How to clear previous expectations on an object?

There are three ways:

You can reset the expectations by using BackToRecord

I have to admit that I never really used it because it is awkward.

// clear expectations, an enum defines which
_stubRepository.BackToRecord(BackToRecordOptions.All);
// go to replay again.
_stubRepository.Replay();

Edit: Now I use it sometimes, it is actually the cleanest way. There should be an extension method (like Stub) which does it – I think it just got forgotten. I would suggest to write your own.

You can use Repeat.Any()

It ‘breaks’ the order of the stubbed definition and “overrides” previous definitions. But it’s somehow implicit. I use it sometimes because it is easy to write.

_stubRepository.Stub(x => x.Contains(null))
  .IgnoreArguments()
  .Return(false)
  .Repeat.Any();

You can create a new mock

Trivial, but explicit and easy to understand. It is only a problem if you want to keep plenty of definitions and only change one call.

_stubRepository = MockRepository.GenerateMock<IRepository>();
_stubRepository.Stub(x => x.Contains(null))
  .IgnoreArguments()
  .Return(false);