[unit-testing] The value of high level unit tests and mock objects

1 Answers

An aside

Just to touch on your bolded statement:

"I should be testing the behavior of the method, not that it's calling the right sequence of methods"

The behaviour of the object-under-test is the sequence of actions it takes. This is actually "behaviour" testing, whereas when you say "behaviour of the method", I think you mean stateful testing, as in, give it an input and verify the correct output.

I make this distinction because some BDD purists go so far as to argue that it is much more meaningful to test what your class should be calling on, rather than what the inputs and outputs are, because if you know fully how your system is behaving, then your inputs and outputs will be correct.

A response

That aside, I personally never write comprehensive tests for the UI layer. If you are using an MVVM, MVP or MVC pattern for your application, then at a "1-developer team" level, it's mind-numbing and counter-productive for me to do so. I can see the bugs in the UI, and yes, mocking behaviours at this level tends to be brittle. I'm much more concerned with making sure that my underlying domain and DAL layers are performing properly.

What is of value at the top level is an integration test. Got a web app? Instead of asserting that your controller methods are returning an ActionResult (test of little value), write an integration test that requests all the pages in your app and makes sure there are no 404's or 403's. Run it once on every deployment.

Final answer

I always follow the 80/20 rule with unit testing. To get that last 20% coverage at the high level you are talking about, is going to be 80% of your effort. For my personal and most of my work projects, this doesn't pay off.

In short, I agree. I would write integration tests, and ignore unit tests for the code you describe.


I am beginning to believe that unit testing high level, well-written code, which requires extensive use of mock objects, has little to no value. I am wondering if this assertion is correct, or am I missing something?

What do I mean by high level? These are the classes and functions near the top of the food chain. Their input and output tends to be user input, and user interface. Most of their work consists of taking user input and making a series of calls to lower-level entities. They often have little or no meaningful return values.

What do I mean by well-written? In this case, I am referring to code that is decoupled from its dependencies (using interfaces and dependency injection), and line by line is at a consistent level of abstraction. There's no tricky algorithms, and few conditionals.

I hate writing unit tests for this kind of code. The unit tests consist almost entirely of mock object setup. Line by line, the unit test read almost like a mirror image of the implementation. In fact, I write the unit tests by looking at the implementation. "First I assert this mock method is called, then I assert this mock method is called...", etc. I should be testing the behavior of the method, not that it's calling the right sequence of methods. Another thing: I have found that these tests are extremely fragile to refactoring. If a test is so brittle it utterly shatters and must be rewritten when the code under test is refactored, then hasn't one of the major benefits of unit testing been lost?

I don't want this post to be flagged as argumentative, or not a question. So I'll state my question directly: What is the correct way to unit test the kind of code I have described, or is it understood that not everything needs a unit test?

If doing TDD you should not write tests after implementation but rather the other way around. This way you'll also avoid the problem of making a test conform to the written code. You probably do have to test certain method calls within those units, but not their sequence (if it's not imperative to the domain problem - business process).

And sometimes it's perfectly feasible not to write a test for a certain method.