From 538a4b2993c9de46c31003db098f98cdfbf49fc5 Mon Sep 17 00:00:00 2001
From: zacfromaustinpowder
 <165237243+zacfromaustinpowder@users.noreply.github.com>
Date: Thu, 13 Nov 2025 16:32:29 +1000
Subject: [PATCH] Added a constructor to FuncMultiValueConverter that takes an
 IReadOnlyList instead of an IEnumerable (#19936)

Co-authored-by: Max Katz <maxkatz6@outlook.com>
---
 .../Data/Converters/FuncMultiValueConverter.cs    | 15 ++++++++++++---
 .../AvaloniaObjectTests_MultiBinding.cs           | 14 ++++++++++++++
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/src/Avalonia.Base/Data/Converters/FuncMultiValueConverter.cs b/src/Avalonia.Base/Data/Converters/FuncMultiValueConverter.cs
index 5084d8e822..c60b5f7053 100644
--- a/src/Avalonia.Base/Data/Converters/FuncMultiValueConverter.cs
+++ b/src/Avalonia.Base/Data/Converters/FuncMultiValueConverter.cs
@@ -13,17 +13,26 @@ 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<IEnumerable<TIn?>, TOut> convert)
+        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))
+        {
+        }
+
         /// <inheritdoc/>
         public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture)
         {
diff --git a/tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_MultiBinding.cs b/tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_MultiBinding.cs
index 757160a0a4..f3cf36998c 100644
--- a/tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_MultiBinding.cs
+++ b/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
         {