Mathias Brandewinder on .NET, F#, VSTO and Excel development, and quantitative analysis / machine learning.
by Mathias 27. January 2010 18:26

Mark Needham recently published a series of posts around TDD, and one caught my attention. He is mocking a series of calls to a method SomeMethod() of a service IService, and doesn’t really care about the arguments, but:

For the sake of the test I only wanted 'service' to return a value of 'aValue' the first time it was called and then 'anotherValue' for any other calls after that.

His solution is to ditch his mocking framework (Rhino.Mocks, as far as I can tell) for that one test, and hand-roll his stub – and his example is a good case for why you might want to do that, sometimes.

However, this got me curious, and I wondered if this was indeed possible using Rhino. As recently as last week, I struggled with mocking repeat calls; but I had never actually considered a situation where one might want to mock a method, focusing only on the fact that the method is called, without paying attention to the specific arguments passed. Fun stuff.

After some digging into the documentation, I came across IgnoreArguments(), which seems to do the job:

public void SpecifyFirstReturnThenReturnSameThingForeverAfter()
    var fakeService = MockRepository.GenerateStub<IService>();
    fakeService.Expect(f => f.SomeMethod(null)).IgnoreArguments().Return("First").Repeat.Once();
    fakeService.Expect(f => f.SomeMethod(null)).IgnoreArguments().Return("SecondAndAfter");
    var first = fakeService.SomeMethod("ABC");
    var second = fakeService.SomeMethod("DEF");
    var third = fakeService.SomeMethod( "GHI" );

    Assert.AreEqual("First", first);
    Assert.AreEqual("SecondAndAfter", second);
    Assert.AreEqual("SecondAndAfter", third);

IgnoreArguments() seems to be a potentially convenient way to make some tests lighter. That being said, arguably, the setup here is cumbersome, and the hand-rolled version is clearer: when you reach the point where you wonder if your mock is doing what you think it should, you enter perilous territory…


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