71,9 @@ namespace Avalonia.Controls
             return new DateTime(d.Year, d.Month, 1, 0, 0, 0);
         }
 
-        [return: NotNullIfNotNull("d")]
-        public static DateTime? DiscardTime(DateTime? d)
+        public static DateTime DiscardTime(DateTime d)
         {
-            if (d == null)
-            {
-                return null;
-            }
-            return d.Value.Date;
+            return d.Date;
         }
 
         public static int EndOfDecade(DateTime date)
@@ -127,28 +122,14 @@ namespace Avalonia.Controls
 
         public static string ToYearMonthPatternString(DateTime date)
         {
-            string result = string.Empty;
-            DateTimeFormatInfo format = GetCurrentDateFormat();
-
-            if (format != null)
-            {
-                result = date.ToString(format.YearMonthPattern, format);
-            }
-
-            return result;
+            var format = GetCurrentDateFormat();
+            return date.ToString(format.YearMonthPattern, format);
         }
 
         public static string ToYearString(DateTime date)
         {
-            string result = string.Empty;
-            DateTimeFormatInfo format = GetCurrentDateFormat();
-
-            if (format != null)
-            {
-                result = date.Year.ToString(format);
-            }
-
-            return result;
+            var format = GetCurrentDateFormat();
+            return date.Year.ToString(format);
         }
     }
 }

src/Avalonia.Controls/CalendarDatePicker/CalendarDatePicker.cs

@@ -51,11 +51,11 @@ namespace Avalonia.Controls
         private bool _isDropDownOpen;
         private DateTime? _selectedDate;
         private string? _text;
-        private bool _suspendTextChangeHandler = false;
-        private bool _isPopupClosing = false;
-        private bool _ignoreButtonClick = false;
-        private bool _isFlyoutOpen = false;
-        private bool _isPressed = false;
+        private bool _suspendTextChangeHandler;
+        private bool _isPopupClosing;
+        private bool _ignoreButtonClick;
+        private bool _isFlyoutOpen;
+        private bool _isPressed;
 
         /// <summary>
         /// Occurs when the drop-down
@@ -185,7 +185,7 @@ namespace Avalonia.Controls
             {
                 _textBox.KeyDown += TextBox_KeyDown;
                 _textBox.GotFocus += TextBox_GotFocus;
-                _textBoxTextChangedSubscription = _textBox.GetObservable(TextBox.TextProperty).Subscribe(txt => TextBox_TextChanged());
+                _textBoxTextChangedSubscription = _textBox.GetObservable(TextBox.TextProperty).Subscribe(_ => TextBox_TextChanged());
 
                 if(SelectedDate.HasValue)
                 {
@@ -292,7 +292,7 @@ namespace Avalonia.Controls
             // Text
             else if (change.Property == TextProperty)
             {
-                var (oldValue, newValue) = change.GetOldAndNewValue<string>();
+                var (_, newValue) = change.GetOldAndNewValue<string?>();
 
                 if (!_suspendTextChangeHandler)
                 {
@@ -595,9 +595,9 @@ namespace Avalonia.Controls
 
         private void Calendar_KeyDown(object? sender, KeyEventArgs e)
         {
-            Calendar? c = sender as Calendar ?? throw new ArgumentException("Sender must be Calendar.", nameof(sender));
-
-            if (!e.Handled && (e.Key == Key.Enter || e.Key == Key.Space || e.Key == Key.Escape) && c.DisplayMode == CalendarMode.Month)
+            if (!e.Handled
+                && sender is Calendar { DisplayMode: CalendarMode.Month }
+                && (e.Key == Key.Enter || e.Key == Key.Space || e.Key == Key.Escape))
             {
                 Focus();
                 IsDropDownOpen = false;

src/Avalonia.Controls/Chrome/TitleBar.cs

@@ -17,28 +17,26 @@ namespace Avalonia.Controls.Chrome
 
         private void UpdateSize(Window window)
         {
-            if (window != null)
+            Margin = new Thickness(
+                window.OffScreenMargin.Left,
+                window.OffScreenMargin.Top,
+                window.OffScreenMargin.Right,
+                window.OffScreenMargin.Bottom);
+
+            if (window.WindowState != WindowState.FullScreen)
             {
-                Margin = new Thickness(
-                    window.OffScreenMargin.Left,
-                    window.OffScreenMargin.Top,
-                    window.OffScreenMargin.Right,
-                    window.OffScreenMargin.Bottom);
+                Height = window.WindowDecorationMargin.Top;
 
-                if (window.WindowState != WindowState.FullScreen)
+                if (_captionButtons != null)
                 {
-                    Height = window.WindowDecorationMargin.Top;
-
-                    if (_captionButtons != null)
-                    {
-                        _captionButtons.Height = Height;
-                    }
+                    _captionButtons.Height = Height;
                 }
-
-                IsVisible = window.PlatformImpl?.NeedsManagedDecorations ?? false;
             }
+
+            IsVisible = window.PlatformImpl?.NeedsManagedDecorations ?? false;
         }
 
+        /// <inheritdoc />
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
             base.OnApplyTemplate(e);
@@ -55,6 +53,7 @@ namespace Avalonia.Controls.Chrome
             }
         }
 
+        /// <inheritdoc />
         protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
         {
             base.OnAttachedToVisualTree(e);
@@ -64,13 +63,13 @@ namespace Avalonia.Controls.Chrome
                 _disposables = new CompositeDisposable(6)
                 {
                     window.GetObservable(Window.WindowDecorationMarginProperty)
-                        .Subscribe(x => UpdateSize(window)),
+                        .Subscribe(_ => UpdateSize(window)),
                     window.GetObservable(Window.ExtendClientAreaTitleBarHeightHintProperty)
-                        .Subscribe(x => UpdateSize(window)),
+                        .Subscribe(_ => UpdateSize(window)),
                     window.GetObservable(Window.OffScreenMarginProperty)
-                        .Subscribe(x => UpdateSize(window)),
+                        .Subscribe(_ => UpdateSize(window)),
                     window.GetObservable(Window.ExtendClientAreaChromeHintsProperty)
-                        .Subscribe(x => UpdateSize(window)),
+                        .Subscribe(_ => UpdateSize(window)),
                     window.GetObservable(Window.WindowStateProperty)
                         .Subscribe(x =>
                         {
@@ -80,11 +79,12 @@ namespace Avalonia.Controls.Chrome
                             PseudoClasses.Set(":fullscreen", x == WindowState.FullScreen);
                         }),
                     window.GetObservable(Window.IsExtendedIntoWindowDecorationsProperty)
-                        .Subscribe(x => UpdateSize(window))
+                        .Subscribe(_ => UpdateSize(window))
                 };
             }
         }
 
+        /// <inheritdoc />
         protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
         {
             base.OnDetachedFromVisualTree(e);

src/Avalonia.Controls/ComboBox.cs

@@ -1,19 +1,19 @@
 using System;
+using System.Diagnostics;
 using System.Linq;
 using Avalonia.Automation.Peers;
-using Avalonia.Reactive;
-using Avalonia.Controls.Generators;
-using Avalonia.Controls.Mixins;
-using Avalonia.Controls.Presenters;
+using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
+using Avalonia.Controls.Selection;
 using Avalonia.Controls.Shapes;
 using Avalonia.Controls.Templates;
+using Avalonia.Controls.Utils;
 using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.Layout;
 using Avalonia.Media;
+using Avalonia.Reactive;
 using Avalonia.VisualTree;
-using Avalonia.Controls.Metadata;
 
 namespace Avalonia.Controls
 {
@@ -219,7 +219,7 @@ namespace Avalonia.Controls
                 }
                 else if (e.Key == Key.Up)
                 {
-                    SelectPrev();
+                    SelectPrevious();
                     e.Handled = true;
                 }
             }
@@ -250,7 +250,7 @@ namespace Avalonia.Controls
                         if (e.Delta.Y < 0)
                             SelectNext();
                         else
-                            SelectPrev();
+                            SelectPrevious();
 
                         e.Handled = true;
                     }
@@ -478,19 +478,40 @@ namespace Avalonia.Controls
             }
         }
 
-        private void SelectNext()
-        {
-            if (ItemCount >= 1)
-            {
-                MoveSelection(NavigationDirection.Next, WrapSelection);
-            }
-        }
+        private void SelectNext() => MoveSelection(SelectedIndex, 1, WrapSelection);
+        private void SelectPrevious() => MoveSelection(SelectedIndex, -1, WrapSelection);
 
-        private void SelectPrev()
+        private void MoveSelection(int startIndex, int step, bool wrap)
         {
-            if (ItemCount >= 1)
+            static bool IsSelectable(object? o) => (o as AvaloniaObject)?.GetValue(IsEnabledProperty) ?? true;
+
+            var count = ItemCount;
+
+            for (int i = startIndex + step; i != startIndex; i += step)
             {
-                MoveSelection(NavigationDirection.Previous, WrapSelection);
+                if (i < 0 || i >= count)
+                {
+                    if (wrap)
+                    {
+                        if (i < 0)
+                            i += count;
+                        else if (i >= count)
+                            i %= count;
+                    }
+                    else
+                    {
+                        return;
+                    }
+                }
+
+                var item = ItemsView[i];
+                var container = ContainerFromIndex(i);
+                
+                if (IsSelectable(item) && IsSelectable(container))
+                {
+                    SelectedIndex = i;
+                    break;
+                }
             }
         }
     }

src/Avalonia.Controls/Control.cs

@@ -2,14 +2,12 @@ using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using Avalonia.Automation.Peers;
-using Avalonia.Controls.Documents;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
 using Avalonia.LogicalTree;
-using Avalonia.Media;
 using Avalonia.Rendering;
 using Avalonia.Styling;
 using Avalonia.Threading;
@@ -211,8 +209,6 @@ namespace Avalonia.Controls
             remove => RemoveHandler(SizeChangedEvent, value);
         }
 
-        public new Control? Parent => (Control?)base.Parent;
-
         /// <inheritdoc/>
         bool IDataTemplateHost.IsDataTemplatesInitialized => _dataTemplates != null;
 

src/Avalonia.Controls/Controls.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using Avalonia.Collections;
 
@@ -13,7 +14,7 @@ namespace Avalonia.Controls
         /// </summary>
         public Controls()
         {
-            ResetBehavior = ResetBehavior.Remove;
+            Configure();
         }
 
         /// <summary>
@@ -21,9 +22,22 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="items">The initial items in the collection.</param>
         public Controls(IEnumerable<Control> items)
-            : base(items)
+        {
+            Configure();
+            AddRange(items); // virtual member call in ctor, ok for our current implementation
+        }
+
+        private void Configure()
         {
             ResetBehavior = ResetBehavior.Remove;
+            Validate = item =>
+            {
+                if (item is null)
+                {
+                    throw new ArgumentNullException(nameof(item),
+                        $"A null control cannot be added to a {nameof(Controls)} collection.");
+                }
+            };
         }
     }
 }

src/Avalonia.Controls/Converters/MenuScrollingVisibilityConverter.cs

@@ -14,7 +14,6 @@ namespace Avalonia.Controls.Converters
         public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture)
         {
             if (parameter == null ||
-                values == null ||
                 values.Count != 4 ||
                 !(values[0] is ScrollBarVisibility visibility) ||
                 !(values[1] is double offset) ||

src/Avalonia.Controls/DefinitionBase.cs

@@ -21,9 +21,9 @@ namespace Avalonia.Controls
         /// <summary>
         /// SharedSizeGroup property.
         /// </summary>
-        public string SharedSizeGroup
+        public string? SharedSizeGroup
         {
-            get { return (string)GetValue(SharedSizeGroupProperty); }
+            get { return GetValue(SharedSizeGroupProperty); }
             set { SetValue(SharedSizeGroupProperty, value); }
         }
 
