Assembly: System.Web (in System.Web.dll)
Page developers can provide custom handlers for Web Parts events by adding an OnSelectedWebPartChanging attribute, OnWebPartClosing attribute, or OnWebPartDeleting attribute to the <asp:webpartmanager> element in a page, and assigning a custom method name to the attribute. A custom handler can also be assigned programmatically. For more information on programmatically assigning an event handler, see NIB: How to: Connect Event Handler Methods to Events.
Available since 2.0