The exception that is thrown when referencing an object that is no longer valid. 
   Namespace:   Autodesk.Revit.Exceptions  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
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Remarks
 The object may no longer exist for many reasons: 
 - The object was explicitly deleted from the database.
 - A change to other database items caused the item to be automatically deleted from the database.
 - The object no longer exists as its creation was undone (by rolling a transaction back).
 
Inheritance Hierarchy
  System Object  
System Exception
Autodesk.Revit.Exceptions ApplicationException
Autodesk.Revit.Exceptions InvalidOperationException
Autodesk.Revit.Exceptions InvalidObjectException
 
 System Exception
Autodesk.Revit.Exceptions ApplicationException
Autodesk.Revit.Exceptions InvalidOperationException
Autodesk.Revit.Exceptions InvalidObjectException