If you think about code in terms of objects that perform roles for each other, then it’s natural to unit test collaborating objects by verifying how they interact. It’s not about substituting external objects, but about discovering the relationships between parts of your code.

I could go on, but there’s http://www.mockobjects.com/book instead.

@Gil – Thanks for commenting! Yeah – I have seen some of the seemingly impossible stuff you can do with it.

Nice post.
Speaking on Typemock’s behalf, we do the same wierd stuff for sealed classes and all types. We don’t discriminate:)

I’ve done a post a while back talking why it’s so wasteful to write your own stubs/mocks. Using any framework (I recommend Isolator of course) is much more productive than writing your own. You can read it here: http://blog.typemock.com/2009/03/why-you-need-isolation-framework.html

Thanks,

Gil Zilberfeld
Typemock