It is recommended that you implement a default protocol extension class, as shown in the Protocol Extension Example. If there is no corresponding protocol extension object for a particular class, ObjectARX searches up the class hierarchy and uses the nearest one that it finds. It is also recommended that you associate the default protocol extension class with AcRxObject or with some other class at the top of the ObjectARX class hierarchy such as AcDbEntity (in this example) or AcDbObject.