@@ -32,20 +32,15 @@ namespace Avalonia.Controls
         /// </summary>
         internal void OnEnterParentTree()
         {
-            this.InheritanceParent = Parent;
+            InheritanceParent = Parent;
             if (_sharedState == null)
             {
                 //  start with getting SharedSizeGroup value. 
                 //  this property is NOT inherited which should result in better overall perf.
-                string sharedSizeGroupId = SharedSizeGroup;
-                if (sharedSizeGroupId != null)
+                if (SharedSizeGroup is { } sharedSizeGroupId && PrivateSharedSizeScope is { } privateSharedSizeScope)
                 {
-                    SharedSizeScope? privateSharedSizeScope = PrivateSharedSizeScope;
-                    if (privateSharedSizeScope != null)
-                    {
-                        _sharedState = privateSharedSizeScope.EnsureSharedState(sharedSizeGroupId);
-                        _sharedState.AddMember(this);
-                    }
+                    _sharedState = privateSharedSizeScope.EnsureSharedState(sharedSizeGroupId);
+                    _sharedState.AddMember(this);
                 }
             }
 
@@ -321,13 +316,12 @@ namespace Avalonia.Controls
             return ((_flags & flags) == flags);
         }
 
-        private static void OnSharedSizeGroupPropertyChanged(AvaloniaObject d, AvaloniaPropertyChangedEventArgs e)
+        private static void OnSharedSizeGroupPropertyChanged(DefinitionBase definition,
+            AvaloniaPropertyChangedEventArgs<string?> e)
         {
-            DefinitionBase definition = (DefinitionBase)d;
-
             if (definition.Parent != null)
             {
-                string sharedSizeGroupId = (string)e.NewValue!;
+                string? sharedSizeGroupId = e.NewValue.Value;
 
                 if (definition._sharedState != null)
                 {
@@ -337,16 +331,14 @@ namespace Avalonia.Controls
                     definition._sharedState = null;
                 }
 
-                if ((definition._sharedState == null) && (sharedSizeGroupId != null))
+                if (definition._sharedState == null
+                    && sharedSizeGroupId != null
+                    && definition.PrivateSharedSizeScope is { } privateSharedSizeScope)
                 {
-                    SharedSizeScope? privateSharedSizeScope = definition.PrivateSharedSizeScope;
-                    if (privateSharedSizeScope != null)
-                    {
-                        //  if definition is not registered and both: shared size group id AND private shared scope 
-                        //  are available, then register definition.
-                        definition._sharedState = privateSharedSizeScope.EnsureSharedState(sharedSizeGroupId);
-                        definition._sharedState.AddMember(definition);
-                    }
+                    //  if definition is not registered and both: shared size group id AND private shared scope
+                    //  are available, then register definition.
+                    definition._sharedState = privateSharedSizeScope.EnsureSharedState(sharedSizeGroupId);
+                    definition._sharedState.AddMember(definition);
                 }
             }
         }
@@ -357,17 +349,15 @@ namespace Avalonia.Controls
         /// b) contains only letters, digits and underscore ('_').
         /// c) does not start with a digit.
         /// </remarks>
-        private static bool SharedSizeGroupPropertyValueValid(string value)
+        private static bool SharedSizeGroupPropertyValueValid(string? id)
         {
             //  null is default value
-            if (value == null)
+            if (id == null)
             {
                 return true;
             }
 
-            string id = (string)value;
-
-            if (!string.IsNullOrEmpty(id))
+            if (id.Length > 0)
             {
                 int i = -1;
                 while (++i < id.Length)
@@ -397,14 +387,11 @@ namespace Avalonia.Controls
         /// existing scope just left. In both cases if the DefinitionBase object is already registered
         /// in SharedSizeState, it should un-register and register itself in a new one.
         /// </remark>
-        private static void OnPrivateSharedSizeScopePropertyChanged(AvaloniaObject d, AvaloniaPropertyChangedEventArgs e)
+        private static void OnPrivateSharedSizeScopePropertyChanged(DefinitionBase definition,
+            AvaloniaPropertyChangedEventArgs<SharedSizeScope?> e)
         {
-            DefinitionBase definition = (DefinitionBase)d;
-
             if (definition.Parent != null)
             {
-                SharedSizeScope privateSharedSizeScope = (SharedSizeScope)e.NewValue!;
-
                 if (definition._sharedState != null)
                 {
                     //  if definition is already registered And shared size scope is changing,
@@ -413,16 +400,14 @@ namespace Avalonia.Controls
                     definition._sharedState = null;
                 }
 
-                if ((definition._sharedState == null) && (privateSharedSizeScope != null))
+                if (definition._sharedState == null
+                    && e.NewValue.Value is { } privateSharedSizeScope
+                    && definition.SharedSizeGroup is { } sharedSizeGroup)
                 {
-                    string sharedSizeGroup = definition.SharedSizeGroup;
-                    if (sharedSizeGroup != null)
-                    {
-                        //  if definition is not registered and both: shared size group id AND private shared scope 
-                        //  are available, then register definition.
-                        definition._sharedState = privateSharedSizeScope.EnsureSharedState(definition.SharedSizeGroup);
-                        definition._sharedState.AddMember(definition);
-                    }
+                    //  if definition is not registered and both: shared size group id AND private shared scope
+                    //  are available, then register definition.
+                    definition._sharedState = privateSharedSizeScope.EnsureSharedState(sharedSizeGroup);
+                    definition._sharedState.AddMember(definition);
                 }
             }
         }
@@ -432,7 +417,7 @@ namespace Avalonia.Controls
         /// </summary>
         private SharedSizeScope? PrivateSharedSizeScope
         {
-            get { return (SharedSizeScope?)GetValue(PrivateSharedSizeScopeProperty); }
+            get { return GetValue(PrivateSharedSizeScopeProperty); }
         }
 
         /// <summary>
@@ -465,7 +450,7 @@ namespace Avalonia.Controls
 
         private SharedSizeState? _sharedState;           //  reference to shared state object this instance is registered with
 
-        [System.Flags]
+        [Flags]
         private enum Flags : byte
         {
             //
@@ -520,11 +505,10 @@ namespace Avalonia.Controls
             /// </summary>
             internal SharedSizeState(SharedSizeScope sharedSizeScope, string sharedSizeGroupId)
             {
-                Debug.Assert(sharedSizeScope != null && sharedSizeGroupId != null);
                 _sharedSizeScope = sharedSizeScope;
                 _sharedSizeGroupId = sharedSizeGroupId;
                 _registry = new List<DefinitionBase>();
-                _layoutUpdated = new EventHandler(OnLayoutUpdated);
+                _layoutUpdated = OnLayoutUpdated;
                 _broadcastInvalidation = true;
             }
 
@@ -568,7 +552,7 @@ namespace Avalonia.Controls
                 {
                     for (int i = 0, count = _registry.Count; i < count; ++i)
                     {
-                        Grid parentGrid = (Grid)(_registry[i].Parent!);
+                        Grid parentGrid = _registry[i].Parent!;
                         parentGrid.Invalidate();
                     }
                     _broadcastInvalidation = false;
@@ -703,7 +687,7 @@ namespace Avalonia.Controls
                         // measure is invalid - it used the old shared size,
                         // which is larger than d's (possibly changed) minSize
                         measureIsValid = (definitionBase.LayoutWasUpdated &&
-                                        MathUtilities.GreaterThanOrClose(definitionBase._minSize, this.MinSize));
+                                        MathUtilities.GreaterThanOrClose(definitionBase._minSize, MinSize));
                     }
 
                     if(!measureIsValid)
@@ -786,8 +770,8 @@ namespace Avalonia.Controls
         /// </description></item>
         /// </list>
         /// </remarks> 
-        public static readonly AttachedProperty<string> SharedSizeGroupProperty =
-            AvaloniaProperty.RegisterAttached<DefinitionBase, Control, string>(
+        public static readonly AttachedProperty<string?> SharedSizeGroupProperty =
+            AvaloniaProperty.RegisterAttached<DefinitionBase, Control, string?>(
                 "SharedSizeGroup",
                 validate: SharedSizeGroupPropertyValueValid);
 
@@ -796,8 +780,8 @@ namespace Avalonia.Controls
         /// </summary>
         static DefinitionBase()
         {
-            SharedSizeGroupProperty.Changed.AddClassHandler<DefinitionBase>(OnSharedSizeGroupPropertyChanged);
-            PrivateSharedSizeScopeProperty.Changed.AddClassHandler<DefinitionBase>(OnPrivateSharedSizeScopePropertyChanged);
+            SharedSizeGroupProperty.Changed.AddClassHandler<DefinitionBase, string?>(OnSharedSizeGroupPropertyChanged);
+            PrivateSharedSizeScopeProperty.Changed.AddClassHandler<DefinitionBase, SharedSizeScope?>(OnPrivateSharedSizeScopePropertyChanged);
         }
 
         /// <summary>

src/Avalonia.Controls/DockPanel.cs

@@ -101,9 +101,6 @@ namespace Avalonia.Controls
                 Size childConstraint;             // Contains the suggested input constraint for this child.
                 Size childDesiredSize;            // Contains the return size from child measure.
 
-                if (child == null)
-                { continue; }
-
                 // Child constraint is the remaining size; this is total size minus size consumed by previous children.
                 childConstraint = new Size(Math.Max(0.0, constraint.Width - accumulatedWidth),
                                            Math.Max(0.0, constraint.Height - accumulatedHeight));
@@ -122,7 +119,7 @@ namespace Avalonia.Controls
                 // will deal with computing our minimum size (parentSize) due to that accumulation.
                 // Therefore, we only need to compute our minimum size (parentSize) in dimensions that this child does
                 //   not accumulate: Width for Top/Bottom, Height for Left/Right.
-                switch (DockPanel.GetDock((Control)child))
+                switch (GetDock(child))
                 {
                     case Dock.Left:
                     case Dock.Right:
@@ -164,8 +161,6 @@ namespace Avalonia.Controls
             for (int i = 0; i < totalChildrenCount; ++i)
             {
                 var child = children[i];
-                if (child == null)
-                { continue; }
 
                 Size childDesiredSize = child.DesiredSize;
                 Rect rcChild = new Rect(
@@ -176,7 +171,7 @@ namespace Avalonia.Controls
 
                 if (i < nonFillChildrenCount)
                 {
-                    switch (DockPanel.GetDock((Control)child))
+                    switch (GetDock(child))
                     {
                         case Dock.Left:
                             accumulatedLeft += childDesiredSize.Width;

src/Avalonia.Controls/Documents/Inline.cs

@@ -13,8 +13,8 @@ namespace Avalonia.Controls.Documents
         /// <summary>
         /// AvaloniaProperty for <see cref="TextDecorations" /> property.
         /// </summary>
-        public static readonly StyledProperty<TextDecorationCollection> TextDecorationsProperty =
-            AvaloniaProperty.Register<Inline, TextDecorationCollection>(
+        public static readonly StyledProperty<TextDecorationCollection?> TextDecorationsProperty =
+            AvaloniaProperty.Register<Inline, TextDecorationCollection?>(
                 nameof(TextDecorations));
 
