Assembly: System.Web (in System.Web.dll)
The WebPartManager control uses this method, and the associated event, to provide an opportunity to cancel a process in which the selected control is changing. For example, the method is called within the BeginWebPartEditing method, and also within the EndWebPartEditing method, to provide a way to cancel the process. If a user clicks the cancel verb on a control that is being edited, the control is no longer selected and any editing changes are not saved, because the method makes it possible to cancel the editing changes.
The method is similarly called during the methods that begin and end the process of connecting a WebPart control to another control, to enable canceling that process.
Page developers can create a custom handler for the SelectedWebPartChanging event by adding the OnSelectedWebPartChanging attribute to the <asp:webpartmanager> element in declarative page markup, and assigning to the attribute the name of a custom method.