Can unit testing be successfully added into an existing production project? If so, how and is it worth it?

Question

I\'m strongly considering adding unit testing to an existing project that is in production. It was started 18 months ago before I could really see any benefit of TDD

Answer 1

It's absolutely worth it. Our app has complex cross-validation rules, and we recently had to make significant changes to the business rules. We ended up with conflicts that prevented the user from saving. I realized it would take forever to sort it out in the applcation (it takes several minutes just to get to the point where the problems were). I'd wanted to introduce automated unit tests and had the framework installed, but I hadn't done anything beyond a couple of dummy tests to make sure things were working. With the new business rules in hand, I started writing tests. The tests quickly identified the conditions that caused the conflicts, and we were able to get the rules clarified.

If you write tests that cover the functionality you're adding or modifying, you'll get an immediate benefit. If you wait for a re-write, you may never have automated tests.

You shouldn't spend a lot of time writing tests for existing things that already work. Most of the time, you don't have a specification for the existing code, so the main thing you're testing is your reverse-engineering ability. On the other hand, if you're going to modify something, you need to cover that functionality with tests so you'll know you made the changes correctly. And of course, for new functionality, write tests that fail, then implement the missing functionality.