         /// <summary>
@@ -28,7 +28,7 @@ namespace Avalonia.Controls.Documents
         /// <summary>
         /// The TextDecorations property specifies decorations that are added to the text of an element.
         /// </summary>
-        public TextDecorationCollection TextDecorations
+        public TextDecorationCollection? TextDecorations
         {
             get { return GetValue(TextDecorationsProperty); }
             set { SetValue(TextDecorationsProperty, value); }
@@ -83,7 +83,8 @@ namespace Avalonia.Controls.Documents
             return new GenericTextRunProperties(new Typeface(FontFamily, fontStyle, fontWeight), FontSize,
                 textDecorations, Foreground, background, BaselineAlignment);
         }
-        
+
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);

src/Avalonia.Controls/Documents/InlineUIContainer.cs

@@ -64,5 +64,23 @@ namespace Avalonia.Controls.Documents
         internal override void AppendText(StringBuilder stringBuilder)
         {
         }
+
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
+        {
+            base.OnPropertyChanged(change);
+
+            if (change.Property == ChildProperty)
+            {
+                if(change.OldValue is Control oldChild)
+                {
+                    LogicalChildren.Remove(oldChild);
+                }
+
+                if(change.NewValue is Control newChild)
+                {
+                    LogicalChildren.Add(newChild);
+                }
+            }
+        }
     }
 }

src/Avalonia.Controls/Documents/Span.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Text;
 using Avalonia.Media.TextFormatting;
@@ -51,6 +52,7 @@ namespace Avalonia.Controls.Documents
             }
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
@@ -68,26 +70,26 @@ namespace Avalonia.Controls.Documents
         {
             base.OnInlineHostChanged(oldValue, newValue);
 
-            if (Inlines is not null)
-            {
-                Inlines.InlineHost = newValue;
-            }
+            Inlines.InlineHost = newValue;
         }
 
         private void OnInlinesChanged(InlineCollection? oldValue, InlineCollection? newValue)
         {
+            void OnInlinesInvalidated(object? sender, EventArgs e)
+                => InlineHost?.Invalidate();
+
             if (oldValue is not null)
             {
                 oldValue.LogicalChildren = null;
                 oldValue.InlineHost = null;
-                oldValue.Invalidated -= (s, e) => InlineHost?.Invalidate();
+                oldValue.Invalidated -= OnInlinesInvalidated;
             }
 
             if (newValue is not null)
             {
                 newValue.LogicalChildren = LogicalChildren;
                 newValue.InlineHost = InlineHost;
-                newValue.Invalidated += (s, e) => InlineHost?.Invalidate();
+                newValue.Invalidated += OnInlinesInvalidated;
             }
         }
     }

src/Avalonia.Controls/Grid.cs

@@ -164,20 +164,21 @@ namespace Avalonia.Controls
         /// <summary>
         /// Returns a ColumnDefinitions of column definitions.
         /// </summary>
+        [MemberNotNull(nameof(_extData))]
         public ColumnDefinitions ColumnDefinitions
         {
             get
             {
-                if (_data == null) { _data = new ExtendedData(); }
-                if (_data.ColumnDefinitions == null) { _data.ColumnDefinitions = new ColumnDefinitions() { Parent = this }; }
+                if (_extData == null) { _extData = new ExtendedData(); }
+                if (_extData.ColumnDefinitions == null) { _extData.ColumnDefinitions = new ColumnDefinitions() { Parent = this }; }
 
-                return (_data.ColumnDefinitions);
+                return (_extData.ColumnDefinitions);
             }
             set
             {
-                if (_data == null) { _data = new ExtendedData(); }
-                _data.ColumnDefinitions = value;
-                _data.ColumnDefinitions.Parent = this;
+                if (_extData == null) { _extData = new ExtendedData(); }
+                _extData.ColumnDefinitions = value;
+                _extData.ColumnDefinitions.Parent = this;
                 InvalidateMeasure();
             }
         }
@@ -185,20 +186,21 @@ namespace Avalonia.Controls
         /// <summary>
         /// Returns a RowDefinitions of row definitions.
         /// </summary>
+        [MemberNotNull(nameof(_extData))]
         public RowDefinitions RowDefinitions
         {
             get
             {
-                if (_data == null) { _data = new ExtendedData(); }
-                if (_data.RowDefinitions == null) { _data.RowDefinitions = new RowDefinitions() { Parent = this }; }
+                if (_extData == null) { _extData = new ExtendedData(); }
+                if (_extData.RowDefinitions == null) { _extData.RowDefinitions = new RowDefinitions() { Parent = this }; }
 
-                return (_data.RowDefinitions);
+                return (_extData.RowDefinitions);
             }
             set
             {
-                if (_data == null) { _data = new ExtendedData(); }
-                _data.RowDefinitions = value;
-                _data.RowDefinitions.Parent = this;
+                if (_extData == null) { _extData = new ExtendedData(); }
+                _extData.RowDefinitions = value;
+                _extData.RowDefinitions.Parent = this;
                 InvalidateMeasure();
             }
         }
@@ -211,7 +213,7 @@ namespace Avalonia.Controls
         protected override Size MeasureOverride(Size constraint)
         {
             Size gridDesiredSize;
-            ExtendedData extData = ExtData;
+            var extData = _extData;
 
             try
             {
@@ -221,17 +223,14 @@ namespace Avalonia.Controls
                 if (extData == null)
                 {
                     gridDesiredSize = new Size();
-                    var children = this.Children;
+                    var children = Children;
 
                     for (int i = 0, count = children.Count; i < count; ++i)
                     {
                         var child = children[i];
-                        if (child != null)
-                        {
-                            child.Measure(constraint);
-                            gridDesiredSize = new Size(Math.Max(gridDesiredSize.Width, child.DesiredSize.Width),
-                                                       Math.Max(gridDesiredSize.Height, child.DesiredSize.Height));
-                        }
+                        child.Measure(constraint);
+                        gridDesiredSize = new Size(Math.Max(gridDesiredSize.Width, child.DesiredSize.Width),
+                                                   Math.Max(gridDesiredSize.Height, child.DesiredSize.Height));
                     }
                 }
                 else
@@ -512,17 +511,14 @@ namespace Avalonia.Controls
             {
                 ArrangeOverrideInProgress = true;
 
-                if (_data == null)
+                if (_extData is null)
                 {
-                    var children = this.Children;
+                    var children = Children;
 
                     for (int i = 0, count = children.Count; i < count; ++i)
                     {
                         var child = children[i];
-                        if (child != null)
-                        {
-                            child.Arrange(new Rect(arrangeSize));
-                        }
+                        child.Arrange(new Rect(arrangeSize));
                     }
                 }
                 else
@@ -532,15 +528,11 @@ namespace Avalonia.Controls
                     SetFinalSize(DefinitionsU, arrangeSize.Width, true);
                     SetFinalSize(DefinitionsV, arrangeSize.Height, false);
 
-                    var children = this.Children;
+                    var children = Children;
 
                     for (int currentCell = 0; currentCell < PrivateCells.Length; ++currentCell)
                     {
                         var cell = children[currentCell];
-                        if (cell == null)
-                        {
-                            continue;
-                        }
 
                         int columnIndex = PrivateCells[currentCell].ColumnIndex;
                         int rowIndex = PrivateCells[currentCell].RowIndex;
@@ -599,7 +591,7 @@ namespace Avalonia.Controls
         {
             double value = 0.0;
 
-            Debug.Assert(_data != null);
+            Debug.Assert(_extData != null);
 
             //  actual value calculations require structure to be up-to-date
             if (!ColumnDefinitionsDirty)
@@ -621,7 +613,7 @@ namespace Avalonia.Controls
         {
             double value = 0.0;
 
-            Debug.Assert(_data != null);
+            Debug.Assert(_extData != null);
 
             //  actual value calculations require structure to be up-to-date
             if (!RowDefinitionsDirty)
@@ -654,18 +646,20 @@ namespace Avalonia.Controls
         /// <summary>
         /// Convenience accessor to ValidDefinitionsUStructure bit flag.
         /// </summary> 
+        [MemberNotNull(nameof(_extData))]
         internal bool ColumnDefinitionsDirty
         {
-            get => ColumnDefinitions?.IsDirty ?? false;
+            get => ColumnDefinitions.IsDirty;
             set => ColumnDefinitions.IsDirty = value;
         }
 
         /// <summary>
         /// Convenience accessor to ValidDefinitionsVStructure bit flag.
         /// </summary>
+        [MemberNotNull(nameof(_extData))]
         internal bool RowDefinitionsDirty
         {
-            get => RowDefinitions?.IsDirty ?? false;
+            get => RowDefinitions.IsDirty;
             set => RowDefinitions.IsDirty = value;
         }
 
@@ -686,8 +680,10 @@ namespace Avalonia.Controls
         /// </summary>
         private void ValidateCellsCore()
         {
-            var children = this.Children;
-            ExtendedData extData = ExtData;
+            Debug.Assert(_extData is not null);
+
+            var children = Children;
+            var extData = _extData!;
 
             extData.CellCachesCollection = new CellCache[children.Count];
             extData.CellGroup1 = int.MaxValue;
@@ -702,10 +698,6 @@ namespace Avalonia.Controls
             for (int i = PrivateCells.Length - 1; i >= 0; --i)
             {
                 var child = children[i];
-                if (child == null)
-                {
-                    continue;
-                }
 
                 CellCache cell = new CellCache();
 
@@ -713,19 +705,19 @@ namespace Avalonia.Controls
                 //  Read indices from the corresponding properties:
                 //      clamp to value < number_of_columns
                 //      column >= 0 is guaranteed by property value validation callback
-                cell.ColumnIndex = Math.Min(GetColumn((Control)child), DefinitionsU.Count - 1);
+                cell.ColumnIndex = Math.Min(GetColumn(child), DefinitionsU.Count - 1);
                 //      clamp to value < number_of_rows
                 //      row >= 0 is guaranteed by property value validation callback
-                cell.RowIndex = Math.Min(GetRow((Control)child), DefinitionsV.Count - 1);
+                cell.RowIndex = Math.Min(GetRow(child), DefinitionsV.Count - 1);
 
                 //  Read span properties:
                 //      clamp to not exceed beyond right side of the grid
                 //      column_span > 0 is guaranteed by property value validation callback
-                cell.ColumnSpan = Math.Min(GetColumnSpan((Control)child), DefinitionsU.Count - cell.ColumnIndex);
+                cell.ColumnSpan = Math.Min(GetColumnSpan(child), DefinitionsU.Count - cell.ColumnIndex);
 
                 //      clamp to not exceed beyond bottom side of the grid
                 //      row_span > 0 is guaranteed by property value validation callback
-                cell.RowSpan = Math.Min(GetRowSpan((Control)child), DefinitionsV.Count - cell.RowIndex);
+                cell.RowSpan = Math.Min(GetRowSpan(child), DefinitionsV.Count - cell.RowIndex);
 
                 Debug.Assert(0 <= cell.ColumnIndex && cell.ColumnIndex < DefinitionsU.Count);
                 Debug.Assert(0 <= cell.RowIndex && cell.RowIndex < DefinitionsV.Count);
@@ -792,7 +784,7 @@ namespace Avalonia.Controls
         {
             if (ColumnDefinitionsDirty)
             {
-                ExtendedData extData = ExtData;
+                var extData = _extData;
 
                 if (extData.ColumnDefinitions == null)
                 {
@@ -818,7 +810,7 @@ namespace Avalonia.Controls
                 ColumnDefinitionsDirty = false;
             }
 
-            Debug.Assert(ExtData.DefinitionsU != null && ExtData.DefinitionsU.Count > 0);
+            Debug.Assert(_extData is { DefinitionsU.Count: > 0 });
         }
 
         /// <summary>
@@ -833,7 +825,7 @@ namespace Avalonia.Controls
         {
             if (RowDefinitionsDirty)
             {
-                ExtendedData extData = ExtData;
+                var extData = _extData;
 
                 if (extData.RowDefinitions == null)
                 {
@@ -859,7 +851,7 @@ namespace Avalonia.Controls
                 RowDefinitionsDirty = false;
             }
 
-            Debug.Assert(ExtData.DefinitionsV != null && ExtData.DefinitionsV.Count > 0);
+            Debug.Assert(_extData is { DefinitionsV.Count: > 0 });
         }
 
