I'm currently trying to grasp MSpec, mainly to learn new ways of (T/B)DD to be able to make an educated decision on which technology to use. Previously, I've mostly (read: only) used the built-in MSTest framework with Moq, so BDD is quite new for me.

I'm writing an ASP.NET MVC app, and I want to implement PRG. Last time I did this, I used action filters to export and import ModelState via TempData, so that I could return a RedirectResult and the validation errors would still be there when the user got the view. I tested that scenario by verifying two things:

a) That the ExportModelStateAttribute I had written was applied (among tests for my controller)
b) That the attribute worked (among tests for action filter attributes)

However, in BDD I've understood I should be even more concerned with behavior, and even less with implementation. This means I should probably just verify that the model state is in tempdata when the action has finished executing - not necessarily that it's done via an attribute.

To further complicate things, attributes are not run when calling the action directly in the test, so I can't just call the action and see if the job's been done.

How should I spec/test this in MSpec?