Mocking sealed classes without public constructors?

Question

The particular class I'm testing depends upon the HttpSessionState object.

The HttpSessionState class has no public constructors. The class under test is only using this object as a NameValue store. The class is used in an ASMX web service to return information for a particular method.

I'm thinking about creating a facade around the HttpSessionState class where I can provide a Dictionary <string, string> instead of the Session object in testing.

Is this a good idea or standard practice?

Answer 1

Yep, as the old saying goes, there's nothing that can't be solved by adding another layer of abstraction. I usually just hide the type behind an interface where the interface's methods are the only ones needed to perform the actions I want on that type.

Just mock the interface that hides HttpSessionState, and do Asserts on the uses of the interface, in Rhino Mocks it's just AssertWasCalled(d => ....) etc.

Answer 2

You can create a sub-class of the HttpSessionStateBase class. This answer shows how to implement this for Moq, but you can still use the MockHttpSession class with your Rhino Mocks (I assume. I haven't used Rhino Mocks).

public class MockHttpSession : HttpSessionStateBase
{
    Dictionary<string, object> sessionStorage = new Dictionary<string, object>();

    public override object this[string name]
    {
        get { return sessionStorage[name]; }
        set { sessionStorage[name] = value; }
    }
}

A fairly extensive discussion about how to mock .NET classes can be found at Scott Hanselman's blog here.

Answer 3

You can mock any type even sealed ones using Microsoft's Moles Isolation framework for .NET. Takes a little work to setup but might be better than adding another layer of abstraction. Mocking HttpContext and HttpSessionState using moles is discussed here. There is another similar discussion here.