Retrieve the DOFType represented within the DOFValue. 
- Lifecycle
 - The data returned is owned by the DOFValue and is only valid as long as the DOFValue remains valid. It must not be destroyed individually.
 
- Parameters
 - 
  
    | self | The DOFValue. This must not be NULL. | 
  
   
- Returns
 - The DOFType of the DOFValue.
 
- Exceptions
 - 
  
    | NULL | is returned if self is NULL.  |