Your question was about how the MS Fakes framework is different from NMock and it appears the other answers have resolved some of that, but here is some more information regarding how they are the same and how they are different. NMock is also similar to RhinoMocks and Moq, so I'm grouping them in with NMock.

There are 3 major differences I see right off between NMock/RhinoMocks/Moq and the MS Fakes Framework:

  • The MS fakes framework uses generated code, much like Accessors in prior versions of Visual Studio instead of generic types. When you want to use the fakes framework for a dependency, you add the assembly that contains the dependency to the references of the test project and then right-click on it to generate the test doubles (stubs or shims). Then when you are testing, you are actually using these generated classes instead. NMock uses generics to accomplish the same thing (i.e. IStudentRepository studentRepository = mocks.NewMock<IStudentRepository>()). In my opinion, the MS Fakes framework approach inhibits code navigation and refactoring from within the tests since you are actually working against a generated class, not your real interface.

  • The MS fakes framework supplies stubs and moles (shims), whereas NMock, RhinoMocks, and Moq all provide stubs and mocks. I really don't understand MS's decision to not include mocks and I am, personally not a fan of moles for reasons described below.

  • With the MS fakes framework, you supply an alternative implementation of the methods you want to stub. Within these alternate implementations, you can specify the return values and track information about how or if the method was called. With NMock, RhinoMocks and Moq, you generate a mock object and then use that object to specify stubbed return values or to track interactions (whether and how the methods were called). I find the MS fakes approach more complex and less expressive.

To clarify the difference in what the frameworks provide: NMock, RhinoMocks and Moq all provide two types of test doubles (stubs and mocks). The fakes framework provides stubs and moles (they call them shims), and unfortunately does not include mocks. In order to understand the differences and similarities between NMock and MS Fakes, it is helpful to understand what these different types of test doubles are:

Stubs: Stubs are used when you need to provide a values for methods or properties that will be asked of your test doubles by the method under test. For example, when my method under test calls the DoesStudentExist() method of the IStudentRepository test double, I want it to return true.

The idea of stubs in NMock and MS fakes is the same, but with NMock you would do something like this:

Stub.On(mockStudentRepository).Method("DoesStudentExist").Will(Return.Value(true));

And with MSFakes you would do somethign like this:

IStudentRepository studentRepository = new DataAccess.Fakes.StubIStudentRepository() // Generated by Fakes.
{
    DoesStudentExistInt32 = (studentId) => { return new Student(); }
};

Notice in the MS Fakes example you create an entirely new implementation for the DoesStudentExist method (Note that it is called DoesStudentExistInt32 because the fakes framework appends the parameter data types to the method names when it generates the stub objects, I think this obscures the clarity of the tests). To be honest the NMock implementation also bugs me because it uses a string to identify the method name. (Forgive me if I've misunderstood how NMock is intended to be used.) This approach really inhibits refactoring and I'd highly recommend RhinoMocks or Moq over NMock for this reason.

Mocks: Mocks are used to verify interaction between your method under test and its dependencies. With NMock, you do this by setting expectations similar to this:

Expect.Once.On(mockStudentRepository).Method("Find").With(123);

This is another reason why I'd prefer RhinoMocks and Moq over NMock, NMock uses the older expectation style whereas RhinoMocks and Moq both support the Arrange/Act/Assert approach where you specify you expected interactions as assertions at the end of the test like this:

stubStudentRepository.AssertWasCalled( x => x.Find(123));

Again, note that RhinoMocks uses a lambda instead of a string to identify the method. The ms fakes framework does not provide mocks at all. This means that in your stubbed out implementations (see the description of stubs above) you have to set variables that you later verify were set correctly. That would look something like this:

bool wasFindCalled = false;

IStudentRepository studentRepository = new DataAccess.Fakes.StubIStudentRepository() 
{
    DoesStudentExistInt32 = (studentId) => 
        { 
            wasFindCalled = true;
            return new Student(); 
        }
};

classUnderTest.MethodUnderTest();

Assert.IsTrue(wasFindCalled);

I find this approach to be a little convoluted since you have to track the call up in the stub and then assert it later in the test. I find the NMock, and especially the RhinoMocks, examples to be more expressive.

Moles (Shims): To be frank, I do not like moles, because of their potential for misuse. One of the things I like so much about unit testing (and TDD in particular) is that testing your code helps you to understand where you have written poor code. This is because testing poorly written code is difficult. This is not true when using moles because moles are actually designed to allow you to test against dependencies that are not injected or to test private methods. They work similarly to stubs, except that you use a ShimsContext like this:

using (ShimsContext.Create())
{
    System.Fakes.ShimDateTime.NowGet = () => { return new DateTime(fixedYear, 1, 1); };
}

My worry with shims is that people will start seeing them as "an easier way to unit test" because it doesn't force you to write code the way you should. For a more complete write-up on this concept see this post of mine:

For more information on some concerns related to the fakes frameworks take a look at these posts:

If you're interested in learning RhinoMocks here's a Pluralsight training video (full disclosure - I wrote this course and get paid royalties for views, but I think it applies to this discussion so I'm including it here):