         /// <summary>
@@ -965,8 +957,7 @@ namespace Avalonia.Controls
             bool ignoreDesiredSizeU,
             bool forceInfinityV)
         {
-            bool unusedHasDesiredSizeUChanged;
-            MeasureCellsGroup(cellsHead, referenceSize, ignoreDesiredSizeU, forceInfinityV, out unusedHasDesiredSizeUChanged);
+            MeasureCellsGroup(cellsHead, referenceSize, ignoreDesiredSizeU, forceInfinityV, out _);
         }
 
         /// <summary>
@@ -994,7 +985,7 @@ namespace Avalonia.Controls
                 return;
             }
 
-            var children = this.Children;
+            var children = Children;
             Hashtable? spanStore = null;
             bool ignoreDesiredSizeV = forceInfinityV;
 
@@ -1101,8 +1092,6 @@ namespace Avalonia.Controls
             int cell,
             bool forceInfinityV)
         {
-
-
             double cellMeasureWidth;
             double cellMeasureHeight;
 
@@ -1144,15 +1133,9 @@ namespace Avalonia.Controls
             }
 
 
-            var child = this.Children[cell];
-            if (child != null)
-            {
-                Size childConstraint = new Size(cellMeasureWidth, cellMeasureHeight);
-                child.Measure(childConstraint);
-            }
-
-
-
+            var child = Children[cell];
+            Size childConstraint = new Size(cellMeasureWidth, cellMeasureHeight);
+            child.Measure(childConstraint);
         }
 
         /// <summary>
@@ -1230,7 +1213,7 @@ namespace Avalonia.Controls
             //  avoid processing when asked to distribute "0"
             if (!MathUtilities.IsZero(requestedSize))
             {
-                DefinitionBase[] tempDefinitions = TempDefinitions; //  temp array used to remember definitions for sorting
+                DefinitionBase?[] tempDefinitions = TempDefinitions; //  temp array used to remember definitions for sorting
                 int end = start + count;
                 int autoDefinitionsCount = 0;
                 double rangeMinSize = 0;
@@ -1288,20 +1271,24 @@ namespace Avalonia.Controls
                         Array.Sort(tempDefinitions, 0, count, s_spanPreferredDistributionOrderComparer);
                         for (i = 0, sizeToDistribute = requestedSize; i < autoDefinitionsCount; ++i)
                         {
+                            var tempDefinition = tempDefinitions[i]!;
+
                             //  sanity check: only auto definitions allowed in this loop
-                            Debug.Assert(tempDefinitions[i].UserSize.IsAuto);
+                            Debug.Assert(tempDefinition.UserSize.IsAuto);
 
                             //  adjust sizeToDistribute value by subtracting auto definition min size
-                            sizeToDistribute -= (tempDefinitions[i].MinSize);
+                            sizeToDistribute -= (tempDefinition.MinSize);
                         }
 
                         for (; i < count; ++i)
                         {
-                            //  sanity check: no auto definitions allowed in this loop
-                            Debug.Assert(!tempDefinitions[i].UserSize.IsAuto);
+                            var tempDefinition = tempDefinitions[i]!;
 
-                            double newMinSize = Math.Min(sizeToDistribute / (count - i), tempDefinitions[i].PreferredSize);
-                            if (newMinSize > tempDefinitions[i].MinSize) { tempDefinitions[i].UpdateMinSize(newMinSize); }
+                            //  sanity check: no auto definitions allowed in this loop
+                            Debug.Assert(!tempDefinition.UserSize.IsAuto);
+
+                            double newMinSize = Math.Min(sizeToDistribute / (count - i), tempDefinition.PreferredSize);
+                            if (newMinSize > tempDefinition.MinSize) { tempDefinition.UpdateMinSize(newMinSize); }
                             sizeToDistribute -= newMinSize;
                         }
 
@@ -1325,24 +1312,28 @@ namespace Avalonia.Controls
                         Array.Sort(tempDefinitions, 0, count, s_spanMaxDistributionOrderComparer);
                         for (i = 0, sizeToDistribute = requestedSize - rangePreferredSize; i < count - autoDefinitionsCount; ++i)
                         {
-                            //  sanity check: no auto definitions allowed in this loop
-                            Debug.Assert(!tempDefinitions[i].UserSize.IsAuto);
+                            var tempDefinition = tempDefinitions[i]!;
 
-                            double preferredSize = tempDefinitions[i].PreferredSize;
+                            //  sanity check: no auto definitions allowed in this loop
+                            Debug.Assert(!tempDefinition.UserSize.IsAuto);
+
+                            double preferredSize = tempDefinition.PreferredSize;
                             double newMinSize = preferredSize + sizeToDistribute / (count - autoDefinitionsCount - i);
-                            tempDefinitions[i].UpdateMinSize(Math.Min(newMinSize, tempDefinitions[i].SizeCache));
-                            sizeToDistribute -= (tempDefinitions[i].MinSize - preferredSize);
+                            tempDefinition.UpdateMinSize(Math.Min(newMinSize, tempDefinition.SizeCache));
+                            sizeToDistribute -= (tempDefinition.MinSize - preferredSize);
                         }
 
                         for (; i < count; ++i)
                         {
-                            //  sanity check: only auto definitions allowed in this loop
-                            Debug.Assert(tempDefinitions[i].UserSize.IsAuto);
+                            var tempDefinition = tempDefinitions[i]!;
 
-                            double preferredSize = tempDefinitions[i].MinSize;
+                            //  sanity check: only auto definitions allowed in this loop
+                            Debug.Assert(tempDefinition.UserSize.IsAuto);
+
+                            double preferredSize = tempDefinition.MinSize;
                             double newMinSize = preferredSize + sizeToDistribute / (count - i);
-                            tempDefinitions[i].UpdateMinSize(Math.Min(newMinSize, tempDefinitions[i].SizeCache));
-                            sizeToDistribute -= (tempDefinitions[i].MinSize - preferredSize);
+                            tempDefinition.UpdateMinSize(Math.Min(newMinSize, tempDefinition.SizeCache));
+                            sizeToDistribute -= (tempDefinition.MinSize - preferredSize);
                         }
 
                         //  sanity check: requested size must all be distributed
@@ -1376,8 +1367,10 @@ namespace Avalonia.Controls
 
                             for (int i = 0; i < count; ++i)
                             {
-                                double deltaSize = (maxMaxSize - tempDefinitions[i].SizeCache) * sizeToDistribute / totalRemainingSize;
-                                tempDefinitions[i].UpdateMinSize(tempDefinitions[i].SizeCache + deltaSize);
+                                var tempDefinition = tempDefinitions[i]!;
+
+                                double deltaSize = (maxMaxSize - tempDefinition.SizeCache) * sizeToDistribute / totalRemainingSize;
+                                tempDefinition.UpdateMinSize(tempDefinition.SizeCache + deltaSize);
                             }
                         }
                         else
@@ -1388,7 +1381,7 @@ namespace Avalonia.Controls
                             //
                             for (int i = 0; i < count; ++i)
                             {
-                                tempDefinitions[i].UpdateMinSize(equalSize);
+                                tempDefinitions[i]!.UpdateMinSize(equalSize);
                             }
                         }
                     }
@@ -1429,7 +1422,7 @@ namespace Avalonia.Controls
             double availableSize)
         {
             int defCount = definitions.Count;
-            DefinitionBase[] tempDefinitions = TempDefinitions;
+            DefinitionBase?[] tempDefinitions = TempDefinitions;
             int minCount = 0, maxCount = 0;
             double takenSize = 0;
             double totalStarWeight = 0.0;
@@ -1560,8 +1553,8 @@ namespace Avalonia.Controls
                         remainingStarWeight = totalStarWeight - takenStarWeight;
                     }
 
-                    double minRatio = (minCount > 0) ? tempDefinitions[minCount - 1].MeasureSize : Double.PositiveInfinity;
-                    double maxRatio = (maxCount > 0) ? tempDefinitions[defCount + maxCount - 1].SizeCache : -1.0;
+                    double minRatio = (minCount > 0) ? tempDefinitions[minCount - 1]!.MeasureSize : Double.PositiveInfinity;
+                    double maxRatio = (maxCount > 0) ? tempDefinitions[defCount + maxCount - 1]!.SizeCache : -1.0;
 
                     // choose the def with larger ratio to the current proportion ("max discrepancy")
                     double proportion = remainingStarWeight / remainingAvailableSize;
@@ -1579,13 +1572,13 @@ namespace Avalonia.Controls
                     double resolvedSize;
                     if (chooseMin == true)
                     {
-                        resolvedDef = tempDefinitions[minCount - 1];
+                        resolvedDef = tempDefinitions[minCount - 1]!;
                         resolvedSize = resolvedDef.MinSize;
                         --minCount;
                     }
                     else
                     {
-                        resolvedDef = tempDefinitions[defCount + maxCount - 1];
+                        resolvedDef = tempDefinitions[defCount + maxCount - 1]!;
                         resolvedSize = Math.Max(resolvedDef.MinSize, resolvedDef.UserMaxSize);
                         --maxCount;
                     }
@@ -1603,12 +1596,12 @@ namespace Avalonia.Controls
 
                     // advance to the next candidate defs, removing ones that have been resolved.
                     // Both counts are advanced, as a def might appear in both lists.
-                    while (minCount > 0 && tempDefinitions[minCount - 1].MeasureSize < 0.0)
+                    while (minCount > 0 && tempDefinitions[minCount - 1]!.MeasureSize < 0.0)
                     {
                         --minCount;
                         tempDefinitions[minCount] = null!;
                     }
-                    while (maxCount > 0 && tempDefinitions[defCount + maxCount - 1].MeasureSize < 0.0)
+                    while (maxCount > 0 && tempDefinitions[defCount + maxCount - 1]!.MeasureSize < 0.0)
                     {
                         --maxCount;
                         tempDefinitions[defCount + maxCount] = null!;
@@ -1637,8 +1630,7 @@ namespace Avalonia.Controls
                     // resolved as 'min'.   Their allocation can be increased to make up the gap.
                     for (int i = minCount; i < minCountPhase2; ++i)
                     {
-                        DefinitionBase def = tempDefinitions[i];
-                        if (def != null)
+                        if (tempDefinitions[i] is { } def)
                         {
                             def.MeasureSize = 1.0;      // mark as 'not yet resolved'
                             ++starCount;
@@ -1653,8 +1645,7 @@ namespace Avalonia.Controls
                     // resolved as 'max'.   Their allocation can be decreased to make up the gap.
                     for (int i = maxCount; i < maxCountPhase2; ++i)
                     {
-                        DefinitionBase def = tempDefinitions[defCount + i];
-                        if (def != null)
+                        if (tempDefinitions[defCount + i] is { } def)
                         {
                             def.MeasureSize = 1.0;      // mark as 'not yet resolved'
                             ++starCount;
@@ -1695,7 +1686,7 @@ namespace Avalonia.Controls
                 totalStarWeight = 0.0;
                 for (int i = 0; i < starCount; ++i)
                 {
-                    DefinitionBase def = tempDefinitions[i];
+                    DefinitionBase def = tempDefinitions[i]!;
                     totalStarWeight += def.MeasureSize;
                     def.SizeCache = totalStarWeight;
                 }
@@ -1703,7 +1694,7 @@ namespace Avalonia.Controls
                 // resolve the defs, in decreasing order of weight
                 for (int i = starCount - 1; i >= 0; --i)
                 {
-                    DefinitionBase def = tempDefinitions[i];
+                    DefinitionBase def = tempDefinitions[i]!;
                     double resolvedSize = (def.MeasureSize > 0.0) ? Math.Max(availableSize - takenSize, 0.0) * (def.MeasureSize / def.SizeCache) : 0.0;
 
