I really appreciated chapter 7 overall, it was a nice walk-through of a simple TDD approach.

However, I'm concerned about the part that tests that an ArgumentNullException is thrown. The way it is written, this unit test is testing the mock, not the actual entity!

Maybe the example wasn't completed accidentally, but code should have been added to the ItemTypeRepository to actually throw an ArgumentNullException when called with "null".

Looking at it further this morning, I think I see where my confusion was.

I was mistakenly thinking the code was doing a setup on the ItemTypeRepository.Save method, when in fact it's on the NHibernate session.Save method.

I'm assuming this method in fact does throw an ArgumentNullException in actuality.