I was recently part of a discussion on SHAPE about automated unit and functional testing. I was talking about buggy test code, and how frustrating (and sadly common) it can be to have automated test code that is unreliable. As a result, I try to keep test case code as simple and short as possible in an attempt to reduce bugs I might inadvertantly create. I do what I can to test the test code, but it can get out of control easily. It’s hard to unit test test code, and then I wonder why do I need tests for my test code? Isn’t that a smell that my test code is too complex?

I was musing that there is nothing more frustrating than buggy test code and buggy test harnesses, and Jerry agreed. He then pointed out something interesting. Jerry said:

There’s a kind of symmetry to all this, so that someone from Mars might not know, without being told, which was the product code and which was the test code. Each tests the other.

I had never thought of test code and its relationship to production code in this way before. This is a powerful and profound way of thinking about how we can “test our tests”, and is especially true with automated xUnit tests. It also strengthens the idea that automated test code needs to be treated as seriously as production code, and deserves the same kind of design, planning, resources and people behind it. Without symmetry, problems inevitably arise in test code especially.