How do I use Moq to mock an extension method?

Question

I am writing a test that depends on the results of an extension method but I don't want a future failure of that extension method to ever break this test. Mocking that result seemed the obvious choice but Moq doesn't seem to offer a way to override a static method (a requirement for an extension method). There is a similar idea with Moq.Protected and Moq.Stub, but they don't seem to offer anything for this scenario. Am I missing something or should I be going about this a different way?

Here is a trivial example that fails with the usual "Invalid expectation on a non-overridable member". This is a bad example of needing to mock an extension method, but it should do.

public class SomeType {     int Id { get; set; } }  var ListMock = new Mock<List<SomeType>>(); ListMock.Expect(l => l.FirstOrDefault(st => st.Id == 5))         .Returns(new SomeType { Id = 5 }); 

As for any TypeMock junkies that might suggest I use Isolator instead: I appreciate the effort since it looks like TypeMock could do the job blindfolded and inebriated, but our budget isn't increasing any time soon.

Answer 1

Extension methods are just static methods in disguise. Mocking frameworks like Moq or Rhinomocks can only create mock instances of objects, this means mocking static methods is not possible.