Rhino + VS debugger == WFT?

We have used the MVP pattern lately in my recent project to increase testability and decouple the UI from our business logic. To enhance our testing environment we have begun using Rhino Mocks and it is an overall satisfying experience even though I am still having a hard time "getting it right" when writing tests.

I want to share with you one of the "What The F***" experiences I had just a few minutes ago: One of our mocking tests failed. Well - insert a few breakpoints and fire up the debugger. And then it happened: For every time I ran my test I had errors on different lines in my source code... Most of the times I had a NullPointerException but at other times the Rhino framework threw mysterious errormessages at me...

I was a little scared to see a unittest behave like this for twenty minutes and really had no clue what was happening but my collegue was able to solve the puzzle once I asked around for a little help. What had happened was that in my MVP Presenter I had set up an mock of my view and some expectations for different properties to be called during OnLoad. They should be called just once so when you start using your debugger's Quickwath on those properties you "use up" the quota of expected calls. You are actually fulfilling the expectation you just set up when quickwatching a property in a test with expectations set Rhino-style... And when the code afterwards tries to execute the code you intended the expectation to test for your test fails because you only set your property up for being accessed just once.

Lesson learned today: When a test which uses mocking fails you - clear all breakpoints and try again. It actually solved the mystery and I was able to fix the initial cause of the error within a few minutes.