Changed is called automatically when a collection item changes in a way that must be reflected by the collection to which the item belongs. For example, when an item's Index property changes, the collection must update the indexes of all other items. Call this method in the implementation of a descendant class when making changes that require the collection to update. 

The AllItems parameter indicates whether the change affects just the collection item (such as a change to its DisplayName property) or whether it requires corresponding changes to other items in the collection (such as a change to the Index property). 

Index 

DisplayName 

Collection