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RevitAPIEventArgs.Cancellable

Property
Description:
Indicates whether an event may be cancelled by an event delegate.
Remarks:
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.
Syntax
public bool Cancellable { get; }