Merge pull request #19252 from unoplatform/dev/xygu/20250116/depobjco…

…l-index-out-range

fix: align DepObjCol indexer behavior with winui

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Given_DependencyObjectCollection.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.InteropServices.ObjectiveC;
 using Microsoft.UI.Xaml;
@@ -28,4 +29,16 @@ public void When_Add_Multiple_And_Invoke()
 
 		Assert.IsTrue(list.SequenceEqual(["One", "Two"]));
 	}
+
+	[TestMethod]
+	public void When_Indexer_Get_IndexOutOfRange()
+	{
+		Assert.IsNull(new DependencyObjectCollection()[int.MaxValue]);
+	}
+
+	[TestMethod]
+	public void When_Indexer_Get_NegativeIndex()
+	{
+		Assert.ThrowsException<ArgumentOutOfRangeException>(() => new DependencyObjectCollection()[-1]);
+	}
 }

src/Uno.UI/UI/Xaml/DependencyObjectCollection.cs

@@ -127,7 +127,7 @@ private protected virtual void ValidateItem(T item) { }
 
 		public T this[int index]
 		{
-			get => _list[index];
+			get => index < _list.Count ? _list[index] : default;
 			set
 			{
 				ValidateItem(value);