                     // min and max should have no effect by now, but just in case...
@@ -2095,7 +2086,7 @@ namespace Avalonia.Controls
             {
                 // DpiScale dpiScale = GetDpi();
                 // double dpi = columns ? dpiScale.DpiScaleX : dpiScale.DpiScaleY;
-                var dpi = (VisualRoot as Layout.ILayoutRoot)?.LayoutScaling ?? 1.0;
+                var dpi = (VisualRoot as ILayoutRoot)?.LayoutScaling ?? 1.0;
                 double[] roundingErrors = RoundingErrors;
                 double roundedTakenSize = 0.0;
 
@@ -2302,8 +2293,7 @@ namespace Avalonia.Controls
         /// </summary>
         private void SetValid()
         {
-            ExtendedData extData = ExtData;
-            if (extData != null)
+            if (_extData is { } extData)
             {
                 //                for (int i = 0; i < PrivateColumnCount; ++i) DefinitionsU[i].SetValid ();
                 //                for (int i = 0; i < PrivateRowCount; ++i) DefinitionsV[i].SetValid ();
@@ -2330,12 +2320,12 @@ namespace Avalonia.Controls
             if (ShowGridLines && (_gridLinesRenderer == null))
             {
                 _gridLinesRenderer = new GridLinesRenderer();
-                this.VisualChildren.Add(_gridLinesRenderer);
+                VisualChildren.Add(_gridLinesRenderer);
             }
 
             if ((!ShowGridLines) && (_gridLinesRenderer != null))
             {
-                this.VisualChildren.Add(_gridLinesRenderer);
+                VisualChildren.Add(_gridLinesRenderer);
                 _gridLinesRenderer = null;
             }
 
@@ -2364,7 +2354,7 @@ namespace Avalonia.Controls
         {
             Grid grid = (Grid)d;
 
-            if (grid.ExtData != null    // trivial grid is 1 by 1. there is no grid lines anyway
+            if (grid._extData != null    // trivial grid is 1 by 1. there is no grid lines anyway
                 && grid.ListenToNotifications)
             {
                 grid.InvalidateVisual();
@@ -2375,13 +2365,11 @@ namespace Avalonia.Controls
 
         private static void OnCellAttachedPropertyChanged(AvaloniaObject d, AvaloniaPropertyChangedEventArgs e)
         {
-            Visual? child = d as Visual;
-
-            if (child != null)
+            if (d is Visual child)
             {
                 Grid? grid = child.GetVisualParent<Grid>();
                 if (grid != null
-                    && grid.ExtData != null
+                    && grid._extData != null
                     && grid.ListenToNotifications)
                 {
                     grid.CellsStructureDirty = true;
@@ -2427,7 +2415,7 @@ namespace Avalonia.Controls
         /// </summary>
         private IReadOnlyList<DefinitionBase> DefinitionsU
         {
-            get { return (ExtData.DefinitionsU!); }
+            get { return _extData!.DefinitionsU!; }
         }
 
         /// <summary>
@@ -2435,17 +2423,19 @@ namespace Avalonia.Controls
         /// </summary>
         private IReadOnlyList<DefinitionBase> DefinitionsV
         {
-            get { return (ExtData.DefinitionsV!); }
+            get { return _extData!.DefinitionsV!; }
         }
 
         /// <summary>
         /// Helper accessor to layout time array of definitions.
         /// </summary>
-        private DefinitionBase[] TempDefinitions
+        private DefinitionBase?[] TempDefinitions
         {
             get
             {
-                ExtendedData extData = ExtData;
+                Debug.Assert(_extData is not null);
+
+                var extData = _extData!;
                 int requiredLength = Math.Max(DefinitionsU.Count, DefinitionsV.Count) * 2;
 
                 if (extData.TempDefinitions == null
@@ -2516,7 +2506,7 @@ namespace Avalonia.Controls
         /// </summary>
         private CellCache[] PrivateCells
         {
-            get { return (ExtData.CellCachesCollection!); }
+            get { return _extData!.CellCachesCollection!; }
         }
 
         /// <summary>
@@ -2582,18 +2572,10 @@ namespace Avalonia.Controls
             set { SetFlags(value, Flags.HasGroup3CellsInAutoRows); }
         }
 
-        /// <summary>
-        /// Returns reference to extended data bag.
-        /// </summary>
-        private ExtendedData ExtData
-        {
-            get { return (_data!); }
-        }
-
         /// <summary>
         /// Returns *-weight, adjusted for scale computed during Phase 1
         /// </summary>
-        static double StarWeight(DefinitionBase def, double scale)
+        private static double StarWeight(DefinitionBase def, double scale)
         {
             if (scale < 0.0)
             {
@@ -2609,17 +2591,17 @@ namespace Avalonia.Controls
         }
 
         // Extended data instantiated on demand, for non-trivial case handling only
-        private ExtendedData? _data;
+        private ExtendedData? _extData;
 
         // Grid validity / property caches dirtiness flags
         private Flags _flags;
         private GridLinesRenderer? _gridLinesRenderer;
 
         // Keeps track of definition indices.
-        int[]? _definitionIndices;
+        private int[]? _definitionIndices;
 
         // Stores unrounded values and rounding errors during layout rounding.
-        double[]? _roundingErrors;
+        private double[]? _roundingErrors;
 
         // 5 is an arbitrary constant chosen to end the measure loop
         private const int c_layoutLoopMaxCount = 5;
@@ -2645,14 +2627,14 @@ namespace Avalonia.Controls
             internal int CellGroup2;                                //  index of the first cell in second cell group
             internal int CellGroup3;                                //  index of the first cell in third cell group
             internal int CellGroup4;                                //  index of the first cell in forth cell group
-            internal DefinitionBase[]? TempDefinitions;              //  temporary array used during layout for various purposes
+            internal DefinitionBase?[]? TempDefinitions;              //  temporary array used during layout for various purposes
                                                                     //  TempDefinitions.Length == Max(definitionsU.Length, definitionsV.Length)
         }
 
         /// <summary>
         /// Grid validity / property caches dirtiness flags
         /// </summary>
-        [System.Flags]
+        [Flags]
         private enum Flags
         {
             //
@@ -2768,7 +2750,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// LayoutTimeSizeType is used internally and reflects layout-time size type.
         /// </summary>
-        [System.Flags]
+        [Flags]
         internal enum LayoutTimeSizeType : byte
         {
             None = 0x00,
@@ -3317,7 +3299,7 @@ namespace Avalonia.Controls
             internal void UpdateRenderBounds(Size arrangeSize)
             {
                 _lastArrangeSize = arrangeSize;
-                this.InvalidateVisual();
+                InvalidateVisual();
             }
 
             private static Size _lastArrangeSize;

src/Avalonia.Controls/Image.cs

@@ -14,8 +14,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Source"/> property.
         /// </summary>
-        public static readonly StyledProperty<IImage> SourceProperty =
-            AvaloniaProperty.Register<Image, IImage>(nameof(Source));
+        public static readonly StyledProperty<IImage?> SourceProperty =
+            AvaloniaProperty.Register<Image, IImage?>(nameof(Source));
 
         /// <summary>
         /// Defines the <see cref="Stretch"/> property.
@@ -42,7 +42,7 @@ namespace Avalonia.Controls
         /// Gets or sets the image that will be displayed.
         /// </summary>
         [Content]
-        public IImage Source
+        public IImage? Source
         {
             get { return GetValue(SourceProperty); }
             set { SetValue(SourceProperty, value); }
@@ -66,6 +66,7 @@ namespace Avalonia.Controls
             set { SetValue(StretchDirectionProperty, value); }
         }
 
+        /// <inheritdoc />
         protected override bool BypassFlowDirectionPolicies => true;
         
         /// <summary>

src/Avalonia.Controls/ItemsControl.cs

@@ -2,7 +2,6 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
-using System.Diagnostics.CodeAnalysis;
 using Avalonia.Automation.Peers;
 using Avalonia.Collections;
 using Avalonia.Controls.Generators;
@@ -17,7 +16,6 @@ using Avalonia.Layout;
 using Avalonia.LogicalTree;
 using Avalonia.Metadata;
 using Avalonia.Styling;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Controls
 {
@@ -91,10 +89,11 @@ namespace Avalonia.Controls
         /// Gets or sets the <see cref="IBinding"/> to use for binding to the display member of each item.
         /// </summary>
         [AssignBinding]
+        [InheritDataTypeFromItems(nameof(Items))]
         public IBinding? DisplayMemberBinding
         {
-            get { return GetValue(DisplayMemberBindingProperty); }
-            set { SetValue(DisplayMemberBindingProperty, value); }
+            get => GetValue(DisplayMemberBindingProperty);
+            set => SetValue(DisplayMemberBindingProperty, value);
         }
         
         private IEnumerable? _items = new AvaloniaList<object>();
@@ -106,7 +105,6 @@ namespace Avalonia.Controls
         private Tuple<int, Control>? _containerBeingPrepared;
         private ScrollViewer? _scrollViewer;
         private ItemsPresenter? _itemsPresenter;
-        private IScrollSnapPointsInfo? _scrolSnapPointInfo;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ItemsControl"/> class.
@@ -134,8 +132,8 @@ namespace Avalonia.Controls
         [Content]
         public IEnumerable? Items
         {
-            get { return _items; }
-            set { SetAndRaise(ItemsProperty, ref _items, value); }
+            get => _items;
+            set => SetAndRaise(ItemsProperty, ref _items, value);
         }
 
         /// <summary>
@@ -143,8 +141,8 @@ namespace Avalonia.Controls
         /// </summary>
         public ControlTheme? ItemContainerTheme
         {
-            get { return GetValue(ItemContainerThemeProperty); }
-            set { SetValue(ItemContainerThemeProperty, value); }
+            get => GetValue(ItemContainerThemeProperty); 
+            set => SetValue(ItemContainerThemeProperty, value);
         }
 
         /// <summary>
@@ -161,8 +159,8 @@ namespace Avalonia.Controls
         /// </summary>
         public ITemplate<Panel> ItemsPanel
         {
-            get { return GetValue(ItemsPanelProperty); }
-            set { SetValue(ItemsPanelProperty, value); }
+            get => GetValue(ItemsPanelProperty);
+            set => SetValue(ItemsPanelProperty, value);
         }
 
         /// <summary>
@@ -171,8 +169,8 @@ namespace Avalonia.Controls
         [InheritDataTypeFromItems(nameof(Items))]
         public IDataTemplate? ItemTemplate
         {
-            get { return GetValue(ItemTemplateProperty); }
-            set { SetValue(ItemTemplateProperty, value); }
+            get => GetValue(ItemTemplateProperty); 
+            set => SetValue(ItemTemplateProperty, value);
         }
 
         /// <summary>
@@ -221,6 +219,7 @@ namespace Avalonia.Controls
         }
 
 
+        /// <inheritdoc />
         public event EventHandler<RoutedEventArgs> HorizontalSnapPointsChanged
         {
             add
@@ -240,6 +239,7 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <inheritdoc />
         public event EventHandler<RoutedEventArgs> VerticalSnapPointsChanged
         {
             add
@@ -264,8 +264,8 @@ namespace Avalonia.Controls
         /// </summary>
         public bool AreHorizontalSnapPointsRegular
         {
-            get { return GetValue(AreHorizontalSnapPointsRegularProperty); }
-            set { SetValue(AreHorizontalSnapPointsRegularProperty, value); }
+            get => GetValue(AreHorizontalSnapPointsRegularProperty); 
+            set => SetValue(AreHorizontalSnapPointsRegularProperty, value);
         }
 
         /// <summary>
@@ -273,8 +273,8 @@ namespace Avalonia.Controls
         /// </summary>
         public bool AreVerticalSnapPointsRegular
         {
-            get { return GetValue(AreVerticalSnapPointsRegularProperty); }
-            set { SetValue(AreVerticalSnapPointsRegularProperty, value); }
+            get => GetValue(AreVerticalSnapPointsRegularProperty); 
+            set => SetValue(AreVerticalSnapPointsRegularProperty, value);
         }
 
