dotnet/wpf · error · InvalidOperationException

SR.Enumerator_CollectionChanged

Error message

SR.Enumerator_CollectionChanged

What it means

The Transform3DCollection enumerator's MoveNext throws InvalidOperationException (SR.Enumerator_CollectionChanged) when the collection's version stamp no longer matches the version captured when the enumerator was created. WPF collections invalidate enumerators on any add/remove to prevent undefined iteration.

Solutions

  1. Materialize the items first (foreach over collection.ToArray() or ToList()) before mutating.
  2. Collect items to remove in a separate list, then remove them after the loop.
  3. Synchronize access across threads (freeze the collection or dispatch mutations to one thread).

Example fix

// before
foreach (var t in collection)
    if (ShouldRemove(t)) collection.Remove(t);
// after
foreach (var t in collection.ToArray())
    if (ShouldRemove(t)) collection.Remove(t);
Defensive patterns

Strategy: try-catch

Validate before calling

var snapshot = collection.ToArray();
foreach (var t in snapshot) { /* safe to mutate collection */ }

Try / catch

try { foreach (var t in collection) { /* ... */ } } catch (InvalidOperationException) { /* collection changed during iteration; restart with snapshot */ }

Prevention

When it happens

Trigger: Calling MoveNext after an element was added, removed, or replaced (indexer set) on the same collection during iteration.

Common situations: Removing items inside a foreach loop; a background thread mutating the collection while the UI thread iterates; building a filtered list by calling Remove while enumerating.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/f6c0cf146c040f94. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media3D/Generated/Transform3DCollection.cs:830

            {
                _list.ReadPreamble();

                if (_version == _list._version)
                {
                    if (_index > -2 && _index < _list._collection.Count - 1)
                    {
                        _current = _list._collection[++_index];
                        return true;
                    }
                    else
                    {
                        _index = -2; // -2 indicates "past the end"
                        return false;
                    }
                }
                else
                {
                    throw new InvalidOperationException(SR.Enumerator_CollectionChanged);
                }
            }

            /// <summary>
            /// Sets the enumerator to its initial position, which is before the
            /// first element in the collection.
            /// </summary>
            public void Reset()
            {
                _list.ReadPreamble();

                if (_version == _list._version)
                {
                    _index = -1;
                }
                else
                {
                    throw new InvalidOperationException(SR.Enumerator_CollectionChanged);

View on GitHub (pinned to 81131a70a4)