The manual mock approach works great here because the IService interface has a single method. If that interface would have many methods, you would end up with a lot of ‘not implemented’ methods. This is typically why one would use a mock framework.

Now the shameless plug: if you would use the Stubs farmework that ship with Pex, you would have almost the same experience as with manual mocks, without having to dangling ‘not implemented’ methods lying around. In a nutshell, Stubs allows you to replace any method of the interface with your delegate, e.g. your example would look like this:

[TestMethod]
void SomeTest() {
int numberOfCalls = 0;
var service = new SIService { // implements IService
SomeMethodString = s => { // Func
if(numberOfCalls++ == 0)
return “aValue”;
else
return “another value”;
}
};
… // the rest of the test
}

Stubs is really just a code generator that produces the SIService type that has a delegate field and a boiler plate implementation for each method:

class SIService : IService {
public Func SomeMethodString;
string IService.SomeMethod(string s) {
var f = this.SomeMethodString;
if (f != null) return f(s);
else throw Exception();
}
}