         /// <summary>
@@ -424,13 +424,12 @@ namespace Avalonia.Controls
         /// <returns>true if the item is (or is eligible to be) its own container; otherwise, false.</returns>
         protected internal virtual bool IsItemItsOwnContainerOverride(Control item) => true;
 
+        /// <inheritdoc />
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
             base.OnApplyTemplate(e);
             _scrollViewer = e.NameScope.Find<ScrollViewer>("PART_ScrollViewer");
             _itemsPresenter = e.NameScope.Find<ItemsPresenter>("PART_ItemsPresenter");
-
-            _scrolSnapPointInfo = _itemsPresenter as IScrollSnapPointsInfo;
         }
 
         /// <summary>
@@ -477,11 +476,13 @@ namespace Avalonia.Controls
             base.OnKeyDown(e);
         }
 
+        /// <inheritdoc />
         protected override AutomationPeer OnCreateAutomationPeer()
         {
             return new ItemsControlAutomationPeer(this);
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
@@ -748,11 +749,13 @@ namespace Avalonia.Controls
             return true;
         }
 
+        /// <inheritdoc />
         public IReadOnlyList<double> GetIrregularSnapPoints(Orientation orientation, SnapPointsAlignment snapPointsAlignment)
         {
             return _itemsPresenter?.GetIrregularSnapPoints(orientation, snapPointsAlignment) ?? new List<double>();
         }
 
+        /// <inheritdoc />
         public double GetRegularSnapPoints(Orientation orientation, SnapPointsAlignment snapPointsAlignment, out double offset)
         {
             offset = 0;

src/Avalonia.Controls/LayoutTransformControl.cs

@@ -28,7 +28,7 @@ namespace Avalonia.Controls
                 .AddClassHandler<LayoutTransformControl>((x, e) => x.OnLayoutTransformChanged(e));
 
             ChildProperty.Changed
-                .AddClassHandler<LayoutTransformControl>((x, e) => x.OnChildChanged(e));
+                .AddClassHandler<LayoutTransformControl>((x, _) => x.OnChildChanged());
 
             UseRenderTransformProperty.Changed
                 .AddClassHandler<LayoutTransformControl>((x, e) => x.OnUseRenderTransformPropertyChanged(e));
@@ -146,7 +146,7 @@ namespace Avalonia.Controls
             return transformedDesiredSize;
         }
 
-        IDisposable? _renderTransformChangedEvent;
+        private IDisposable? _renderTransformChangedEvent;
 
         private void OnUseRenderTransformPropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
@@ -167,8 +167,7 @@ namespace Avalonia.Controls
                             .Subscribe(
                                 (x) =>
                                 {
-                                    var target2 = x.Sender as LayoutTransformControl;
-                                    if (target2 != null)
+                                    if (x.Sender is LayoutTransformControl target2)
                                     {
                                         target2.LayoutTransform = target2.RenderTransform;
                                     }
@@ -182,7 +181,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void OnChildChanged(AvaloniaPropertyChangedEventArgs e)
+        private void OnChildChanged()
         {
             if (null != TransformRoot)
             {
@@ -206,18 +205,18 @@ namespace Avalonia.Controls
         /// <summary>
         /// Actual DesiredSize of Child element (the value it returned from its MeasureOverride method).
         /// </summary>
-        private Size _childActualSize = default;
+        private Size _childActualSize;
 
         /// <summary>
         /// RenderTransform/MatrixTransform applied to TransformRoot.
         /// </summary>
-        private MatrixTransform _matrixTransform = new MatrixTransform();
+        private readonly MatrixTransform _matrixTransform = new();
 
         /// <summary>
         /// Transformation matrix corresponding to _matrixTransform.
         /// </summary>
         private Matrix _transformation;
-        private IDisposable? _transformChangedEvent = null;
+        private IDisposable? _transformChangedEvent;
 
         /// <summary>
         /// Returns true if Size a is smaller than Size b in either dimension.
@@ -263,10 +262,7 @@ namespace Avalonia.Controls
             // Get the transform matrix and apply it
             _transformation = RoundMatrix(LayoutTransform.Value, DecimalsAfterRound);
 
-            if (null != _matrixTransform)
-            {
-                _matrixTransform.Matrix = _transformation;
-            }
+            _matrixTransform.Matrix = _transformation;
 
             // New transform means re-layout is necessary
             InvalidateMeasure();

src/Avalonia.Controls/ListBox.cs

@@ -104,6 +104,7 @@ namespace Avalonia.Controls
         public void UnselectAll() => Selection.Clear();
 
         protected internal override Control CreateContainerForItemOverride() => new ListBoxItem();
+        protected internal override bool IsItemItsOwnContainerOverride(Control item) => item is ListBoxItem;
 
         /// <inheritdoc/>
         protected override void OnGotFocus(GotFocusEventArgs e)

src/Avalonia.Controls/MaskedTextBox.cs

@@ -178,12 +178,11 @@ namespace Avalonia.Controls
                 }
 
             }
-
-
         }
 
         Type IStyleable.StyleKey => typeof(TextBox);
 
+        /// <inheritdoc />
         protected override void OnGotFocus(GotFocusEventArgs e)
         {
             if (HidePromptOnLeave == true && MaskProvider != null)
@@ -193,6 +192,7 @@ namespace Avalonia.Controls
             base.OnGotFocus(e);
         }
 
+        /// <inheritdoc />
         protected override async void OnKeyDown(KeyEventArgs e)
         {
             if (MaskProvider == null)
@@ -271,15 +271,17 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <inheritdoc />
         protected override void OnLostFocus(RoutedEventArgs e)
         {
-            if (HidePromptOnLeave == true && MaskProvider != null)
+            if (HidePromptOnLeave && MaskProvider != null)
             {
                 Text = MaskProvider.ToString(!HidePromptOnLeave, true);
             }
             base.OnLostFocus(e);
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             void UpdateMaskProvider()
@@ -357,6 +359,8 @@ namespace Avalonia.Controls
             }
             base.OnPropertyChanged(change);
         }
+
+        /// <inheritdoc />
         protected override void OnTextInput(TextInputEventArgs e)
         {
             _ignoreTextChanges = true;
@@ -423,7 +427,7 @@ namespace Avalonia.Controls
             return startPosition;
         }
 
-        private void RefreshText(MaskedTextProvider provider, int position)
+        private void RefreshText(MaskedTextProvider? provider, int position)
         {
             if (provider != null)
             {

src/Avalonia.Controls/NativeControlHost.cs

@@ -16,19 +16,17 @@ namespace Avalonia.Controls
         private IPlatformHandle? _nativeControlHandle;
         private bool _queuedForDestruction;
         private bool _queuedForMoveResize;
-        private readonly List<Visual> _propertyChangedSubscriptions = new List<Visual>();
+        private readonly List<Visual> _propertyChangedSubscriptions = new();
 
+        /// <inheritdoc />
         protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
         {
             _currentRoot = e.Root as TopLevel;
             var visual = (Visual)this;
             while (visual != null)
             {
-                if (visual is Visual v)
-                {
-                    v.PropertyChanged += PropertyChangedHandler;
-                    _propertyChangedSubscriptions.Add(v);
-                }
+                visual.PropertyChanged += PropertyChangedHandler;
+                _propertyChangedSubscriptions.Add(visual);
 
                 visual = visual.GetVisualParent();
             }
@@ -42,15 +40,13 @@ namespace Avalonia.Controls
                 EnqueueForMoveResize();
         }
 
+        /// <inheritdoc />
         protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
         {
             _currentRoot = null;
-            if (_propertyChangedSubscriptions != null)
-            {
-                foreach (var v in _propertyChangedSubscriptions)
-                    v.PropertyChanged -= PropertyChangedHandler;
-                _propertyChangedSubscriptions.Clear();
-            }
+            foreach (var v in _propertyChangedSubscriptions)
+                v.PropertyChanged -= PropertyChangedHandler;
+            _propertyChangedSubscriptions.Clear();
             UpdateHost();
         }
 
@@ -128,7 +124,7 @@ namespace Avalonia.Controls
             return new Rect(position.Value, bounds.Size);
         }
 
-        void EnqueueForMoveResize()
+        private void EnqueueForMoveResize()
         {
             if(_queuedForMoveResize)
                 return;

src/Avalonia.Controls/NativeMenu.Export.cs

@@ -12,10 +12,10 @@ namespace Avalonia.Controls
 
         public static bool GetIsNativeMenuExported(TopLevel tl) => tl.GetValue(IsNativeMenuExportedProperty);
         
-        private static readonly AttachedProperty<NativeMenuInfo> s_nativeMenuInfoProperty =
-            AvaloniaProperty.RegisterAttached<NativeMenu, TopLevel, NativeMenuInfo>("___NativeMenuInfo");
-        
-        class NativeMenuInfo
+        private static readonly AttachedProperty<NativeMenuInfo?> s_nativeMenuInfoProperty =
+            AvaloniaProperty.RegisterAttached<NativeMenu, TopLevel, NativeMenuInfo?>("___NativeMenuInfo");
+
+        private sealed class NativeMenuInfo
         {
             public bool ChangingIsExported { get; set; }
             public ITopLevelNativeMenuExporter? Exporter { get; }
@@ -33,7 +33,7 @@ namespace Avalonia.Controls
             }
         }
 
-        static NativeMenuInfo GetInfo(TopLevel target)
+        private static NativeMenuInfo GetInfo(TopLevel target)
         {
             var rv = target.GetValue(s_nativeMenuInfoProperty);
             if (rv == null)
@@ -45,18 +45,18 @@ namespace Avalonia.Controls
             return rv;
         }
 
-        static void SetIsNativeMenuExported(TopLevel tl, bool value)
+        private static void SetIsNativeMenuExported(TopLevel tl, bool value)
         {
             GetInfo(tl).ChangingIsExported = true;
             tl.SetValue(IsNativeMenuExportedProperty, value);
         }
 
-        public static readonly AttachedProperty<NativeMenu> MenuProperty
-            = AvaloniaProperty.RegisterAttached<NativeMenu, AvaloniaObject, NativeMenu>("Menu");
+        public static readonly AttachedProperty<NativeMenu?> MenuProperty
+            = AvaloniaProperty.RegisterAttached<NativeMenu, AvaloniaObject, NativeMenu?>("Menu");
 
-        public static void SetMenu(AvaloniaObject o, NativeMenu menu) => o.SetValue(MenuProperty, menu);
+        public static void SetMenu(AvaloniaObject o, NativeMenu? menu) => o.SetValue(MenuProperty, menu);
 
-        public static NativeMenu GetMenu(AvaloniaObject o) => o.GetValue(MenuProperty);
+        public static NativeMenu? GetMenu(AvaloniaObject o) => o.GetValue(MenuProperty);
         
         static NativeMenu()
         {

src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs

@@ -553,7 +553,7 @@ namespace Avalonia.Controls.Platform
             }
         }
 
-        protected static IMenuItem? GetMenuItem(Control? item)
+        protected static IMenuItem? GetMenuItem(StyledElement? item)
         {
             while (true)
             {

src/Avalonia.Controls/Primitives/AdornerLayer.cs

@@ -34,8 +34,8 @@ namespace Avalonia.Controls.Primitives
         public static readonly AttachedProperty<Control?> AdornerProperty =
             AvaloniaProperty.RegisterAttached<AdornerLayer, Visual, Control?>("Adorner");
 
