I've been going through the process of cleaning up our controller code to make each action as testable. Generally speaking, this hasn't been too difficult--where we have opportunity to use a fixed object, like say FormsAuthentication, we generally introduce some form of wrapper as appropriate and be on our merry way.

For reasons not particularly germaine to this conversation, when it came to dealing with usage of HttpContext, we decided to use the newly created HttpContextWrapper class rather than inventing something homegrown. One thing we did introduce was the ability to swap in a HttpContextWrapper (like say, for unit testing). This was wholly inspired by the way Oren Eini handles unit testing with DateTimes (see article, a pattern we also use)

public static class FooHttpContext
{
    public static Func<HttpContextWrapper> Current = () 
         => new HttpContextWrapper(HttpContext.Current);

    public static void Reset()
    {
        Current = () => new HttpContextWrapper(HttpContext.Current);
    }
}

Nothing particularly fancy. And it works just fine in our controller code. The kicker came when we go to write unit tests. We're using Moq as our mocking framework, but alas

var context = new Mock<HttpContextWrapper>() 

breaks since HttpContextWrapper doesn't have a parameterless ctor. And what does it take as a ctor parameter? A HttpContext object. So I find myself in a catch 22.

I'm using the prescribed way to decouple HttpContext--but I can't mock a value in because the original HttpContext object was sealed and therefore difficult to test. I can map HttpContextBase, which both derive from--but that doesn't really get me what I'm after. Am I just missing the point somewhere with regard to HttpContextWrapper? I can find ways to work around the issue, but we are kind of fond of remaining consistent in decoupling using the Function delegate pattern--but it seems like we're not fully grokking intent of the wrapper.