dotnet/wpf · error · InvalidOperationException

SR.AddedItemNotInCollection

Error message

SR.AddedItemNotInCollection

What it means

An Add notification did not specify an index (-1), so ListCollectionView searched the source list with IndexOf and did not find the announced item at all. It throws InvalidOperationException with SR.AddedItemNotInCollection because the 'added' item is not actually present in the source.

Solutions

1. Insert the item into the source collection BEFORE raising the Add event.
2. Raise the event with the actual index instead of -1 to skip IndexOf.
3. Verify the same object instance (or a value-equal one per the collection's comparer) is in the source list.
4. If the item genuinely is not in the collection, raise Reset or no event at all.

Example fix

// before (event before mutation)
OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, item, -1));
Items.Add(item);
// after
Items.Add(item);
OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, item, Items.Count - 1));

if (args.Action == NotifyCollectionChangedAction.Add && args.NewStartingIndex == -1 && !sourceList.Contains(args.NewItems[0])) { /* item missing; raise Reset after inserting */ }

try { /* process add */ } catch (InvalidOperationException) { view.Refresh(); }

When it happens

Trigger: Raising NotifyCollectionChangedAction.Add with NewStartingIndex == -1 for an item that was never inserted into (or was later removed from) the underlying IList — the IndexOf(item) fallback returns -1.

Common situations: Event raised before the actual insert completed; item removed between raise and handling; adding an item to a filtered copy rather than the real source collection; EqualsEx/object identity mismatch so IndexOf can't locate an 'equal' item.

Understand the failure class

Background: "Not found" and "does not exist" errors: why "Task not found", "No such folder", and "Can't find" fire when a lookup comes back empty — this error's family across 14 libraries.

Related errors

• SR.AddedItemNotAtIndex (formatted with index)
• SR.BindingListCannotCustomFilter
• SR.BindingListCanOnlySortByOneProperty
• SR.CannotMoveToUnknownPosition
• SR.CannotMoveToUnknownPosition

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c5751e3df1855b21. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Data/ListCollectionView.cs:2680

            IList ilFull = (AllowsCrossThreadChanges ? ShadowCollection : SourceCollection) as IList;

            // validate input
            if (index < -1 || index > ilFull.Count)
                throw new InvalidOperationException(SR.Format(SR.CollectionChangeIndexOutOfRange, index, ilFull.Count));

            if (action == NotifyCollectionChangedAction.Add)
            {
                if (index >= 0)
                {
                    if (!System.Windows.Controls.ItemsControl.EqualsEx(item, ilFull[index]))
                        throw new InvalidOperationException(SR.Format(SR.AddedItemNotAtIndex, index));
                }
                else
                {
                    // event didn't specify index - determine it the hard way
                    index = ilFull.IndexOf(item);
                    if (index < 0)
                        throw new InvalidOperationException(SR.AddedItemNotInCollection);
                }
            }

            // if there's no sort or filter, use the index into the full array
            if (!UsesLocalArray)
            {
                if (IsAddingNew)
                {
                    if (NewItemPlaceholderPosition != NewItemPlaceholderPosition.None &&
                        index > _newItemIndex)
                    {
                        --index;        // the new item has been artificially moved elsewhere
                    }
                }

                return (index < 0) ? index : index + delta;
            }

View on GitHub (pinned to 81131a70a4)