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RevitEventArgs.Cancellable

Property
Description:
Indicates whether an event may be cancelled by an event delegate.
Remarks:
If 'Cancellable' is true, event delegates may cancel the command that was announced by the event. To do so, a delegate would set the 'Cancel' property to True. Typically, 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 virtual bool Cancellable { get; }