I've heard people say TDD automatically creates good designs. More recently, I've heard David Hansson say it creates design damage. Who's right?
Neither. TDD doesn't create design. You do.
TDD Can Lead to Better Design
There are a few ways I've seen TDD lead to better design:
A good test suite allows you to refactor, which allows you to improve your design over time.
The TDD cycle is very detail-oriented and requires you to make some design decisions when writing tests, rather than when writing production code. I find this helps me think about design issues more deeply.
TDD makes some design problems more obvious.
None of these force you to create better design, but if you're working hard to create good design, I find that these things make it easier to get there.
TDD Can Lead to Worse Design
There are a few ways I've seen TDD lead to worse design:
TDD works best with fast tests and rapid feedback. In search of speed, some people use mocks in a way that locks their production code in place. Ironically, this makes refactoring very difficult, which prevents designs from being improved.
Also in search of speed, some people make very elaborate dependency-injection tools and structures, as well as unnecessary interfaces or classes, just so they can mock out dependencies for testing. This leads to overly complex, hard to understand code.
TDD activates people's desire to get a "high score" by having a lot of tests. This can push people to write worthless or silly tests, or use multiple tests where a single test would do.
None of these are required by TDD, but they're still common. The first two are obvious solutions to the sorts of design problems TDD exposes.
They're also very poor solutions, and you can (and should) choose not to do these things. It is possible to create fast tests and rapid feedback without these mistakes, and you can see us take that approach in the screencast.
So Do Your Job
TDD doesn't create good design. You do. TDD can help expose design smells. You have to pay attention and fix them. TDD can push you toward facile solutions. You have to be careful not to make your design worse just to make testing better.
So pay attention. Think about design. And if TDD is pushing you in a direction that makes your code worse, stop. Take a walk. Talk to a colleague. And look for a better way.