Last year I wrote the same test in 24 different ways as a little advent calendar for you to enjoy. This year I'm going to do something similar but also a little bit different. I'm going to start with a simple piece of code and then show how this code can be tested for thread safety in different ways. I think it's pretty common to think that thread safety is one of those things that are really hard to test and something you typically handle by good practices and code reviews. Very similar to logging in an application. Most people don't TDD their logs. But why not? Personally I've experienced situations where fixing a bug means making something thread safe or adding a better log message. When I do this I want a test to reproduce the bug and verify the fix. These tests are typically hard to write since the code was not written to be easily tested in regards to logging or thread safety. But it doesn't have to be that way. For example there is a testing framework that basically works with verifying logs (TextTest) and thread safety can absolutely be designed using TDD. Remember; if you think it is hard, you're probably doing something wrong. Let's decide after Christmas if test driving thread safety is better than the standard way of just adding it where you know it is needed.