Tips for writing great unit tests

  • Make each test orthogonal (i.e., independent) to all the others
    Any given behaviour should be specified in one and only one test. Otherwise if you later change that behaviour, you’ll have to change multiple tests. The corollaries of this rule include:
    Don’t make unnecessary assertions

          Which specific behaviour are you testing? It’s counterproductive to Assert() anything that’s also asserted by another test: it just increases the frequency of pointless failures without improving unit test coverage one bit. This also applies to unnecessary Verify() calls – if it isn’t the core behaviour under test, then stop making observations about it! Sometimes, TDD folks express this by saying “

    have only one logical assertion per test

        Remember, unit tests are a design specification of how a certain behaviour should work, not a list of observations of


      the code happens to do.

    • Test only one code unit at a time
      Your architecture must support testing units (i.e., classes or very small groups of classes) independently, not all chained together. Otherwise, you have lots of overlap between tests, so changes to one unit can cascade outwards and cause failures everywhere.
      If you can’t do this, then your architecture is limiting your work’s quality – consider using Inversion of Control.
    • Mock out all external services and state
      Otherwise, behaviour in those external services overlaps multiple tests, and state data means that different unit tests can influence each other’s outcome.
      You’ve definitely taken a wrong turn if you have to run your tests in a specific order, or if they only work when your database or network connection is active.
      (By the way, sometimes your architecture might mean your code touches static variables during unit tests. Avoid this if you can, but if you can’t, at least make sure each test resets the relevant statics to a known state before it runs.)
    • Avoid unnecessary preconditions
      Avoid having common setup code that runs at the beginning of lots of unrelated tests. Otherwise, it’s unclear what assumptions each test relies on, and indicates that you’re not testing just a single unit.
      An exception: Sometimes I find it useful to have a common setup method shared by a very smallnumber of unit tests (a handful at the most) but only if all those tests require all of those preconditions. This is related to the context-specification unit testing pattern, but still risks getting unmaintainable if you try to reuse the same setup code for a wide range of tests.

    (By the way, I wouldn’t count pushing multiple data points through the same test (e.g., using NUnit’s [TestCase] API) as violating this orthogonality rule. The test runner may display multiple failures if something changes, but it’s still only one test method to maintain, so that’s fine.)

  • Don’t unit-test configuration settings
    By definition, your configuration settings aren’t part of any unit of code (that’s why you extracted the setting out of your unit’s code). Even if you could write a unit test that inspects your configuration, it merely forces you to specify the same configuration in an additional redundant location. Congratulations: it proves that you can copy and paste!Personally I regard the use of things like filters in ASP.NET MVC as being configuration. Filters like [Authorize] or [RequiresSsl] are configuration options baked into the code. By all means write an integration test for the externally-observable behaviour, but it’s meaningless to try unit testing for the filter attribute’s presence in your source code – it just proves that you can copy and paste again. That doesn’t help you to design anything, and it won’t ever detect any defects.
  • Name your unit tests clearly and consistently
    If you’re testing how ProductController’s Purchase action behaves when stock is zero, then maybe have a test fixture class called PurchasingTests with a unit test called ProductPurchaseAction_IfStockIsZero_RendersOutOfStockView(). This name describes the subject(ProductController’s Purchase action), the scenario (stock is zero), and the result (renders “out of stock” view). I don’t know whether there’s an existing name for this naming pattern, though I know others follow it. How about S/S/R?Avoid non-descriptive unit tests names such as Purchase() or OutOfStock(). Maintenance is hard if you don’t know what you’re trying to maintain.



One comment

  1. Re-Searcher... · July 9, 2013

    Reblogged this on Researcher's Blog and commented:
    Tips for writing great unit tests

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