Assembly: System.Web (in System.Web.dll)
The method raises the WebPartAdding event to indicate that a control is in the process of being added. The method also provides the option to cancel the adding process. If the control is successfully added, the WebPartAdded event follows.
Page developers can provide a custom handler for the associated event by adding the OnWebPartAdding attribute to the <asp:webpartmanager> element on a Web page, and then assigning a custom method name to the attribute.Notes to Inheritors
Developers who create derived WebPartManager classes can override the method to customize the event handling.