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

Mocking classes (in contrast to mocking interfaces) is bad because the mock still has a real class in the background, it is inherited from, and it is possible that real implementation is executed during the test.

When you mock (or stub or whatever) an interface, there is no risk of having code executed you actually wanted to mock.

Mocking classes also forces you to make everything, that could possibly be mocked, to be virtual, which is very intrusive and could lead to bad class design.

If you want to decouple classes, they should not know each other, this is the reason why it makes sense to mock (or stub or whatever) one of them. So implementing against interfaces is recommended anyway, but this is mentioned here by others enough.