How do I mock HttpResponseBase.End()?

Question

I'm using Moq to create a mock object of HttpResponseBase. I need to be able to test that HttpResponseBase.End() was called in my library. To do this, I specify some text before the call and some text after. Then I check that only the text before the call to End() is present in HttpResponseBase.Output.

The problem is, I can't figure out how to mock HttpResponseBase.End() so that it stops processing, like it does in ASP.NET.

public static HttpResponseBase CreateHttpResponseBase() {
    var mock = new Mock<HttpResponseBase>();
    StringWriter output = new StringWriter();

    mock.SetupProperty(x => x.StatusCode);
    mock.SetupGet(x => x.Output).Returns(output);
    mock.Setup(x => x.End()) /* what do I put here? */;
    mock.Setup(x => x.Write(It.IsAny<string>()))
        .Callback<string>(s => output.Write(s));

    return mock.Object;
}

Answer 1

It is a bit unclear to me what it is you are trying to achieve, but from your description, it sounds like you are attempting to get your Abstraction to behave like a particular implementation. In other words, because HttpResponse.End() has a certain behavior, you want your Mock to have the same behavior?

In general, that is not particularly easy to do with Moq, since it has no concept of ordered expectations (unlike RhinoMocks). There is, however, a feature request for it.

You might be able to use a Callback together with setting up the End method to toggle a flag that determines any further behavior of the Mock, but it's not going to be particularly pretty. I'm thinking about something like this:

bool ended = false;
var mock = new Mock<HttpResponseBase>();
mock.Setup(x => x.End()).Callback(() => ended = true);
// Other setups involving 'ended' and Callbacks

Then have all other Setups have dual implementatations based on whether ended is true or false.

It would be pretty damn ugly, so I'd seriously reconsider my options at this point. There are at least two directions you can take:

1. Make a Fake implementation of HttpResponseBase instead of using Moq. It sounds like you are expecting such specific behavior of the implementation that a Test Double with embedded logic sounds like a better option. Put shortly, a Fake is a Test Double that can contain semi-complex logic that mimics the intended production implementation. You can read more about Fakes and other Test Doubles in the excellent xUnit Test Patterns book.
2. Reconsider your initial assumptions. It sounds to me like you are tying your client very closely to a particular behavior of HttpResponseBase, so you may be violating the Liskov Substitution Principle. However, I may be mistaken, as a method called 'End' carries certain connotations beyond the purely semantic, but still, I'd personally consider if a better design was possible.