how auto-modify the entity newly appended to db?

like the title mentioned above,

If i want to auto-modify the entity newly appended to db, i.e for newly appended entity ,I want to change it's colorIndex to 0;

or according to the type of the entity,for AcDbline I change the length,for circle ,I change the radius.

I know for catching every entity appended Msg I can use the AcDbDatabaseReactor subclass.

and override    

virtual void objectAppended(const AcDbDatabase* dwg,const AcDbObject* dbObj);

 But the in that callback function, dbObj is read-only.

So can you help me ??