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The event occurs prior to adding a new item to a collection, typically in data-binding scenarios. The handler of this event can supply the new item to be added, overriding the standard action of the collection class. This is accomplished by setting the property of the parameter to this new item. Typically this item must be of a type expected by the recipient collection, or the collection will throw an exception of type .

This event is commonly used in data-binding scenarios, within classes such as and .

When you create an delegate, you identify the method that will handle the event. To associate the event with your event handler, add an instance of the delegate to the event. The event handler is called whenever the event occurs, unless you remove the delegate. For more information about event-handler delegates, see Events and Delegates.

Represents the method that will handle the event.