Fix ListState.AddAsync callback firing with null value

src/Uno.Extensions.Reactive.Tests/Operators/Given_ListFeedSelection.cs

@@ -805,4 +805,70 @@ public MyAggregateRoot(int key)
 
 		public int? MyEntityKey { get; init; }
 	}
+
+	[TestMethod]
+	public async Task When_AddAsyncThenTrySelectAsync_Then_SelectionStateDoesNotReceiveIntermediateNone()
+	{
+		// This test verifies the fix for issue #2942
+		// When AddAsync is called followed by TrySelectAsync, the selection state's
+		// ForEach callback should only fire with the selected value, not with null first
+
+		var selection = State<int>.Empty(this).Record();
+		var list = ListState.Async(this, async _ => ImmutableList<int>.Empty);
+		var sut = ListFeedSelection<int>.Create(list, selection.Feed, "sut").Record();
+
+		await sut.WaitForMessages(1);
+
+		// Add an item to the list and then select it
+		await list.AddAsync(42, CT);
+		await list.TrySelectAsync(42, CT);
+
+		// The selection state should only get the selected value (42), not None/null first
+		await selection.Should().BeAsync(r => r
+			.Message(Data.None, Error.No, Progress.Final)  // Initial empty state
+			.Message(42, Error.No, Progress.Final)         // Selected value (should not have intermediate None)
+		);
+
+		await sut.Should().BeAsync(r => r
+			.Message(Data.None, Error.No, Progress.Final, Selection.Empty)
+			.Message(m => m
+				.Changed(Changed.Data)
+				.Current(Items.Some(42), Error.No, Progress.Final, Selection.Empty))
+			.Message(m => m
+				.Changed(Changed.Selection & MessageAxes.SelectionSource)
+				.Current(Items.Some(42), Error.No, Progress.Final, Selection.Items(42)))
+		);
+	}
+
+	[TestMethod]
+	public async Task When_AddAsyncThenTrySelectAsync_WithInitialData_Then_SelectionStateDoesNotReceiveIntermediateNone()
+	{
+		// Same as above but with an initial list that already has data
+
+		var selection = State<int>.Empty(this).Record();
+		var list = ListState.Async(this, async _ => Enumerable.Range(0, 5).ToImmutableList());
+		var sut = ListFeedSelection<int>.Create(list, selection.Feed, "sut").Record();
+
+		await sut.WaitForMessages(1);
+
+		// Add an item to the list and then select it
+		await list.AddAsync(42, CT);
+		await list.TrySelectAsync(42, CT);
+
+		// The selection state should only get the selected value (42), not None/null first
+		await selection.Should().BeAsync(r => r
+			.Message(Data.None, Error.No, Progress.Final)  // Initial empty state
+			.Message(42, Error.No, Progress.Final)         // Selected value (should not have intermediate None)
+		);
+
+		await sut.Should().BeAsync(r => r
+			.Message(Items.Range(5), Error.No, Progress.Final, Selection.Empty)
+			.Message(m => m
+				.Changed(Changed.Data)
+				.Current(Items.Some(0, 1, 2, 3, 4, 42), Error.No, Progress.Final, Selection.Empty))
+			.Message(m => m
+				.Changed(Changed.Selection & MessageAxes.SelectionSource)
+				.Current(Items.Some(0, 1, 2, 3, 4, 42), Error.No, Progress.Final, Selection.Items(42)))
+		);
+	}
 }

src/Uno.Extensions.Reactive/Operators/ListFeedSelection.cs

@@ -14,6 +14,9 @@ namespace Uno.Extensions.Reactive.Operators;
 internal sealed class ListFeedSelection<TSource, TOther> : IListState<TSource>, IStateImpl
 {
 	internal static MessageAxis<object?> SelectionUpdateSource { get; } = new(MessageAxes.SelectionSource, _ => null) { IsTransient = true };
+
+	// Sentinel marker to indicate selection was cleared due to data changes (not an explicit user action)
+	private static readonly object InternalSelectionClear = new();
 
 	private readonly IListFeed<TSource> _source;
 	private readonly IState<TOther> _selectionState;
@@ -93,11 +96,13 @@ public async ValueTask UpdateMessageAsync(Action<MessageBuilder<IImmutableList<T
 						// TODO: This is only to ensure reliability, we should detect changes on the collection and update the SelectionInfo accordingly!
 						u.Set(MessageAxis.Selection, MessageAxisValue.Unset, null);
 
+						// Mark this as an internal clear due to data changes, not an explicit selection change
+						u.Set(SelectionUpdateSource, InternalSelectionClear);
 						selectionHasChanged = true;
 					}
-
-					if (selectionHasChanged)
+					else if (selectionHasChanged)
 					{
+						// This is an explicit selection change from the caller
 						u.Set(SelectionUpdateSource, this);
 					}
 				},
@@ -130,7 +135,9 @@ private IState<IImmutableList<TSource>> Enable()
 
 		Context
 			.GetOrCreateSource(impl)
-			.Where(msg => msg.Changes.Contains(MessageAxis.Selection) && msg.Current.Get(SelectionUpdateSource) != _selectionState)
+			.Where(msg => msg.Changes.Contains(MessageAxis.Selection) 
+				&& msg.Current.Get(SelectionUpdateSource) != _selectionState
+				&& msg.Current.Get(SelectionUpdateSource) != InternalSelectionClear)
 			.ForEachAwaitWithCancellationAsync(SyncFromListToState, ConcurrencyMode.AbortPrevious, _ct.Token);
 
 		return impl;