Is it a good idea to write tests for legacy code?

The code works perfectly, so why mess it up?

Stack Exchange – Nov 16, 2013 9:15 am | 44

Credit: Stack Exchange

Don’t unit test just to unit test

Karl Bielefeldt answers (14 votes):

Not all unit tests have equal benefit. The benefit of a unit test comes when it fails. The less likely it is to fail, the less beneficial it is. New or recently changed code is more likely to contain bugs than rarely changed code that is well tested in production. Therefore, unit tests on new or recently changed code are more likely to be more beneficial.

Not all unit tests have equal cost, either. It’s much easier to unit test trivial code you designed yourself today than complex code someone else designed a long time ago. Also, testing during development usually saves development time. On legacy code that cost savings is no longer available.

In an ideal world, you’d have all the time you need to unit test legacy code, but in the real world, at some point it stands to reason that the costs of adding unit tests to legacy code will outweigh the benefits. The trick is to identify that point. Your version control can help by showing you the most recently changed and most frequently changed code, and you can start by putting those under unit test. Also, when you make changes going forward, put those changes and closely related code under unit test.

Following that method, eventually you will have pretty good coverage in the most beneficial areas. If you instead spend months getting unit tests in place before resuming revenue-generating activities, that might be a desirable software maintenance decision, but it’s a lousy business decision.

Touch not the legacy

gbjbaanb answers (2 votes):

OK, I’m going to give the contrary opinion….

Adding tests to an existing, working system is going to alter that system, unless the system is all written with mocking in mind from the start. I doubt it, though it’s quite possible it has good separation of all components with easily definable boundaries that you can slip your mock interfaces into. But if it isn’t, then you are going to have to make what are quite significant (relatively speaking) changes that may well break things. In the best case, you’re going to spend a load of time writing these tests, time that could be better spent writing a load of detailed design documents, impact analysis documents, or solution configuration documents instead. After all, that’s the work your boss wants done more than unit tests. Isn’t it?

Anyway, I would not add any unit tests whatsoever.

I would concentrate on external, automated testing tools that will give you a reasonable coverage without changing a thing. Then, when you come to make modifications… that’s when you can start adding unit tests inside the codebase.

Little by little, test

Telastyn answers (7 votes):

Is adding tests a good idea?

Absolutely, though I find it a little hard to believe that the code is clean and working well and using modern techniques and simply has no unit tests. Are you sure they’re not sitting in a separate solution?

Anyways, if you’re going to extend/maintain the code then true unit tests are invaluable to that process.

If so, how would one even start on something like this?

One step at a time. If you’re unfamiliar with unit testing, then learn a bit. Once you’re comfortable with the concepts, pick one little section of the code and write tests for it. Then the next, and the next. Code coverage can help you find spots you’ve missed.

It’s probably best to pick dangerous/risky/vital things to test first, but you might be more effective testing something straight-forward to get into a groove first—especially if you/the team isn’t used to the codebase and/or unit testing.

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