Fix SelectionChanged not raised on collection Reset by NathanDrake2406 · Pull Request #20942 · AvaloniaUI/Avalonia

NathanDrake2406 · 2026-03-19T05:47:52Z

What does the pull request do?

Fixes SelectingItemsControl.SelectionChanged not being raised when an items collection sends a Reset notification that clears the current selection. This covers controls such as ListBox bound to an ObservableCollection that is cleared.

What is the current behavior?

When a selected item is removed by a Reset, SelectionModel reports the loss through LostSelection. SelectingItemsControl only uses that callback to recover AlwaysSelected selection, so no public routed SelectionChanged event is raised for reset-to-empty cases.

What is the updated/expected behavior with this PR?

SelectionChanged is raised when a reset causes the control to lose its selected items. The event reports the previously selected items in RemovedItems and does not report removals when a reset preserves the selection.

Validated with:

dotnet run --project tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj -- --filter-class "Avalonia.Controls.UnitTests.Primitives.SelectingItemsControlTests"
dotnet run --project tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj -- --filter-class "Avalonia.Controls.UnitTests.Selection.InternalSelectionModelTests"
dotnet run --project tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj -- --filter-namespace "Avalonia.Controls.UnitTests.Selection"

How was the solution implemented (if it's not obvious)?

SelectingItemsControl now keeps a snapshot of the last selected items at the control boundary. When an items Reset starts, the control captures that existing snapshot. If SelectionModel.LostSelection is committed for that reset, the control raises the routed SelectionChanged event using the captured items as RemovedItems.

This keeps SelectionModel reset semantics intact and avoids diffing the reset collection contents.

Checklist

Added unit tests (if possible)?
Added XML documentation to any related classes?
Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None.

Obsoletions / Deprecations

None.

Fixed issues

Fixes #20897

avaloniaui-bot · 2026-03-19T06:22:49Z

You can test this PR using the following package version. 12.0.999-cibuild0063669-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

MrJul · 2026-04-29T17:00:56Z

@NathanDrake2406 Is there something missing that you converted this PR to a draft?

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avaloniaui-bot · 2026-04-29T17:39:18Z

You can test this PR using the following package version. 12.1.999-cibuild0065147-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

…deselected When a collection bound to a SelectingItemsControl (e.g. ListBox) was cleared via NotifyCollectionChangedAction.Reset, the SelectionChanged event was not raised despite the selection being lost. The root cause was in InternalSelectionModel.OnSourceReset: the base SelectionModel.OnSourceReset() directly reset _selectedIndex to -1 before any Operation could capture the old selection state. The subsequent SyncFromSelectedItems created an Operation that saw no change (old and new both -1), so CommitOperation never fired SelectionChanged. The fix snapshots _writableSelectedItems before sync, diffs against the post-sync state to find items that were actually lost (not merely re-selected at a new index after reorder), and injects them as DeselectedItems on the pending Operation — following the same pattern used by OnSelectionRemoved for individual item removals. Fixes AvaloniaUI#20897

avaloniaui-bot · 2026-04-30T10:08:33Z

You can test this PR using the following package version. 12.1.999-cibuild0065164-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

Copilot

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

MrJul · 2026-04-30T15:02:16Z

I had a deeper look at this.

From my understanding, the current behavior is expected from the SelectionModel side: the SelectionModel.LostSelection event is raised instead of SelectionModel.SelectionChanged. Imo, we shouldn't try to diff after a reset as this might be costly: after all, there's a reason a Reset was received.

However, the SelectionModel.LostSelection event doesn't trigger result in any event in the SelectingItemsControl, which is the original issue, #20897.

An alternative would be to add a flag or operation type to SelectionChangedEventArgs, informing the user that it's a complete reset.

cc @grokys for additional thoughts.

NathanDrake2406 · 2026-04-30T16:58:17Z

@codex review the implementation and what @MrJul said.

The Reset diff in InternalSelectionModel used a HashSet to detect which previously-selected items were still present after sync. Selection allows duplicates (same instance or equal items at multiple indices), so set semantics collapsed duplicates into one entry and under-reported deselections when only some occurrences were lost. Track counts per item plus a null counter and decrement per match, so RemovedItems reflects the actual number of lost selections. Adds a duplicate-items Reset test covering the regression.

