Assembly: System.Web (in System.Web.dll)
The event is raised by the OnWebPartAdding method after the process of adding a control to a zone has begun. This event provides an opportunity to cancel the process before it is complete. If the adding process is successful, this event is followed by the WebPartAdded event.
Page developers can provide custom handlers for the event by adding the OnWebPartAdding attribute to the <asp:webpartmanager> element in a page, and assigning a custom method name to the attribute.