A colleague of mine sent me an article on ArsTechnica that was a short discussion on whether it is a good idea to write tests for legacy code. This was made up from a collection of posts on Stack Exchange. The original question was as follows:
“Suppose one had a relatively large program (say 900k SLOC in C#), all commented/documented thoroughly, well organized and working well. The entire code base was written by a single senior developer who no longer with the company. All the code is testable as is and IoC is used throughout—except for some strange reason they did not write any unit tests. Now, your company wants to branch the code and wants unit tests added to detect when changes break the core functionality.
Is adding tests a good idea? If so, how would one even start on something like this? “
Original question posted by Paul over at Stack Exchange
It’s a good question, so I thought I would write down my thoughts on it. I am a firm believer in Test Driven Development (TDD) and this is much easier when you are working on a nice new green field project (writing a new system). Unfortunately we don’t always have the luxury of working on new systems and we have to maintain older legacy systems, or brown field applications. If you have a large brown field system, I personally do not think there is much value in getting your team of developers to sit there and wrap the whole system in tests. Whilst it may feel nice to know the application is covered in tests, the level of effort and expense in doing so is likely to be very high indeed. If the system has been around for a long time, then it is most probably working fine and your users are happy with it (this isn’t always the case, but is more often the case).
What I do feel is valuable though is adding some tests when you need to change/extend part of the code base. As the system already exists and you can’t reliable determine that the code is doing what it should be doing you can write what Michael Feathers describes in his book “Working Effectively with Legacy Code” calls characterization tests. A characterization test is:
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