-        private static readonly AttachedProperty<AdornedElementInfo> s_adornedElementInfoProperty =
-            AvaloniaProperty.RegisterAttached<AdornerLayer, Visual, AdornedElementInfo>("AdornedElementInfo");
+        private static readonly AttachedProperty<AdornedElementInfo?> s_adornedElementInfoProperty =
+            AvaloniaProperty.RegisterAttached<AdornerLayer, Visual, AdornedElementInfo?>("AdornedElementInfo");
 
         private static readonly AttachedProperty<AdornerLayer?> s_savedAdornerLayerProperty =
             AvaloniaProperty.RegisterAttached<Visual, Visual, AdornerLayer?>("SavedAdornerLayer");
@@ -159,8 +159,8 @@ namespace Avalonia.Controls.Primitives
                 return;
             }
 
-            AdornerLayer.SetAdornedElement(adorner, visual);
-            AdornerLayer.SetIsClipEnabled(adorner, false);
+            SetAdornedElement(adorner, visual);
+            SetIsClipEnabled(adorner, false);
 
             ((ISetLogicalParent) adorner).SetParent(visual);
             layer.Children.Add(adorner);
@@ -177,6 +177,7 @@ namespace Avalonia.Controls.Primitives
             ((ISetLogicalParent) adorner).SetParent(null);
         }
 
+        /// <inheritdoc />
         protected override Size MeasureOverride(Size availableSize)
         {
             foreach (var child in Children)
@@ -199,6 +200,7 @@ namespace Avalonia.Controls.Primitives
             return default;
         }
 
+        /// <inheritdoc />
         protected override Size ArrangeOverride(Size finalSize)
         {
             foreach (var child in Children)
@@ -217,7 +219,7 @@ namespace Avalonia.Controls.Primitives
                     }
                     else
                     {
-                        ArrangeChild((Control) child, finalSize);
+                        ArrangeChild(child, finalSize);
                     }
                 }
             }

src/Avalonia.Controls/Primitives/OverlayPopupHost.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using Avalonia.Reactive;
 using Avalonia.Controls.Primitives.PopupPositioning;
 using Avalonia.Interactivity;
 using Avalonia.Media;
@@ -18,8 +17,8 @@ namespace Avalonia.Controls.Primitives
             PopupRoot.TransformProperty.AddOwner<OverlayPopupHost>();
 
         private readonly OverlayLayer _overlayLayer;
-        private PopupPositionerParameters _positionerParameters = new PopupPositionerParameters();
-        private ManagedPopupPositioner _positioner;
+        private readonly ManagedPopupPositioner _positioner;
+        private PopupPositionerParameters _positionerParameters;
         private Point _lastRequestedPosition;
         private bool _shown;
 
@@ -29,13 +28,16 @@ namespace Avalonia.Controls.Primitives
             _positioner = new ManagedPopupPositioner(this);
         }
 
+        /// <inheritdoc />
         public void SetChild(Control? control)
         {
             Content = control;
         }
 
+        /// <inheritdoc />
         public Visual? HostedVisualTreeRoot => null;
 
+        /// <inheritdoc />
         public Transform? Transform
         {
             get => GetValue(TransformProperty);
@@ -48,23 +50,27 @@ namespace Avalonia.Controls.Primitives
             set { /* Not currently supported in overlay popups */ }
         }
 
-        protected internal override Interactive? InteractiveParent => Parent;
+        /// <inheritdoc />
+        protected internal override Interactive? InteractiveParent => (Interactive?)VisualParent;
 
+        /// <inheritdoc />
         public void Dispose() => Hide();
 
-
+        /// <inheritdoc />
         public void Show()
         {
             _overlayLayer.Children.Add(this);
             _shown = true;
         }
 
+        /// <inheritdoc />
         public void Hide()
         {
             _overlayLayer.Children.Remove(this);
             _shown = false;
         }
 
+        /// <inheritdoc />
         public void ConfigurePosition(Visual target, PlacementMode placement, Point offset,
             PopupAnchor anchor = PopupAnchor.None, PopupGravity gravity = PopupGravity.None,
             PopupPositionerConstraintAdjustment constraintAdjustment = PopupPositionerConstraintAdjustment.All,
@@ -75,6 +81,7 @@ namespace Avalonia.Controls.Primitives
             UpdatePosition();
         }
 
+        /// <inheritdoc />
         protected override Size ArrangeOverride(Size finalSize)
         {
             if (_positionerParameters.Size != finalSize)
@@ -123,17 +130,18 @@ namespace Avalonia.Controls.Primitives
        
         public static IPopupHost CreatePopupHost(Visual target, IAvaloniaDependencyResolver? dependencyResolver)
         {
-            var platform = TopLevel.GetTopLevel(target)?.PlatformImpl?.CreatePopup();
-            if (platform != null)
-                return new PopupRoot((TopLevel)target.GetVisualRoot()!, platform, dependencyResolver);
-            
-            var overlayLayer = OverlayLayer.GetOverlayLayer(target);
-            if (overlayLayer == null)
-                throw new InvalidOperationException(
-                    "Unable to create IPopupImpl and no overlay layer is found for the target control");
+            if (TopLevel.GetTopLevel(target) is { } topLevel && topLevel.PlatformImpl?.CreatePopup() is { } popupImpl)
+            {
+                return new PopupRoot(topLevel, popupImpl, dependencyResolver);
+            }
 
+            if (OverlayLayer.GetOverlayLayer(target) is { } overlayLayer)
+            {
+                return new OverlayPopupHost(overlayLayer);
+            }
 
-            return new OverlayPopupHost(overlayLayer);
+            throw new InvalidOperationException(
+                "Unable to create IPopupImpl and no overlay layer is found for the target control");
         }
     }
 }

src/Avalonia.Controls/Primitives/Popup.cs

@@ -120,7 +120,7 @@ namespace Avalonia.Controls.Primitives
         public static readonly StyledProperty<bool> TopmostProperty =
             AvaloniaProperty.Register<Popup, bool>(nameof(Topmost));
 
-        private bool _isOpenRequested = false;
+        private bool _isOpenRequested;
         private bool _isOpen;
         private bool _ignoreIsOpenChanged;
         private PopupOpenState? _openState;
@@ -377,9 +377,9 @@ namespace Avalonia.Controls.Primitives
             popupHost.SetChild(Child);
             ((ISetLogicalParent)popupHost).SetParent(this);
 
-            if (InheritsTransform && placementTarget is Control c)
+            if (InheritsTransform)
             {
-                TransformTrackingHelper.Track(c, PlacementTargetTransformChanged)
+                TransformTrackingHelper.Track(placementTarget, PlacementTargetTransformChanged)
                     .DisposeWith(handlerCleanup);
             }
             else
@@ -518,6 +518,7 @@ namespace Avalonia.Controls.Primitives
             Close();
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
@@ -579,7 +580,7 @@ namespace Avalonia.Controls.Primitives
             var scaleX = 1.0;
             var scaleY = 1.0;
 
-            if (InheritsTransform && placementTarget.TransformToVisual(topLevel) is Matrix m)
+            if (InheritsTransform && placementTarget.TransformToVisual(topLevel) is { } m)
             {
                 scaleX = Math.Sqrt(m.M11 * m.M11 + m.M12 * m.M12);
                 scaleY = Math.Sqrt(m.M11 * m.M11 + m.M12 * m.M12);
@@ -623,6 +624,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected override AutomationPeer OnCreateAutomationPeer()
         {
             return new PopupAutomationPeer(this);
@@ -723,7 +725,7 @@ namespace Avalonia.Controls.Primitives
 
                     while (e is object && (!e.Focusable || !e.IsEffectivelyEnabled || !e.IsVisible))
                     {
-                        e = e.Parent;
+                        e = e.VisualParent as Control;
                     }
 
                     if (e is object)
@@ -850,7 +852,7 @@ namespace Avalonia.Controls.Primitives
 
             var popupHost = _openState.PopupHost;
 
-            return popupHost != null && ((Visual)popupHost).IsVisualAncestorOf(visual);
+            return ((Visual)popupHost).IsVisualAncestorOf(visual);
         }
 
         public bool IsPointerOverPopup => ((IInputElement?)_openState?.PopupHost)?.IsPointerOver ?? false;

src/Avalonia.Controls/Primitives/PopupRoot.cs

@@ -72,12 +72,12 @@ namespace Avalonia.Controls.Primitives
         /// <remarks>
         /// Popup events are passed to their parent window. This facilitates this.
         /// </remarks>
-        protected internal override Interactive? InteractiveParent => Parent;
+        protected internal override Interactive? InteractiveParent => (Interactive?)Parent;
 
         /// <summary>
         /// Gets the control that is hosting the popup root.
         /// </summary>
-        Visual? IHostedVisualTreeRoot.Host => Parent;
+        Visual? IHostedVisualTreeRoot.Host => VisualParent;
 
         /// <summary>
         /// Gets the styling parent of the popup root.

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -3,16 +3,16 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
-using System.Xml.Linq;
-using Avalonia.Controls.Generators;
 using Avalonia.Controls.Selection;
+using Avalonia.Controls.Utils;
 using Avalonia.Data;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
+using Avalonia.Metadata;
 using Avalonia.Threading;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Controls.Primitives
 {
@@ -66,6 +66,19 @@ namespace Avalonia.Controls.Primitives
                 (o, v) => o.SelectedItem = v,
                 defaultBindingMode: BindingMode.TwoWay, enableDataValidation: true);
 
+        /// <summary>
+        /// Defines the <see cref="SelectedValue"/> property
+        /// </summary>
+        public static readonly StyledProperty<object?> SelectedValueProperty =
+            AvaloniaProperty.Register<SelectingItemsControl, object?>(nameof(SelectedValue),
+                defaultBindingMode: BindingMode.TwoWay);
+
+        /// <summary>
+        /// Defines the <see cref="SelectedValueBinding"/> property
+        /// </summary>
+        public static readonly StyledProperty<IBinding?> SelectedValueBindingProperty =
+            AvaloniaProperty.Register<SelectingItemsControl, IBinding?>(nameof(SelectedValueBinding));
+
         /// <summary>
         /// Defines the <see cref="SelectedItems"/> property.
         /// </summary>
@@ -129,6 +142,8 @@ namespace Avalonia.Controls.Primitives
         private bool _ignoreContainerSelectionChanged;
         private UpdateState? _updateState;
         private bool _hasScrolledToSelectedItem;
+        private BindingHelper? _bindingHelper;
+        private bool _isSelectionChangeActive;
 
         /// <summary>
         /// Initializes static members of the <see cref="SelectingItemsControl"/> class.
@@ -143,8 +158,8 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         public event EventHandler<SelectionChangedEventArgs>? SelectionChanged
         {
-            add { AddHandler(SelectionChangedEvent, value); }
-            remove { RemoveHandler(SelectionChangedEvent, value); }
+            add => AddHandler(SelectionChangedEvent, value); 
+            remove => RemoveHandler(SelectionChangedEvent, value);
         }
 
         /// <summary>
@@ -152,8 +167,8 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         public bool AutoScrollToSelectedItem
         {
-            get { return GetValue(AutoScrollToSelectedItemProperty); }
-            set { SetValue(AutoScrollToSelectedItemProperty, value); }
+            get => GetValue(AutoScrollToSelectedItemProperty);
+            set => SetValue(AutoScrollToSelectedItemProperty, value);
         }
 
         /// <summary>
@@ -209,6 +224,28 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <summary>
+        /// Gets the <see cref="IBinding"/> instance used to obtain the 
+        /// <see cref="SelectedValue"/> property
+        /// </summary>
+        [AssignBinding]
+        [InheritDataTypeFromItems(nameof(Items))]
+        public IBinding? SelectedValueBinding
+        {
+            get => GetValue(SelectedValueBindingProperty);
+            set => SetValue(SelectedValueBindingProperty, value);
+        }
+
+        /// <summary>
+        /// Gets or sets the value of the selected item, obtained using 
+        /// <see cref="SelectedValueBinding"/>
+        /// </summary>
+        public object? SelectedValue
+        {
+            get => GetValue(SelectedValueProperty);
+            set => SetValue(SelectedValueProperty, value);
+        }
+
         /// <summary>
         /// Gets or sets the selected items.
         /// </summary>
@@ -255,6 +292,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets or sets the model that holds the current selection.
         /// </summary>
+        [AllowNull]
         protected ISelectionModel Selection
         {
             get
@@ -322,8 +360,8 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         public bool IsTextSearchEnabled
         {
-            get { return GetValue(IsTextSearchEnabledProperty); }
-            set { SetValue(IsTextSearchEnabledProperty, value); }
+            get => GetValue(IsTextSearchEnabledProperty);
+            set => SetValue(IsTextSearchEnabledProperty, value);
         }
 
