What causes "Transitions collection cannot be reset." in AvaloniaUI/Avalonia?

Remove transitions individually (foreach then Remove) instead of Clear(), or replace the collection instance rather than resetting it.

AvaloniaUI/Avalonia · error · NotSupportedException

Transitions collection cannot be reset.

Error message

Transitions collection cannot be reset.

What it means

Thrown by Animatable.OnTransitionsCollectionChanged with NotifyCollectionChangedAction.Reset. The transitions system maintains per-transition state and cannot be rebuilt from a generic reset (Clear()), so Avalonia explicitly disallows resetting the Transitions collection via NotSupportedException.

Source

Thrown at src/Avalonia.Base/Animation/Animatable.cs:215

            if (!_transitionsEnabled)
            {
                return;
            }

            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Add:
                    AddTransitions(e.NewItems!);
                    break;
                case NotifyCollectionChangedAction.Remove:
                    RemoveTransitions(e.OldItems!);
                    break;
                case NotifyCollectionChangedAction.Replace:
                    RemoveTransitions(e.OldItems!);
                    AddTransitions(e.NewItems!);
                    break;
                case NotifyCollectionChangedAction.Reset:
                    throw new NotSupportedException("Transitions collection cannot be reset.");
            }
        }

        private void AddTransitions(IList items)
        {
            if (!_transitionsEnabled)
            {
                return;
            }

            _transitionState ??= new Dictionary<ITransition, TransitionState>();

            for (var i = 0; i < items.Count; ++i)
            {
                var t = (ITransition)items[i]!;

                _transitionState.Add(t, new TransitionState
                {

View on GitHub (pinned to 11c5427268)

Solutions

Remove transitions individually (foreach then Remove) instead of Clear(), or replace the collection instance rather than resetting it.
If binding Transitions from a VM, reassign the bound property to a new collection rather than Clear()-ing the existing one.
Avoid Reset notifications from your source collection; use Remove notifications when emptying.
If you need a fresh set, assign a new TransitionCollection and let the setter swap it.

Example fix

// before
animatable.Transitions.Clear(); // raises Reset -> throws

// after
while (animatable.Transitions.Count > 0)
    animatable.Transitions.RemoveAt(0);
// or:
animatable.Transitions = new TransitionCollection();

Defensive patterns

Strategy: validation

Validate before calling

// never Clear() the transitions collection — remove items or reassign
if (animatable.Transitions is { Count: > 0 } tc)
    while (tc.Count > 0) tc.RemoveAt(tc.Count - 1);
animatable.Transitions = new TransitionCollection();

Prevention

Never call Transitions.Clear(); remove items individually or reassign the collection.
When binding Transitions, replace the VM property with a new collection rather than Clear()-ing.
Ensure source collections raise Remove, not Reset, when emptied.
Audit XAML/code-behind for Clear() on transition collections.

When it happens

Trigger: Calling Animatable.Transitions.Clear() (which raises a Reset action) or reassigning/rebinding a collection that triggers Reset. Any data binding or code that issues CollectionChanged with Action=Reset on the Transitions collection hits this.

Common situations: XAML/binding a transitions collection then clearing it; calling .Clear() on Animatable.Transitions in code-behind; a TwoWay/OneWay binding whose source raises NotifyCollectionChangedAction.Reset (e.g. an ObservableCollection re-init); merging/resetting a CompositeCollection.

Related errors

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/3ca82b8216e2df26. Report an issue: GitHub.