dotnet/wpf · error · NotSupportedException

SR.Format(SR.UnexpectedCollectionChangeAction, e.Action)

Error message

SR.Format(SR.UnexpectedCollectionChangeAction, e.Action)

What it means

When ItemCollection syncs its internal clone with the underlying view, it clones the change-log for Add/Remove/Replace/Reset actions. Any other NotifyCollectionChangedAction reaches a default case that throws NotSupportedException(SR.UnexpectedCollectionChangeAction, e.Action) — an internal-invariant guard against malformed change notifications.

Solutions

Fix the custom INotifyCollectionChanged implementation to raise only valid actions with correctly built EventArgs
Use ObservableCollection or a well-tested observable collection implementation
If you control the event, ensure e.Action is Add, Remove, Replace, or Reset

Example fix

// before
RaiseCollectionChanged(new NotifyCollectionChangedEventArgs((NotifyCollectionChangedAction)9));
// after
RaiseCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));

Defensive patterns

Strategy: try-catch

Validate before calling

if (Enum.IsDefined(typeof(NotifyCollectionChangedAction), e.Action) && e.Action != NotifyCollectionChangedAction.Move) { /* handled */ }

Type guard

bool isSupportedAction = e.Action is NotifyCollectionChangedAction.Add or NotifyCollectionChangedAction.Remove or NotifyCollectionChangedAction.Replace or NotifyCollectionChangedAction.Reset;

Try / catch

try { /* sync with view */ } catch (NotSupportedException ex) { /* bad INotifyCollectionChanged source: replace or fix source */ }

Prevention

Use ObservableCollection rather than hand-rolled INotifyCollectionChanged
Raise only well-formed NotifyCollectionChangedEventArgs
Test custom collections against ItemsControl consumers

When it happens

Trigger: A source collection (or custom INotifyCollectionChanged implementation) raising NotifyCollectionChangedEventArgs with an unexpected or invalid Action value that the clone logic does not handle.

Common situations: Implementing INotifyCollectionChanged yourself and firing events with wrong/misbuilt args; third-party observable collections that raise nonstandard notifications.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/a9f81e5d76d7f34e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/ItemCollection.cs:2229

                        {
                            clone.RemoveAt(e.OldStartingIndex);
                        }
                        for (int i = 0; i < e.NewItems.Count; i++)
                        {
                            clone.Insert(e.NewStartingIndex + i, (T)e.NewItems[i]);
                        }
                    }
                    break;

                // this arm also handles cases where the two collections have gotten
                // out of sync (typically because exceptions prevented a previous sync
                // from happening)
                case NotifyCollectionChangedAction.Reset:
                    CloneList(clone, origin);
                    break;

                default:
                    throw new NotSupportedException(SR.Format(SR.UnexpectedCollectionChangeAction, e.Action));
            }
        }

        private void CloneList(IList clone, IList master)
        {
            // if either party is null, do nothing.  Allowing null lets the caller
            // avoid a lazy instantiation of the Sort/Group description collection.
            if (clone == null || master == null)
                return;

            if (clone.Count > 0)
            {
                clone.Clear();
            }

            for (int i = 0, n = master.Count; i < n; ++i)
            {
                clone.Add(master[i]);

View on GitHub (pinned to 81131a70a4)