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

I have similar experiences with unit tests - when you change the contract of one class often you need to change loads of other tests as well (which will actually pass in many cases, which makes it even more difficult). That is why I always use higher level tests as well:

1. Acceptance tests - test a couple or more classes. These tests are usually aligned to user stores that need to be implemented - so you test that the user story "works". These don't need to connect to a DB or other external systems, but may.
2. Integration tests - mainly to check external system connectivity, etc.
3. Full end-to-end tests - test the whole system

Please note that even if you have 100% unit test coverage, you are not even guaranteed that your application starts! That is why you need higher level tests. There are so many different layers of tests because the lower you test something, the cheaper it usually is (in terms of development, maintaining test infrastructure as well as execution time).

As a side note - because of the problem you mentioned using unit tests teaches you to keep your components as decoupled as possible and their contracts as small as possible - which is definitely a good practise!