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Using unit tests to unlock quality (Pt I)

When I started working as a developer my mentor taught me to write unit tests with each changeset, so I did. After switching team, my new lead & mentor had us doing the same and I learnt new techniques to write more complex unit tests. When a couple of newer members joined the team, getting unit tests written was something I pushed hard. After all, it was good practice that all good software engineers do.

One of my strengths, or so I thought, was writing unit tests for any and every method. No matter how ugly to code that the test was for was, as a (very small) team we had great coverage… even if it became a running joke that maintaining the tests was often most of a user story.

In hindsight I realise that I was wrong on two accounts.

Not all developers are writing unit tests anywhere to the level that I thought.

It surprised me when we kept having regressions in sections of code. I asked why unit tests weren’t catching them. The simple answer was the code was too hard to unit test.

Now in the developer’s defence here, this is a very old code base that they were building upon and there was no existing coverage but I want to talk about the idea what code can be too hard or “not possible”.

One of the most common challenges that I’ve seen is with calling APIs (Windows / first or 3rd party) or where your method relies on an external entity. Some examples might be using DirectX, accessing the file system or calling an API for a third party system.

The solution is, in theory, pretty simple. Mocking. Rather than calling DirectX directly, have a wrapper and call that. Keep your logic separate from the API calls and you can test it. This is good for developing maintainable code as well as good for your testing. There may be the odd exception where your wrapper might complicate things too much, but that should be a rarity not a norm.

The other reason for not using testing is where timing issue make the tests flakey. Now this is a good reason to not automate something as I believe that a flakey test is worse than no test. However again in most cases I have found the mocking is again the solution. In projects where I’ve been a developer we always have wrappers for our timers so that if we want to test the behaviour in response a timer elapsing, we just invoke the timer.

I’ve found dependency injection to be really useful in making my code testable. We’ve also used reflection as well where you can insert your mock into a created object. You can also set certain properties so that if you’ve got a private member for “isAlive” then you can test “personUnderTest.PokeWith(stick)” with different values for “isAlive”, without having to include steps like “personUnderTest.ThrowOffBridge()” in your setup (meaning changes to ThrowOffBridge can affect PokeWith).

Another thing that I’ve found a little unsettling is “it’s all pushed, I’ve just some unit tests to write.”

No, no no.

There’s a few big issues here:

  • It assumes that your code would pass unit testing before trying.
  • It assumes that your code is testable.
  • If either of those are not true then you will have to re-write the functional code, dev test it again then get it through review again.
  • It can lead you to write unit tests to pass, rather than to test.

My other learning is how bad my tests and code were.

Some of the methods that we wrote were massive and complicated. This meant that in order to unit test one part of the code, I needed to mock and setup absolutely loads of other code. The worst part was making changes. Because we decided that one small part of the business logic needed changing, I was fixing up dozens of unit tests. It was nasty.

I really have learnt the value in keeping things small and ensuring that your methods are serving one function, not “go do everything”.
Some words on how keeping things small is better.

The other major mistake that I made was being what I thought was clever in creating tests that I could set a bunch of inputs on different parameters then the expected output. For example changing how some of my mocks would be setup based on logic in my unit test. Only needing one unit test to cover a bunch of different business logic is genius right?

No. No it is not. It meant that I had tests that were very hard to debug when they failed. It also made it really awkward when we made a tweak or extension to the behaviour.

Lesson learnt: Keep your code and tests simple!

In my next post I will explore more on the technique(s) that I’ve been using to improve my unit tests.

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