title: Why is it so bad to mock classes?
body: <p>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.</p>
  
  <p>He says that only interfaces should be mocked, otherwise it's an architecture fault.</p>
  
  <p>I wonder why this statement (I fully trust him) is so correct? I don't know it and would like to be convinced.</p>
  
  <p>Did I miss the point of mocking (yes, I read <a href="http://www.martinfowler.com/articles/mocksArentStubs.html" rel="noreferrer">Martin Fowler's article</a>)</p>