One of the less stated benefits of doing TDD is that it concentrates your mind on the problem.
Recently I was looking at a section of code which was creating a Lucene search string from criteria passed in from the user. I needed to add some new functionality, but it was a bit higgledy-piggledy so following the broken windows principle I decided to refactor it to the builder pattern before I added my new feature.
I ran NCover on the code and it reported 70% coverage which considering what I knew about the history of the system was surprisingly high. I decided there was enough coverage, paired with a colleague and dived in. At first we were doing well. Our changes broke the tests a few times which reassured us about the coverage, but after a while it felt that we were losing a bit of steam. We’d done a lot of refactoring but it didn’t feel like we’d got very far.
Skip to a couple of days later – I’d not been able to touch the code and neither had my colleague who had now been taken off to deal with another project, so I pulled in the other dev on my team who is probably the most fanatical TDD nut I’ve ever met. Immediately he was uncomfortable, he’d quickly spotted the tests were really only integration tests (something I had put to the back of my mind) and got jittery as we continued from where I’d left off. I didn’t like to see him in so much pain so quickly relented and let him write new tests even though I felt it was just going to slow us down, maybe even to the point where we wouldn’t get it finished. However he assured me that although it felt slow now we’d be racing along soon enough.
Not only was he right about that, but as we were doing it the objective was so much clearer. Writing the new tests meant we were forced to really think about each move we made so our refactoring had clear direction. We spent about the same time working on the problem but probably made twice as much progress as my previous attempt and ended up with a proper unit test suite around the code.
Some day soon people are probably going to do away with the need to do TDD, which I think will be a shame as it never ceases to amaze me how many benefits it has.