Test to see if the set is empty. 
   Namespace:   Autodesk.Revit.DB.Architecture  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
Remarks
 If the set is empty then True will be returned, otherwise False. 
   Namespace:   Autodesk.Revit.DB.Architecture  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0)