How do you mock a Sealed class?

Solution 1:

For .NET, you could use something like TypeMock, which uses the profiling API and allows you to hook into calls to nearly anything.

Solution 2:

My general rule of thumb is that objects that I need to mock should have a common interface too. I think this is right design-wise and makes tests a lot easier (and is usually what you get if you do TDD). More about this can be read in the Google Testing Blog latest post (See point 9).

Also, I've been working mainly in Java in the past 4 years and I can say that I can count on one hand the number of times I've created a final (sealed) class. Another rule here is I should always have a good reason to seal a class, as opposed to sealing it by default.

Solution 3:

I believe that Moles, from Microsoft Research, allows you to do that. From the Moles page:

Moles may be used to detour any .NET method, including non-virtual/static methods in sealed types.

UPDATE: there is a new framework called "Fakes" in the upcoming VS 11 release that is designed to replace Moles:

The Fakes Framework in Visual Studio 11 is the next generation of Moles & Stubs, and will eventually replace it. Fakes is different from Moles, however, so moving from Moles to Fakes will require some modifications to your code. A guide for this migration will be available at a later date.

Requirements: Visual Studio 11 Ultimate, .NET 4.5

Solution 4:

The problem with TypeMock is that it excuses bad design. Now, I know that it is often someone else's bad design that it's hiding, but permitting it into your development process can lead very easily to permitting your own bad designs.

I think if you're going to use a mocking framework, you should use a traditional one (like Moq) and create an isolation layer around the unmockable thing, and mock the isolation layer instead.