The Cargo Cult of Test Driven Development

Test Driven Development(TDD) is an agile practice that has been slowly gaining traction. In the last year I have encountered quite a few podcasts and blog posts that touted TDD as a better way to develop high quality software. I try to stay on top of trends in software development, and I really did attempt to use TDD, but in the end gave up as it just didn’t mesh with my own personal style.

I know many folks will say that you really need to practice TDD for 3 months to a year to start reaping the benefits, as was mentioned in this interesting discussion on the server side. That to me is a cop out. To me it is kind of like saying caviar is an acquired taste, when truthfully it just tastes like garbage.

For me, there is a big disconnect between everything you read on other blogs and podcasts about TDD and what you experience when actually trying to apply TDD in real life. I think it has a lot to do with the fact that proponents of TDD compare it to writing software without testing at all, as opposed to comparing it to a more structured approach that uses tests when they make sense.

After thinking through this and other issues that I touch in this blog post, I have come to the conclusion that TDD is in fact a cargo cult.

According to Wikipedia, Cargo Cult Programming refers to over-applying a design principle blindly without understanding the reasons behind that design principle in the first place. An example would be a novice being taught that commenting code is good, and then adding comments for lines that are self-explanatory or need no comment.

If you replaced “comments” in that last example with “unit tests”, you can begin to see how test driven development is a cargo cult.

Before we get in too deep, let me state empathetically that I am not against unit testing. They are an essential tool in a developer’s tool box. But when testing becomes dogma, that’s when I start to have problems.

Test First? How about Design First?

My biggest problem with TDD is that if feels like “just in time design.” Write a test to fulfill a single use case and design your api to satisfy one part of a use case. When the next use case has different requirements of your objects, modify your api and refactor. Repeat 1,000 times until you have high quality software.

I find refactoring in this fashion incredibly draining. To me, it is a lot of work, with very little pay off. I know there are plenty of tools that make refactoring easier, but as the complexity of your project grows, there is still a lot of pain in refactoring your code base. Don’t get me wrong, refactoring is a necessary part of any development process, especially when it will improve the maintainability of your codebase. I just don’t find it a terribly exciting task.

The other big problem I have with this approach is that when developing using TDD, you need to repeatedly switch contexts between coding and designing. As Joel Spolsky pointed out, context switching is considered harmful.  I find that my frame of mind when designing is different than when I am coding. Having to constantly switch back and forth between designing and coding hampers my ability to get into the “zone:” that awesome place where productivity increases exponentially.

I find that if I spent time upfront really thinking about what the interfaces should look like and how they interact and behave before writing a line of code, the entire process flows better. At the very least, hashing out a class diagram and really trying to understand the responsibilities and collaborators (via CRC cards) of the objects you are building really provides value. Spending that kind of time up front also helps you see the kind of patterns that might bring additional strategic value to your design. When you are doing pure TDD, you are more in a tactical mind set, focused on solving your use cases and getting your tests to pass.

Everything Doesn’t Need to Have its Own Unit Test

I will admit that if you are looking to attain 100% code coverage, TDD is probably the way to go. But what does 100% code coverage actually buy you?

It doesn’t actually prove that your software is bug free. Testing every line of your code and every execution pathway are two entirely separate things. Test Quality is infinitely more useful than Test Quantity.

Also no amount of unit testing will ever replace user acceptance testing. Just because you wrote your code to spec, doesn’t mean that the spec didn’t make any faulty assumptions.

Another fact of life is that not all dependencies can be tested. Maybe you don’t know or can’t find out all the ways in which a dependency will behave. Maybe your class has a dependency on an object that can not be instantiated outside the environment it lives. SharePoint and other Web API’s come to mind. And while I am aware of Mock object frameworks, there gets to be a point where the effort required to mock an object is not worth the value it will bring from both an efficiency and quality point of view.

So why aim for 100% code coverage? I have read plenty of blog posts that say the difference between 99% code coverage and 100% coverage is immense. Some people will even test simple property getters and setters. If this isn’t overkill, I don’t know what is. To me this epitomizes cargo cult thinking.

When I develop software, I write plenty of unit tests. I don’t write them first, but I also don’t write them last. I write them as I write code complex enough that it warrants one. I also don’t just sit back to run unit tests and watch them pass or fail. I actively set breakpoints and debug them. I don’t always trust that a test that passes a test is actually functioning properly unless I step through it line by line. There are plenty of tools that integrate with your IDE to make this easy.

Debugging is something that I actually enjoy. Stepping through code, using locals and watch windows make developing fun. Why let the testing framework have all the fun.

No Silver Bullets for Software Quality

Writing Software is Hard and I am by no means an expert in anything, let alone TDD. If you find that TDD makes it easier for you to write software, by all means adopt it.

But I am fairly convinced that it is not a silver bullet that can be used on all projects and all situations. As it is with everything, the real skill is in identifying where and when it makes sense to use TDD and not trying to fit a round peg into a square hole.

If you agree or disagree with anything I said, leave a comment or meet me July 1st 2008 at the Fairfield/Westchester Dot Net user group meeting where I am participating in a panel discussion on testing.

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