Error COBCH1728 ("type-name1 does not have public implementation of method method-name of interface type-name2") is also produced with this error.
In the following example, this error is returned for System.Windows.Forms.Control:
      $set ilref"System.Windows.Forms"
       class-id MainView
                 inherits type System.Windows.Forms.Form
                 implements type IMainView.
       end class.
       interface-id IMainView public.
       method-id Show. 
       end method.
       end interface.