Rhino Mocks - MethodInvocation.ReturnValue vs IMethodOptions.Return

Question

Playing around with Rhino Mocks, i just stumbled upon a strange behaviour, which I cannot understand.

Setting up my Stub the following way works as expected - 42 is returned

var generator = MockRepository.GenerateStub<INumberGenerator>();
generator.Stub(x => x.GetNumber()).Return(42);

Then I tried using the WhenCalled(Action<MethodInvocation> action) extension, which I used so far only for void functions:

generator.Stub(x => x.GetNumber()).WhenCalled(mi => mi.ReturnValue = 42);

This code throws me an exception Method 'INumberGenerator.GetNumber();' requires a return value or an exception to throw.. Ok, I've added .Return(55) - and now things start to get weird:

generator.Stub(x => x.GetNumber()).WhenCalled(mi => mi.ReturnValue = 42).Return(55);

Which now returns 42, instead of 55. Note that the MethodInvocation.ReturnValue only works when adding the .Return(..) statement which then has absolutely no effect on the returnvalue - which confuses me a lot. Can anybody explain this behaviour?

Answer 1

Rhino Mocks requires you to specify a Return() call at the end of an Expect() chain (in your case, when you're setting expectation for your stub), so that it can determine the type of the return result.

So, in effect, the return value in WhenCalled() is only used. Any value in Returns() would do (and not be used) as long as it is of the same type.

Not explicitly mentioned in the documentation I think.