I was asked to write a post about my views on unit testing because it is a hot subject at the moment.
I’m very ambivalent about unit testing and always have been for many different reasons. Although testing is very important, I often find unit testing to be a time consuming liability but it depends on the project.
Good for static classes
Over the years I’ve build many helper class libraries which is used in many projects and can be considered as business independent fundamental classes. All the classes in those libraries are static and therefore cannot be considered to be entities, but merely a logical placeholder for static methods that works independently.
To unit test such libraries is a must have. They are simple to write and maintain and unit testing is important because the libraries are used in a vast amount of different projects.
Object orientation and business entities
A unit is the smallest testable part of an application which means that for object orientated applications/libraries it will always be a class or business entity. Of course you can test the individual methods, functions and constructors but it doesn’t make much sense since the business entity is to be considered as a single unit and also works as such in real scenarios.
For me it makes more sense to conduct use-case testing on business entities. It works much like unit testing, but is constructed so they test a complete flow of operations on the class. For instance, if I had a class called Dog, then I would write a use-case test that simulated how a real person would use the class. First Feed the Dog, then Pet it and then Walk it before it needs to Sleep.
For that purpose it doesn’t really matter if you use unit testing frameworks such as xUnit or write it in a console application in the same Visual Studio solution. I like both approaches equally, but most often write a console application because I find it simpler.
Financial applications used by banks or government applications needs to be tested extremely well. Those types of applications deal with people’s personal data and money and must never ever fail. The same goes for applications that ships on CD's or DVD's unless they are updated automatically over the Internet.
There needs to be automatic tests running every day and both unit testing and use-case testing is very important. The 80/20 rule does not apply here; it needs to be as close to 100% code coverage as possible and each class needs to be use-case tested for dozens of scenarios.
To test this properly you need professional testers and QA’s because the test is as important as the application itself.
What about no test
The most common thing is to have no test but the application itself. Admit it; you do this all the time as well. Is it a dumb idea to use the application to test its dependent libraries and itself and not doing unit or use-case testing? No it definitely is not. The end user application/website is the ultimate test, but there is a serious problem with not having an automated test harness. If an error occurs you can spend more time finding the problem than it takes to fix it. For smaller projects though, I find it perfectly acceptable to test from the GUI without a test harness.
Also, it is very difficult to test GUI’s automatically so it will be a good idea in most cases to use a test harness for every project in the application stack and then create a procedure for testing the actual GUI. In most cases it has to be done by humans manually.
Test driven development
TDD is one of the hyped methodologies that I never really understood. I can’t get my head around writing the test before the class it tests. In C# it doesn’t make sense to me, because if I write a test to call a non-existing class and its methods and properties, then it will not compile. Then when I write the actual class I often compile to check for errors. That doesn’t work either, because the test references some methods I haven’t written yet and as so cannot compile.
Besides, in most cases I don’t know 100% what properties and methods the class needs and which of them is public or private. Then I need to get back and change the test and thereby looses the point of writing the test before the class.
User driven development
For smaller projects with few dependencies user driven development is for me the best way to test. Consider a rather small project like BlogEngine.NET with one web project and one class library. The class library is only used by the web project and nowhere else. That means that the class library has no classes, methods or properties that is not used by the web project so by testing the web project, the class library get’s 100% code coverage.
If a new feature needs to be implemented in both the web project and class library I use the web project to test the class library. That’s because I’m as much a user as a developer of the project. If you are not the end user, then you should test as much as you can on the web project and then get a co-worker or the customer to test when you feel confident it works. It’s always a good idea to let the customer test as early in the process as possible.
Simple code is simple to test
If you always make sure your code is simple, clean and refactored, then testing is equally simple to write. Complex code needs complex testing and the maintenance of the tests will increase with every change to the code. Every time you change a little thing, you need to rewrite many tests if your code is complex. That goes for both unit testing and use-case testing.
I find unit testing to be good for very few types of projects – helper class libraries and never-fail-applications. User driven development with use-case testing is my absolute favourite. I don’t see the big difference in using a xUnit framework or using a console application, because you only get the benefit from a test project when something fails. For automatic testing though, you need a testing framework that logs the results.
This is how I would manually test different projects.
- Helper class libraries – unit testing in xUnit or console by the 80/20 rule
- Never-fail-applications – automatic unit and use-case testing written by QA’s. Full coverage
- GUI’s – test yourself, then a co-worker and then let the customer test
- Business object layer – use-case testing in xUnit or console by the 80/20 rule
- Data access layer – unit test in xUnit by the 80/20 rule
Again, it is very important to note that it all depends on the type and size of the project. If you don’t have a build- or versioning server to handle automatic unit testing, then you have to do it manually. If you have the ability to automate unit testing, a xUnit framework is the way to go but full code coverage is not always necessary and not all projects need unit testing.