Id of the document that is about to be closed. 
   Namespace:   Autodesk.Revit.DB.Events  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2010 
Syntax
Remarks
 This Id is only used to identify the document for the duration of this event and the DocumentClosed event which follows. It serves as synchronization means for the pair of events.