Property to check whether the instance can be extended and return the interface for extension operation. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
| C# | 
|---|
|  | 
| Visual Basic | 
|---|
|  | 
| Visual C++ | 
|---|
|  | 
Remarks
 If the family instance can not support extension operation, it returns    a null reference (  Nothing  in Visual Basic)