ListBox and other SelectingItemsControl callers did not receive SelectionChanged when a Reset cleared the selected items. The selection model reports this path through LostSelection, but the control only used that callback for AlwaysSelected recovery. Track the last selected items at the control boundary, capture that snapshot for Reset notifications, and raise the routed SelectionChanged event when LostSelection commits during that reset. This avoids diffing reset contents while preserving the removed-items payload for clear/reset-to-empty cases.

avaloniaui-bot · 2026-04-30T18:21:53Z

You can test this PR using the following package version. 12.1.999-cibuild0065178-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

grokys · 2026-05-12T12:40:54Z

I agree with @MrJul that we shouldn't be trying to diff or restore selection after a reset - we explicitly don't try to do that in the selection model because it involves copying an arbitrarily large collection.

We should be raising SelectionChanged on a reset however, I agree. It should simply reflect the new selection state at the time that it's raised IMO.

MrJul · 2026-05-26T17:27:43Z

                UpdateSelectedValueFromItem();
            }

+            _selectedItemsSnapshot = Selection.SelectedItems.ToArray();


Memory: keep a single List for the snapshot instead of creating a new array each time the selection changes.

Copilot

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

+        private object?[] _selectedItemsSnapshot = Array.Empty<object>();
+        private object?[]? _selectedItemsBeforeReset;


+            _selectedItemsSnapshot = Selection.SelectedItems.ToArray();
+            _selectedItemsBeforeReset = null;


+            if (_selectedItemsBeforeReset?.Length > 0)
+            {
+                RaiseEvent(new SelectionChangedEventArgs(
+                    SelectionChangedEvent,
+                    _selectedItemsBeforeReset,
+                    Array.Empty<object>()));
+                _selectedItemsBeforeReset = null;
+            }


- Replace per-change ToArray() snapshot with persistent List<object?> to avoid allocations on every selection change (review: MrJul). - Read snapshot in PreCollectionChanged instead of Selection.SelectedItems because the source is already empty by the time Reset fires. - Align LostSelection event-raising with SelectionChanged path: use BuildEventRoute + HasHandlers guard to avoid allocating args when no handlers are attached (review: copilot). - Harden existing Reset tests to Assert.Single to catch double-fire.

Copilot

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

+                    var ev = new SelectionChangedEventArgs(
+                        SelectionChangedEvent,
+                        _selectedItemsBeforeReset,
+                        Array.Empty<object?>());


+        private void OnItemsViewPreCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
+        {
+            if (e.Action == NotifyCollectionChangedAction.Reset && _selectedItemsSnapshot.Count > 0)
+            {
+                _selectedItemsBeforeReset = _selectedItemsSnapshot.ToArray();
+            }
+        }


…apshot - Extract RaiseSelectionChanged helper so both the normal (SelectionChanged) and reset (LostSelection) paths share the same BuildEventRoute/HasHandlers guard and SelectionChangedEventArgs construction (review: copilot). - Move _selectedItemsBeforeReset clear outside the conditional in OnSelectionModelLostSelection so the field is always nulled after LostSelection, preventing accidental reuse (review: copilot). - Add comment documenting the snapshot lifecycle in OnItemsViewPreCollectionChanged.

Copilot

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

+            _selectedItemsSnapshot.Clear();
+            _selectedItemsSnapshot.AddRange(Selection.SelectedItems);
+            _selectedItemsBeforeReset = null;

-            if (route.HasHandlers)
-            {
-                var ev = new SelectionChangedEventArgs(
-                    SelectionChangedEvent,
-                    e.DeselectedItems.ToArray(),
-                    e.SelectedItems.ToArray());
-                RaiseEvent(ev);
-            }
+            RaiseSelectionChanged(e.DeselectedItems.ToArray(), e.SelectedItems.ToArray());


+        private readonly List<object?> _selectedItemsSnapshot = new();
+        private object?[]? _selectedItemsBeforeReset;


…confirmed SelectionChangedEventArgs materialized arrays via ToArray() at the call site before checking whether the routed event had any registered handlers. This allocated needlessly in the common case of no external subscribers. The event items (IReadOnlyList<object?>) already implement IList via ReadOnlySelectionListBase. RaiseSelectionChanged now accepts IReadOnlyList<object?> and casts to IList inside the HasHandlers gate, falling back to ToArray() only for non-IList enumerables.

avaloniaui-bot · 2026-05-27T09:02:49Z

You can test this PR using the following package version. 12.1.999-cibuild0065810-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

MrJul

LGTM!