         /// <summary>
@@ -332,8 +370,8 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         public bool WrapSelection
         {
-            get { return GetValue(WrapSelectionProperty); }
-            set { SetValue(WrapSelectionProperty, value); }
+            get => GetValue(WrapSelectionProperty); 
+            set => SetValue(WrapSelectionProperty, value);
         }
 
         /// <summary>
@@ -345,8 +383,8 @@ namespace Avalonia.Controls.Primitives
         /// </remarks>
         protected SelectionMode SelectionMode
         {
-            get { return GetValue(SelectionModeProperty); }
-            set { SetValue(SelectionModeProperty, value); }
+            get => GetValue(SelectionModeProperty); 
+            set => SetValue(SelectionModeProperty, value);
         }
 
         /// <summary>
@@ -399,6 +437,7 @@ namespace Avalonia.Controls.Primitives
             return null;
         }
 
+        /// <inheritdoc />
         protected override void ItemsCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             base.ItemsCollectionChanged(sender!, e);
@@ -409,12 +448,14 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
         {
             base.OnAttachedToVisualTree(e);
             AutoScrollToSelectedItemIfNecessary();
         }
 
+        /// <inheritdoc />
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
             base.OnApplyTemplate(e);
@@ -431,6 +472,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected internal override void PrepareContainerForItemOverride(Control element, object? item, int index)
         {
             base.PrepareContainerForItemOverride(element, item, index);
@@ -447,12 +489,14 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected override void ContainerIndexChangedOverride(Control container, int oldIndex, int newIndex)
         {
             base.ContainerIndexChangedOverride(container, oldIndex, newIndex);
             MarkContainerSelected(container, Selection.IsSelected(newIndex));
         }
 
+        /// <inheritdoc />
         protected internal override void ClearContainerForItemOverride(Control element)
         {
             base.ClearContainerForItemOverride(element);
@@ -463,7 +507,7 @@ namespace Avalonia.Controls.Primitives
                 KeyboardNavigation.SetTabOnceActiveElement(panel, null);
             }
 
-            if (element is ISelectable selectable)
+            if (element is ISelectable)
                 MarkContainerSelected(element, false);
         }
 
@@ -498,7 +542,8 @@ namespace Avalonia.Controls.Primitives
                 DataValidationErrors.SetError(this, error);
             }
         }
-        
+
+        /// <inheritdoc />
         protected override void OnInitialized()
         {
             base.OnInitialized();
@@ -509,6 +554,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected override void OnTextInput(TextInputEventArgs e)
         {
             if (!e.Handled)
@@ -551,6 +597,7 @@ namespace Avalonia.Controls.Primitives
             base.OnTextInput(e);
         }
 
+        /// <inheritdoc />
         protected override void OnKeyDown(KeyEventArgs e)
         {
             base.OnKeyDown(e);
@@ -582,6 +629,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
@@ -592,7 +640,7 @@ namespace Avalonia.Controls.Primitives
             }
             if (change.Property == ItemsProperty && _updateState is null && _selection is object)
             {
-                var newValue = change.GetNewValue<IEnumerable>();
+                var newValue = change.GetNewValue<IEnumerable?>();
                 _selection.Source = newValue;
 
                 if (newValue is null)
@@ -609,6 +657,60 @@ namespace Avalonia.Controls.Primitives
             {
                 WrapFocus = WrapSelection;
             }
+            else if (change.Property == SelectedValueProperty)
+            {
+                if (_isSelectionChangeActive)
+                    return;
+
+                if (_updateState is not null)
+                {
+                    _updateState.SelectedValue = change.NewValue;
+                    return;
+                }
+
+                SelectItemWithValue(change.NewValue);
+            }
+            else if (change.Property == SelectedValueBindingProperty)
+            {
+                var idx = SelectedIndex;
+
+                // If no selection is active, don't do anything as SelectedValue is already null
+                if (idx == -1)
+                {
+                    return;
+                }
+
+                var value = change.GetNewValue<IBinding>();
+                if (value is null)
+                {
+                    // Clearing SelectedValueBinding makes the SelectedValue the item itself
+                    SelectedValue = SelectedItem;
+                    return;
+                }
+
+                var selectedItem = SelectedItem;
+
+                try
+                {
+                    _isSelectionChangeActive = true;
+
+                    if (_bindingHelper is null)
+                    {
+                        _bindingHelper = new BindingHelper(value);
+                    }
+                    else
+                    {
+                        _bindingHelper.UpdateBinding(value);
+                    }
+
+                    // Re-evaluate SelectedValue with the new binding
+                    SelectedValue = _bindingHelper.Evaluate(selectedItem);
+                }
+                finally
+                {
+                    _isSelectionChangeActive = false;
+                }
+            }
         }
 
         /// <summary>
@@ -695,7 +797,7 @@ namespace Avalonia.Controls.Primitives
             {
                 if (multi)
                 {
-                    if (Selection.IsSelected(index) == true)
+                    if (Selection.IsSelected(index))
                     {
                         Selection.Deselect(index);
                     }
@@ -716,12 +818,10 @@ namespace Avalonia.Controls.Primitives
                 Selection.Select(index);
             }
 
-            if (Presenter?.Panel != null)
+            if (Presenter?.Panel is { } panel)
             {
                 var container = ContainerFromIndex(index);
-                KeyboardNavigation.SetTabOnceActiveElement(
-                    (InputElement)Presenter.Panel,
-                    container);
+                KeyboardNavigation.SetTabOnceActiveElement(panel, container);
             }
         }
 
@@ -815,6 +915,10 @@ namespace Avalonia.Controls.Primitives
                     new BindingValue<IList?>(SelectedItems));
                 _oldSelectedItems = SelectedItems;
             }
+            else if (e.PropertyName == nameof(ISelectionModel.Source))
+            {
+                ClearValue(SelectedValueProperty);
+            }
         }
 
         /// <summary>
@@ -845,6 +949,11 @@ namespace Avalonia.Controls.Primitives
                 Mark(i, false);
             }
 
+            if (!_isSelectionChangeActive)
+            {
+                UpdateSelectedValueFromItem();
+            }
+
             var route = BuildEventRoute(SelectionChangedEvent);
 
             if (route.HasHandlers)
@@ -871,6 +980,109 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        private void SelectItemWithValue(object? value)
+        {
+            if (ItemCount == 0 || _isSelectionChangeActive)
+                return;
+
+            try
+            {
+                _isSelectionChangeActive = true;
+                var si = FindItemWithValue(value);
+                if (si != AvaloniaProperty.UnsetValue)
+                {
+                    SelectedItem = si;
+                }
+                else
+                {
+                    SelectedItem = null;
+                }
+            }
+            finally
+            {
+                _isSelectionChangeActive = false;
+            }
+        }
+
+        private object FindItemWithValue(object? value)
+        {
+            if (ItemCount == 0 || value is null)
+            {
+                return AvaloniaProperty.UnsetValue;
+            }
+
+            var items = Items;
+            var binding = SelectedValueBinding;
+
+            if (binding is null)
+            {
+                // No SelectedValueBinding set, SelectedValue is the item itself
+                // Still verify the value passed in is in the Items list
+                var index = items!.IndexOf(value);
+
+                if (index >= 0)
+                {
+                    return value;
+                }
+                else
+                { 
+                    return AvaloniaProperty.UnsetValue;
+                }
+            }
+
+            _bindingHelper ??= new BindingHelper(binding);
+
+            // Matching UWP behavior, if duplicates are present, return the first item matching
+            // the SelectedValue provided
+            foreach (var item in items!)
+            {
+                var itemValue = _bindingHelper.Evaluate(item);
+
+                if (itemValue.Equals(value))
+                {
+                    return item;
+                }
+            }
+
+            return AvaloniaProperty.UnsetValue;
+        }
+
+        private void UpdateSelectedValueFromItem()
+        {
+            if (_isSelectionChangeActive)
+                return;
+
+            var binding = SelectedValueBinding;
+            var item = SelectedItem;
+
+            if (binding is null || item is null)
+            {
+                // No SelectedValueBinding, SelectedValue is Item itself
+                try
+                {
+                    _isSelectionChangeActive = true;
+                    SelectedValue = item;
+                }
+                finally
+                {
+                    _isSelectionChangeActive = false;
+                }
+                return;
+            }
+
+            _bindingHelper ??= new BindingHelper(binding);
+
+            try
+            {
+                _isSelectionChangeActive = true;
+                SelectedValue = _bindingHelper.Evaluate(item);
+            }
+            finally
+            {
+                _isSelectionChangeActive = false;
+            }
+        }
+
         private void AutoScrollToSelectedItemIfNecessary()
         {
             if (AutoScrollToSelectedItem &&
@@ -940,7 +1152,7 @@ namespace Avalonia.Controls.Primitives
 
         private void UpdateContainerSelection()
         {
-            if (Presenter?.Panel is Panel panel)
+            if (Presenter?.Panel is { } panel)
             {
                 foreach (var container in panel.Children)
                 {
@@ -1037,6 +1249,13 @@ namespace Avalonia.Controls.Primitives
                     Selection.Clear();
                 }
 
+                if (state.SelectedValue.HasValue)
+                {
+                    var item = FindItemWithValue(state.SelectedValue.Value);
+                    if (item != AvaloniaProperty.UnsetValue)
+                        state.SelectedItem = item;
+                }
+
                 if (state.SelectedIndex.HasValue)
                 {
                     SelectedIndex = state.SelectedIndex.Value;
@@ -1098,6 +1317,7 @@ namespace Avalonia.Controls.Primitives
         {
             private Optional<int> _selectedIndex;
             private Optional<object?> _selectedItem;
+            private Optional<object?> _selectedValue;
 
             public int UpdateCount { get; set; }
             public Optional<ISelectionModel> Selection { get; set; }
@@ -1122,6 +1342,54 @@ namespace Avalonia.Controls.Primitives
                     _selectedIndex = default;
                 }
             }
+
+            public Optional<object?> SelectedValue
+            {
+                get => _selectedValue;
+                set
+                {
+                    _selectedValue = value;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Helper class for evaluating a binding from an Item and IBinding instance
+        /// </summary>
+        private class BindingHelper : StyledElement
+        {
+            public BindingHelper(IBinding binding)
+            {
+                UpdateBinding(binding);
+            }
+
+            public static readonly StyledProperty<object> ValueProperty =
+                AvaloniaProperty.Register<BindingHelper, object>("Value");
+
+            public object Evaluate(object? dataContext)
+            {
+                dataContext = dataContext ?? throw new ArgumentNullException(nameof(dataContext));
+
+                // Only update the DataContext if necessary
+                if (!dataContext.Equals(DataContext))
+                    DataContext = dataContext;
+
+                return GetValue(ValueProperty);
+            }
+
+            public void UpdateBinding(IBinding binding)
+            {
+                _lastBinding = binding;
+                var ib = binding.Initiate(this, ValueProperty);
+                if (ib is null)
+                {
+                    throw new InvalidOperationException("Unable to create binding");
+                }
+
+                BindingOperations.Apply(this, ValueProperty, ib, null);
+            }
+
+            private IBinding? _lastBinding;
         }
     }
 }