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GIVEN my article WHEN a reader reads it THEN the reader writes better tests

jamoyjamie profile image Jamie Read ใƒป4 min read

GIVEN a developer who hates writing tests

I'm not one for testing, that's evidenced by my most used side project having around 5% test coverage. I just can't bring myself to slog through and write every test case of the code I'm writing particularly when I don't have much time to work on side projects anymore.

WHEN placed in a software house that likes testing

However, in my current employment I'm expected to have 80% code coverage as part of the minimum to calling something 'stable' - hence I actually spend most of my working time writing unit, integration, and system tests.

AND he must write lots of tests in his job

Testing is something my company are very good at. We have hundreds of thousands of lines of code spread across hundreds of different repositories across tens of different projects and on any given day I could be working in any part of it.

The fact of the matter is that the tests save us countless headaches and make the insane amount of context switching significantly easier for us. Because of the high test coverage I know that I can hack and slash at code, and even perform massive refactors if I do so desire, and at the end of my rampage I can run the tests and know within minutes if there has been any changes to the code's behaviour.

I just don't need to worry about breaking code while I edit it.

THEN he learns some neat tricks in testing

This is the point where I tell you about the wonderous world of behaviour driven tests.

We have a convention to write every test with the GIVEN-WHEN-THEN structure. Here's an example from my side project:

public void TestReadStringDeserializesTheDataCorrectly()
    // GIVEN a buffer of serialized data
    mockMessageBuffer.Setup(m => m.Buffer).Returns(new byte[] { 0, 0, 0, 6, 65, 0, 66, 0, 67, 0 });
    mockMessageBuffer.Setup(m => m.Offset).Returns(0);
    mockMessageBuffer.Setup(m => m.Count).Returns(10);

    // WHEN I read a string from the reader
    string result = reader.ReadString();

    // THEN the value is as expected
    Assert.AreEqual("ABC", result);

I see this test and immediately know where to look for everything I need:

  • In the GIVEN section we setup any mocks we need, load in any test data from disk if we require it, and do any preparation we have to.

  • In the WHEN section we run the code being tested (and as best as we possibly can, only the code being tested) and obtain a result.

  • In the THEN section we assert that the behaviour of the code was as we expect.

Having this simple structure makes understanding what a test does trivial. I can look through hundreds of tests in development and code review and instantly grasp what a test is doing in a matter of seconds because every test in our company has this same, behaviour driven format.

We also find this style creeps out into other parts of our tests as well:

public void Initialize()
    // GIVEN the object cache is disabled

    // AND a DarkRiftReader under test
    reader = DarkRiftReader.Create(mockMessageBuffer.Object);

You can start to see how we build more complex tests as well here. It's encouraged that if you do multiple things in any section that you document them with an AND section:

public void TestExtraLargeMemoryBlocksArePooledCorrectly()
    // GIVEN a memory pool with no previously pooled memory

    // WHEN I return an extra large memory block
    byte[] oldBlock = new byte[5000];

    // AND I request a new extra large memory block
    byte[] newBlock = memoryPool.GetInstance(4000);

    // THEN my memory block is the same
    Assert.AreSame(oldBlock, newBlock);

(I take a shortcut here, I should probably assign oldBlock in a GIVEN, but then this is really just as understandable)

AND can apply them anywhere

It also helps when writing tests!

Thinking about behaviour is a lot easier in English - that's why things like pseudo code and rubber duck debugging exist - so why not write tests in your native language and then use that as a template?

public void TestGetInstanceUsesPoolWhenHasInstances()
    // GIVEN a pool with a single element

    // WHEN I get an instance from the pool

    // THEN a new instance is not generated

    // AND the returned instance is the pooled object
public void TestGetInstanceUsesPoolWhenHasInstances()
    // GIVEN a pool with a single element
    object pooledObject = new object();

    // WHEN I get an instance from the pool
    object result = objectPool.GetInstance();

    // THEN a new instance is not generated

    // AND the returned instance is the pooled object
    Assert.AreSame(pooledObject, result);

The exact order of the GIVEN-WHEN-THEN also doesn't matter, we often find that in the most complex of tests we need to assert behaviour in multiple places (THEN), or perform some step on the class under test (WHEN) before we can continue the setup (GIVEN):

Scenario: I can disconnect from a server
    Given I have a running server
    And 1 client connected
    Then all clients should be connected
    And the server should have 1 client
    When I disconnect client 0
    Then 0 clients should be connected
    And 1 client should be disconnected
    And the server should have 0 clients
    And there should be no recycling warnings


You have no idea what my side project does, I never mentioned it, but you were hopefully still able to understand what these tests did, despite them covering some of the most complex and wacky optimisations I've had to add to it!

Adopting this behaviour, for me, makes testing significantly less cumbersome. For the developers I work with, it makes understanding my tests faster and easier. And in the merge review we can review the test behaviour using the comments and then check the code against the comments to make reviews of large quantities of tests a little more manageable.

I hope this article has made you think, and maybe you'll write a quick GIVEN-WHEN-THEN before you write your next test ๐Ÿ™‚

Discussion (7)

Editor guide
stealthmusic profile image
Jan Wedel

I like the idea of given/when/then but I donโ€™t like to use it in Java JUnit e.g. because itโ€™s not support as semantic or syntactic part of the test framework. Iโ€™m really opposed to use comments especially together with text explaining what the next lines do.

Some JavaScript test frameworks support it and especially languages that support meta programming.
I want the description to be part of the tests. Otherwise they will be obsolete very quickly...

jamoyjamie profile image
Jamie Read Author

This, like many things requires buy in from your team; otherwise, as you say, the comments will be obsolete quickly.

You need to be able to trust that your team will check for things like this in code review so that you can avoid the exact situation you described. If there isn't that trust then this will probably become more of a distraction and hinderance.

It's worth saying though, since the tests test behaviour, the comments shouldn't change that frequently otherwise your system behaviour is changing which will probably then be breaking backwards compatibility. Sure the exact lines of code may change with a refactor etc. but the behaviour described in the comments should ordinarily be static, and if not then obvious how they should be changing buy the commit message etc.

jbristow profile image
Jon Bristow

Welcome to BDD style testing!

vip3rousmango profile image
Al Romano

This is great! I'll have to try this out with our team and see it in action.

jamoyjamie profile image
Jamie Read Author

Do let me know how well it works for you!

rafalpienkowski profile image
Rafal Pienkowski

If you didn't use BDD before you should take a look at the SpecFlow Framework. Have fun.

jamoyjamie profile image
Jamie Read Author

That's exactly the framework the last test is written in!

Writing SpecFlow tests like that one was one of the first steps I took in adding tests for this side project. It took a little while to get the framework up and running but I knew that I actually could enjoy writing these forms of tests so it gave me that initial boost moving forward.

I still need to port the tests over to the .NET Core version. Unfortunately I can't get it working on CI until then!