Mocking the DAL/Repository that I pass to the BLL/ViewModels for testing purposes is not much different than creating an in-memory DAL. Right now I'm using an in-memory DAL in a new project instead of mocking it and it's working very well so far (even easier than setting up the mocks I'd say).
Since I haven't seen this done in online/book examples I thought I'd ask. Is there a problem with writing an in-memory DAL for testing the BLL/ViewModels?
On my architecture all components share a common library with all the relevant interfaces. Things are late bound which allows me to swap any component for another of the same interface.