Added a constructor to FuncMultiValueConverter that takes an IReadOnlyList instead of an IEnumerable (#19936)

Co-authored-by: Max Katz <maxkatz6@outlook.com>

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

@@ -13,15 +13,24 @@ namespace Avalonia.Data.Converters
     /// <typeparam name="TOut">The output type.</typeparam>
     public class FuncMultiValueConverter<TIn, TOut> : IMultiValueConverter
     {
-        private readonly Func<IEnumerable<TIn?>, TOut> _convert;
+        private readonly Func<IReadOnlyList<TIn?>, TOut> _convert;
 
         /// <summary>
-        /// Initializes a new instance of the <see cref="FuncValueConverter{TIn, TOut}"/> class.
+        /// Initializes a new instance of the <see cref="FuncMultiValueConverter{TIn, TOut}"/> class.
+        /// </summary>
+        /// <param name="convert">The convert function.</param>
+        public FuncMultiValueConverter(Func<IReadOnlyList<TIn?>, TOut> convert)
+        {
+            _convert = convert;
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="FuncMultiValueConverter{TIn, TOut}"/> class.
         /// </summary>
         /// <param name="convert">The convert function.</param>
         public FuncMultiValueConverter(Func<IEnumerable<TIn?>, TOut> convert)
+            : this(new Func<IReadOnlyList<TIn?>, TOut>(convert))
         {
-            _convert = convert;
         }
 
         /// <inheritdoc/>

tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_MultiBinding.cs

@@ -153,6 +153,20 @@ namespace Avalonia.Base.UnitTests
 
             Assert.Equal(",Bar,Baz", value);
         }
+        
+        [Fact]
+        public void MultiValueConverter_Supports_Indexing_The_Parameters()
+        {
+            var target = new FuncMultiValueConverter<string, string>(v => v[0]);
+
+            object value = target.Convert(new[] { "Foo", "Bar", "Baz" }, typeof(string), null, CultureInfo.InvariantCulture);
+
+            Assert.Equal("Foo", value);
+
+            value = target.Convert(new[] { null, "Bar", "Baz" }, typeof(string), null, CultureInfo.InvariantCulture);
+
+            Assert.Null(value);
+        }
 
         private struct StringValueTypeWrapper
         {