dotnet/wpf · error · InvalidOperationException

throw new…

Error message

throw new InvalidOperationException(SR.Format(SR.AddedItemNotAtIndex, index));

What it means

While processing a Remove notification, EnumerableCollectionView verifies that the item reported as removed actually sits at the expected index in its internal snapshot before deleting it. If _snapshot[index] does not equal args.OldItems[i] (via ItemsControl.EqualsEx), the snapshot and source collection have diverged and AddedItemNotAtIndex is thrown.

Solutions

Ensure every mutation of the source collection raises a matching, correctly-indexed CollectionChanged event.
Make item equality (EqualsEx uses Equals) stable — don't mutate items' identity fields used by Equals.
Raise Reset for bulk mutations so the view rebuilds its snapshot.
Force a Refresh/rebind of the view to resynchronize the snapshot.

Example fix

// before
_items[0] = newItem; // silent mutation, later Remove at 0 mismatches snapshot
// after
_items[0] = newItem;
OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Replace, newItem, oldItem, 0));

Defensive patterns

Strategy: validation

Validate before calling

// before raising Remove, verify the item is still at the reported index
if (!Equals(collection[oldStartingIndex], removedItem))
    collectionChangedArgs = new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset);

Try / catch

// recover by forcing a rebuild
try { ProcessChange(args); } catch (InvalidOperationException) { view.Refresh(); }

Prevention

Capture OldItems before mutating the underlying list.
Never mutate collection contents without raising notifications.
Use stable Equals semantics on items.
Raise Reset when indices can't be guaranteed.

When it happens

Trigger: Source collection raised Remove with OldStartingIndex pointing at an item that differs from args.OldItems[i] in the view's snapshot — e.g. the source mutated its items without raising corresponding notifications, or raised events out of order.

Common situations: Custom collection that changes item order or content without raising change notifications while an ItemsControl is bound; missed events because the collection was modified before the view subscribed.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/dc36f7ffc11cc8b8. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/Data/EnumerableCollectionView.cs:426

                    }
                    else
                    {   // insert
                        for (int i = args.NewItems.Count - 1; i >= 0; --i)
                        {
                            _snapshot.Insert(args.NewStartingIndex, args.NewItems[i]);
                        }
                    }
                    break;

                case NotifyCollectionChangedAction.Remove:
                    if (args.OldStartingIndex < 0)
                        throw new InvalidOperationException(SR.RemovedItemNotFound);

                    for (int i = args.OldItems.Count - 1, index = args.OldStartingIndex + i; i >= 0; --i, --index)
                    {
                        if (!System.Windows.Controls.ItemsControl.EqualsEx(args.OldItems[i], _snapshot[index]))
                            // replace error message with a better one
                            throw new InvalidOperationException(SR.Format(SR.AddedItemNotAtIndex, index));
                        _snapshot.RemoveAt(index);
                    }
                    break;

                case NotifyCollectionChangedAction.Replace:
                    for (int i = args.NewItems.Count - 1, index = args.NewStartingIndex + i; i >= 0; --i, --index)
                    {
                        if (!System.Windows.Controls.ItemsControl.EqualsEx(args.OldItems[i], _snapshot[index]))
                            // replace error message with a better one
                            throw new InvalidOperationException(SR.Format(SR.AddedItemNotAtIndex, index));
                        _snapshot[index] = args.NewItems[i];
                    }
                    break;

                case NotifyCollectionChangedAction.Move:
                    if (args.NewStartingIndex < 0)
                        throw new InvalidOperationException(SR.CannotMoveToUnknownPosition);

View on GitHub (pinned to 81131a70a4)