Is it recommended to mock concrete class?

Question

Most of the examples given in mocking framework website is to mock Interface. Let say NSubstitute that I\'m currently using, all their mocking examples is to mock interface.

Answer 1

Its not that it is recommended, it's that you can do this if you have no other choice.

Usually well designed project rely on defining interfaces for your separate components so you can tests each of them in isolation by mocking the other ones. But if you are working with legacy code /code that you are not allowed to change and still want to test your classes then you have no choice and you cannot be criticized for it (assuming you made the effort to try to switch these components to interfaces and were denied the right to).