Sunday, May 23, 2010

Why it is faster to developer with unit tests

[This was originally posted at]

I keep hinting at this with various blog posts over the years, so I wanted to just come out and openly defend it.

It is faster for the average developer to develop non-dependent C# code with unit tests than without.

By non-dependent, I mean code that doesn't have external dependencies, like to the database, UI controls, FTP servers, and the like. These types of UI/Functional/Integration tests are tricky and expensive, and I fully emphasize why projects may punt on them. But code like algorithms, validation, and data manipulation can often be refactored to in-memory C# methods.

Let's walk through a practical example. Say you have an aspx page that collects user input, loads a bunch of data, and eventually manipulates that data with some C# method (like getting the top N items from an array):

    public static T[] SelectTopN(T[] aObj, int intTop)
      if (aObj == null)
        return null;
      if (aObj.Length <= intTop || aObj.Length == 0 || intTop <= 0)
        return aObj;

      //do real work:
      T[] aNew = new T[intTop];
      for (int i = 0; i < intTop; i++)
        aNew[i] = aObj[i];

      return aNew;

This is the kind of low-hanging fruit, obvious method that should absolutely be tested. Yet many devs don't unit test it. Yes it looks simple, but there's actually a lot of real code that can easily be refactored to this type of testable method (and it's usually this type of method that has some sort of "silly" error). There's a lot that could go wrong: null inputs, boundary cases for the length of the array, bad indexes on an array, mapping values to the new array. Sure, real code would be more complicated, which just reinforces the need for unit testing even more so.

Here's the thing - the first time the average developer writes a new method like that, they will miss something. Somehow, the dev needs to test it.

So how does the average programmer test it? By setting up the database, starting the app, and navigating 5 levels deep. Oops, missed the third-null; try again. That's 3 minute wasted. Oops, had an off-by-one in the loop; try again. 6 minutes wasted. Finally, set everything back up, testing the feature, score! 15 minutes later, the dev has verified a positive flow works. The dev is busy and under pressure, got that task done, so they move on. 4 weeks later (after the dev has forgotten everything), QA comes and says "somewhere there's bug", and the dev spends an hour tracking it down, and it was because the dev didn't handle when the array has a length less than the "Select Top N", and the method throws an out-of-range exception. Then the dev makes the fix, hopes they didn't break anything else, and waits a day (?) for that change to be deployed to QA so a test engineer can verify it. Have mercy if that algorithm was a 200-line spaghetti code mess ("there wasn't time to code it right"), and it's like a rubix cube where every change fixes one side only to break another. Have even more mercy if the error is caught in production - not QA.

Unit test is faster because it stubs out context. Instead of take 60 seconds (or 5 minutes, or an hour of hunting a month later!) to set up data and stepping through the app, you just jump straight to it. Because unit tests are so cheap, the dev can quickly try all the boundary conditions (outside of the few positive flows that the application normally runs when the dev is testing their code). This means that QA and Prod often don't find a new boundary condition that the dev just "didn't have time" to check for. Because unit tests are run continually throughout the day, the CI build instantly detects when someone else's change breaks the code.

Especially with MSTest or NUnit, unit test tools are effectively free. Setting up the test harness project takes 60 seconds. Even if you start with only 5% code coverage for just the public static utility methods - it's still 5% better than nothing.

Of course, the "trick" is to write your code such that more and more of it can be unit tested. That's why dependency injection, mocking, or even refactoring to helper public-static helper utilities is so helpful.

Over the last 5 years, I've heard a lot of objections to avoid testing even simple C# methods, but I don't think they save time:

Objection against unit testing being fasterRebuttal
You're writing more code, so it's actually slowerIt's not typing that takes the time, but thinking.
I can test it faster by just running the appWhat does "it" really mean? You're not testing the whole app, just a handful of common scenarios (and you're only running the app occasionally on your machine, as opposed to unit tests than run every day on a verified build server)
"Unit testing" is one more burden for developers to learn, which slows us downUnit tests are ultimately just a class library in the language of  your choice. They're not a new tool or a new language. The only "learning curve" is that conceptually it requires you write code that can be instantiated and run in a test method - i.e. you need to dogfood your own code, which a dev should be prepared to do anyway.
My code is already perfect, there are no bugs, so there is not need to write such unit tests.Ok, so you know your code is perfect (for the sake of argument) - but how will you prove that to others? And how will you "protect" your perfect code from changes caused by other developers? If a dev is smart enough to write perfect code the first time, then the extra time needed to write the unit test will be trivial.
If I write unit tests, when I change my code, then I need to go update all my tests.Having that safety net of tests for when you do change your code is one of the benefits of unit tests. Play out the scene - you change your code, 7 tests fail - that's highlighted what would likely break in production. Better to find out about breaking changes from your unit tests rather than from angry customers.
Devs will just write bad tests that don't actually add value, so it's a waste of time.Any tool can be abused. Use common sense criteria to help devs write good tests - like code coverage and checking boundary cases.
I just don't have timeFor non-dependent C# code, you don't have time not too. Here's the thing - the code has got to be tested anyway. What is your faster alternative to prove that the code indeed works, and that it wasn't broken by someone else after you moved on?


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