Unit tests - The benefit from unit tests with contract changes?

Question

Recently I had an interesting discussion with a colleague about unit tests. We were discussing when maintaining unit tests became less productive, when your contracts change.

Answer 1

One principle that I rely on here is removing duplication. I generally don't have many different fakes or mocks implementing this contract (I use more fakes than mocks partly for this reason). When I change the contract it is natural to inspect every implementation of that contract, production code or test. It bugs me when I find I'm making this kind of change, my abstractions should have been better thought out perhaps etc, but if the test codes is too onerous to change for the scale of the contract change then I have to ask myself if these also are due some refactoring.