I think that's not right way to test units. You should test the interface resp. the right execution of the specification of the unit and not testing implementat

I think that's not right way to test units. You should test the interface resp. the right execution of the specification of the unit and not testing implementation details. That means you're either testing the return value or testing the inputs which were modified by the unit. If the behavior of a unit changes in such a way that it does something else with its inputs, it's a specification change which requires other test cases.