UnitTesting old code

[Luke Redpath]

I've been working on my own class library over the past couple of weeks, but its only in the past few days that I've been getting my head around Unit Testing. I'm happy to start giving it a go, but I have one question: is it worth going back and writing tests for all of the classes I've already written.

If yes, how would you go about doing this (bearing in mind that the idea is you would usually write the tests first).

Cheers.

[McGruff]

Yes - definitely.

Testing will lead you to think more carefully about the classes: I bet you find a few bugs you didn't know about.

Testing can also suggest improvements: changing classes to make them easier to test usually leads to a better design (one of the many wonderful things about testing).

As you develop your library it will always remain in a functional state if you run all tests after each edit and always fix any failing tests (regression testing). For that to work everything will need to be covered by tests, of course.

how would you go about doing this

Just write the tests. Try to write the minimum you need to get an inductive proof. The tests are a constraint on the design: add everything you need to check but don't add anything more than you have to. Over-constraining the design could impede later refactorings.

Once you've got the hang of testing you won't ever want to work any other way again.

[sweatje]

You might just do it as you make additions or need to refactor the existing libraries.

If you can justify it to yourself as a learning excesize, then I would say wrap some of your existing working code in tests, but if you find it to be feeling like drudge work, and discouraging you from writing test, move on to using tests in your new development work where you can realize much more tangable immediate benefits.