If Cancellable returns true, event delegates may cancel the command that was announced by the event. To do so, a delegate may call the Cancel() method if it is available. Typically, single-events and post-events (e.g. DocumentPrinted) are not cancellable, while most pre-events (e.g. DocumentPrinting) are cancellable, except for special conditions and situations, such as when the Revit application is being closed or if an event is raised during another event.