Why is it so bad to mock classes?

Question

I recently discussed with a colleague about mocking. He said that mocking classes is very bad and should not be done, only in few cases.

He says that only interfaces

Answer 1

The answer, like most questions about practices, is "it depends".

Overuse of mocks can lead to tests that don't really test anything. It can also lead to tests which are virtual re-implementations of the code under test, tightly bound to a specific implementation.

On the other hand, judicious use of mocks and stubs can lead to unit tests which are neatly isolated and test one thing and one thing alone - which is a good thing.

It's all about moderation.