In his post Unit tests are the new documentation Scott sees unit tests as an undue burden much like documentation. Documentation, typically written after the fact, inevitably falls sway to the laws of entropy and becomes hopelessly outdated.
On one level, I do agree that unit tests are the new documentation, but I would add new and improved! Unit tests are a great way to provide code samples to document how an API is to be used. Though agile proponents point out that the code (when well written) is the documentation, well named methods and variables only tell you what it does and what they are used for, but not how to use it. There are several ways that I see unit tests as better than your traditional documentation.
Unit Tests Are Closer To The Code\ First of all, unlike traditional documentation sitting in a binder or word doc on the corporate network, your unit tests are typically in a project within the same solution the unit tests test. It is even possible to place the unit tests within the code being tested, but I personally avoid this approach. So it isn’t as close to the code as the code itself, but it is far closer than a word doc sitting on the network somewhere.
Unit Tests Are Hard to Ignore\ Once you’ve adopted unit testing and made it a habit, having unit tests get out of synch with your code is hard to ignore. For one thing, unlike documentation, unit tests compile. If they don’t compile, well you know they are out of date. Secondly, if the unit tests compile, but do not pass, then you know they are out of date. Documentation doesn’t provide any sort of notice.
Unit Tests Are Written As You Go\ The holy grail of documentation is to write it as you go, but you rarely see that in practice. However in places I have worked that have adopted Test Driven Development, I have not seen this same problem with unit tests. Unit tests are written as you develop. It becomes part of the development process. In part, once a developer sees the value in unit tests, it is not hard to integrate it into his/her programming style.
The Question of Discipline\ Many who question or are unfamiliar with unit testing assume that it requires a lot of discipline and will be the first thing to fall out during tight deadlines. But this isn’t necessarily true when TDD is adopted as good software practice. It doesn’t require much more discipline than using source control. These days, I hardly realize source control requires any discipline at all because it is so well integrated into my work style. But if you think about it, it really does take discipline to do it properly. Especially if you employ branching. Likewise TDD does take some discipline, but it becomes second nature. And the benefits are worth it. Documentation, though providing some benefits, requires much more discipline to keep in synch.