The exception thrown when Revit cancels a background operation. Third-party developers are not expected to catch and handle this exception. Instead, if allowed to propagate back to Revit code, it will be handled by Revit. 
   Namespace:   Autodesk.Revit.Exceptions  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 17.0.0.0 (17.0.1090.0) 
  Since:  2017 
Syntax
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Inheritance Hierarchy
  System Object  
System Exception
Autodesk.Revit.Exceptions ApplicationException
Autodesk.Revit.Exceptions BackgroundTaskCancelledException
 
 System Exception
Autodesk.Revit.Exceptions ApplicationException
Autodesk.Revit.Exceptions BackgroundTaskCancelledException