* fix: raise SelectionChanged event on collection Reset when items are deselected When a collection bound to a SelectingItemsControl (e.g. ListBox) was cleared via NotifyCollectionChangedAction.Reset, the SelectionChanged event was not raised despite the selection being lost. The root cause was in InternalSelectionModel.OnSourceReset: the base SelectionModel.OnSourceReset() directly reset _selectedIndex to -1 before any Operation could capture the old selection state. The subsequent SyncFromSelectedItems created an Operation that saw no change (old and new both -1), so CommitOperation never fired SelectionChanged. The fix snapshots _writableSelectedItems before sync, diffs against the post-sync state to find items that were actually lost (not merely re-selected at a new index after reorder), and injects them as DeselectedItems on the pending Operation — following the same pattern used by OnSelectionRemoved for individual item removals. Fixes AvaloniaUI#20897 * fix: use multiset diff to report lost duplicate selections on Reset The Reset diff in InternalSelectionModel used a HashSet to detect which previously-selected items were still present after sync. Selection allows duplicates (same instance or equal items at multiple indices), so set semantics collapsed duplicates into one entry and under-reported deselections when only some occurrences were lost. Track counts per item plus a null counter and decrement per match, so RemovedItems reflects the actual number of lost selections. Adds a duplicate-items Reset test covering the regression. * fix: raise SelectionChanged for reset-lost selection ListBox and other SelectingItemsControl callers did not receive SelectionChanged when a Reset cleared the selected items. The selection model reports this path through LostSelection, but the control only used that callback for AlwaysSelected recovery. Track the last selected items at the control boundary, capture that snapshot for Reset notifications, and raise the routed SelectionChanged event when LostSelection commits during that reset. This avoids diffing reset contents while preserving the removed-items payload for clear/reset-to-empty cases. * fix: address review feedback on SelectionChanged Reset snapshot - Replace per-change ToArray() snapshot with persistent List<object?> to avoid allocations on every selection change (review: MrJul). - Read snapshot in PreCollectionChanged instead of Selection.SelectedItems because the source is already empty by the time Reset fires. - Align LostSelection event-raising with SelectionChanged path: use BuildEventRoute + HasHandlers guard to avoid allocating args when no handlers are attached (review: copilot). - Harden existing Reset tests to Assert.Single to catch double-fire. * fix: consolidate SelectionChanged raising and defend against stale snapshot - Extract RaiseSelectionChanged helper so both the normal (SelectionChanged) and reset (LostSelection) paths share the same BuildEventRoute/HasHandlers guard and SelectionChangedEventArgs construction (review: copilot). - Move _selectedItemsBeforeReset clear outside the conditional in OnSelectionModelLostSelection so the field is always nulled after LostSelection, preventing accidental reuse (review: copilot). - Add comment documenting the snapshot lifecycle in OnItemsViewPreCollectionChanged. * perf: defer SelectionChangedEventArgs allocations until handlers are confirmed SelectionChangedEventArgs materialized arrays via ToArray() at the call site before checking whether the routed event had any registered handlers. This allocated needlessly in the common case of no external subscribers. The event items (IReadOnlyList<object?>) already implement IList via ReadOnlySelectionListBase. RaiseSelectionChanged now accepts IReadOnlyList<object?> and casts to IList inside the HasHandlers gate, falling back to ToArray() only for non-IList enumerables.

MrJul added bug backport-candidate-11.3.x Consider this PR for backporting to 11.3 branch labels Mar 19, 2026

NathanDrake2406 marked this pull request as draft April 14, 2026 08:10

NathanDrake2406 marked this pull request as ready for review April 29, 2026 17:03

Copilot AI review requested due to automatic review settings April 29, 2026 17:03

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NathanDrake2406 force-pushed the fix/listbox-selectionchanged-on-reset branch from ecab00d to f2e8388 Compare April 30, 2026 09:24

NathanDrake2406 force-pushed the fix/listbox-selectionchanged-on-reset branch from 68eb1d7 to 7db50f8 Compare April 30, 2026 09:32

Merge branch 'master' into fix/listbox-selectionchanged-on-reset

916a680

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Comment thread src/Avalonia.Controls/Selection/InternalSelectionModel.cs Outdated

NathanDrake2406 added 2 commits May 1, 2026 03:25

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MrJul added backport-candidate-12.0.x Consider this PR for backporting to 12.0 branch and removed backport-candidate-11.3.x Consider this PR for backporting to 11.3 branch labels May 27, 2026

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Merged via the queue into AvaloniaUI:master with commit b0877fe May 27, 2026
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MrJul added backported-12.0.x and removed backport-candidate-12.0.x Consider this PR for backporting to 12.0 branch labels May 28, 2026

		private object?[] _selectedItemsSnapshot = Array.Empty<object>();
		private object?[]? _selectedItemsBeforeReset;

		_selectedItemsSnapshot = Selection.SelectedItems.ToArray();
		_selectedItemsBeforeReset = null;

		private readonly List<object?> _selectedItemsSnapshot = new();
		private object?[]? _selectedItemsBeforeReset;

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