csharp

Notify Collection Changed Event Args Reference

If you're like me and find yourself forgetting how the event args work in INotifyCollectionChanged, then here's a quick reference that might be useful.

Brandon Baker

Jun 30, 2020 • 1 min read

I end up using INotifyCollectionChanged frequently, but not frequently enough to reliably recall how NotifyCollectionChangedEventArgs works when it is raised from the INotifyCollectionChanged.CollectionChanged event. There's a nice post by Stephen Cleary that summarizes how this works, but I wanted to take a minute to try to improve the formatting.

Add

The Action is NotifyCollectionChangedAction.Add when items are added to the collection.

Property	Description
`NewItems`	Contains any items that were added.
`NewStartingIndex`	Index where new items were added, otherwise `-1`.

Remove

The Action is NotifyCollectionChangedAction.Remove when items are removed from the collection.

Property	Description
`OldItems`	Contains any items that were removed.
`OldStartingIndex`	Index where where items were removed, otherwise `-1`.

Replace

The Action is NotifyCollectionChangedAction.Replace when old items are replaced with new items.

Property	Description
`NewItems`	Contains items that are replacing old items.
`NewStartingIndex`	Equal to `OldStartingIndex`.
`OldItems`	Contains the items that were replaced by new items.
`OldStartingIndex`	Contains the index from which the old items were removed.

Move

The Action is NotifyCollectionChangedAction.Move when one or more items are moved within the collection. This operation is logically treated as a Remove followed by an Add, so NewStartIndex is interpreted as though the items had already been removed.

Property	Description
`NewItems`	`SequenceEqual` to `OldItems`, though they may be different instances.
`NewStartingIndex`	Contains the index to which the items were moved.
`OldItems`	Contains the items that were moved.
`OldStartingIndex`	Contains the index from which the items were moved.

Reset

The Action is NotifyCollectionChangedAction.Reset when a collection is completely reset. None of the properties in the event args will have values.