What causes "SR.Enumerator_CollectionChanged" in dotnet/wpf?

Collect items to change into a separate list during iteration, then apply Add/Remove after the loop.

dotnet/wpf · error · InvalidOperationException

SR.Enumerator_CollectionChanged

Error message

SR.Enumerator_CollectionChanged

What it means

DrawingCollection's Enumerator.MoveNext throws InvalidOperationException (SR.Enumerator_CollectionChanged) when the collection's version stamp changed since the enumerator was created. WPF collections use fail-fast enumeration: any Add/Insert/Remove invalidates outstanding enumerators instead of risking undefined behavior.

Solutions

Collect items to change into a separate list during iteration, then apply Add/Remove after the loop.
Use an index-based for loop iterating backwards when removing items.
Snapshot the collection first: foreach (var d in collection.ToArray()) so the enumerator operates on a copy.

Example fix

// before
foreach (var d in collection) { if (d is PenRemoved) collection.Remove(d); } // throws
// after
foreach (var d in collection.OfType<PenRemoved>().ToList()) collection.Remove(d);

Defensive patterns

Strategy: try-catch

Validate before calling

var snapshot = collection.Cast<object>().ToArray(); // enumerate the snapshot, not the live collection

Type guard

null

Try / catch

try { foreach (var d in collection) { /* ... */ } } catch (InvalidOperationException ex) when (ex.Message.Contains("changed")) { /* restart with a snapshot */ }

Prevention

Never Add/Remove inside foreach over a WPF Freezable collection
Batch mutations: collect changes, apply after the loop
Use ToArray()/ToList() snapshots for read passes
Marshal cross-thread mutations to the UI thread before enumerating

When it happens

Trigger: Iterating a DrawingCollection with foreach while adding or removing items inside the loop body; modifying the collection from another thread or from an event handler fired during iteration.

Common situations: Removing a drawing when a condition is met mid-foreach; UI code that mutates a DrawingGroup's children inside a rendering/layout callback while a LINQ/foreach pass is in progress.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/106a957a30540c70. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Generated/DrawingCollection.cs:832

            {
                _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)