Code Coverage Alone Probably Won’t Ensure Your Code is Fully Tested.

For this week’s CS-443 self-directed professional development blog entry I read a blog post written by Mark Seemann, a professional programmer/ software architect from Copenhagen Denmark. The blog post I read is entitled “Code coverage is a useless target measure,” and I found it to be quite relevant to the material we’ve been discussing in class the past couple of weeks, especially regarding path testing and data-flow testing. In this blog post, Seemann urges project managers and test developers not to just set a “code coverage goal” as a means for measuring whether their code is completely tested or not. Seemann explains that he finds this to be a sort of “perverse incentive” as it could encourage developers to write bad unit tests for the simple purpose of just covering the code as the project’s deadline approaches and the pressure on them increases. He provides some examples of what this scenario might look like using the C# programming language.

In his examples, Seemann shows that it is pretty easy to achieve 100% code coverage for a specific class, however, that doesn’t mean the code in that class is sufficiently tested for correct functionality. In his first example test, Seemann shows that it is possible to write an essentially useless test by using a try/catch block and no assertions existing solely for the purpose of covering code. Next, he gives an example of a test with an assertion that might seem like a legitimate test but Seemann shows that “[the test] doesn’t prevent regressions, or [prove] that the System Under Test works as intended.” Finally, Seemann gives a test in which he uses multiple boundary values and explains that even though it is a much better test, it hasn’t increased code coverage over the previous two tests. Hence, Seemann concludes that in order to show that software is working as it is supposed to, you need to do more than just make sure all the code is covered by unit tests, you need to write multiple tests for certain portions of code and ensure correct outputs are generated when given boundary-value inputs as well.

The reason I chose Mark’s blog post for my entry this week was because I thought it related to the material we’ve been discussing recently in class, especially data-flow testing. I think it is important for us to remember, when we are using code based testing techniques, that writing unit tests to simply cover the code are not sufficient to ensuring software is completely functional. Therefore, it’s probably a good idea to use a combination of code and specification based techniques when writing unit tests.

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