The document discusses techniques for writing effective unit tests, including: 1. Writing tests that validate externally visible behavior rather than implementation details. 2. Providing clear failure messages that indicate expected and actual values. 3. Organizing tests into classes that describe scenarios and intentions. 4. Using context and theory classes to group related tests and parameters. 5. Prioritizing developer experience and ensuring tests are easy to change.

5. 1

7. 2

8. 3

10. 4

11. void ItemCanBeOrdered()
{
// setup ...
PressSubmitButton();
AssertOrderWasSubmitted();
}

12. void ItemCanBeOrdered()
{
// setup ...
PressSubmitButton();
Sleep(250);
AssertOrderWasSubmitted();
}

13. void ItemCanBeOrdered()
{
// setup ...
PressSubmitButton();
WaitUntil(() => OrderWasSubmitted(),
timeoutAfter: 2.Seconds());
}

16. 5

17. Optimise

18. Reduce

19. Acceptance

20. Be reactive
instead

21. 6

22. class StringList
{
public int Count { get; private set; }
public void Add(string item) { }
public void RemoveAt(int index) { }
}

23. public class InvoiceSender
{
IEmailSender emailSender;
IInvoiceGenerator invoiceGenerator;
public InvoiceSender(
IEmailSender emailSender,
IInvoiceGenerator invoiceGenerator)
{
this.emailSender = emailSender;
this.invoiceGenerator = invoiceGenerator;
}
public void SendInvoice(ICustomer customer)
{
var invoice = invoiceGenerator.GenerateFor(customer);
emailSender.Send(
customer.EmailAddress,
invoice.ToString());
}
}

24. Test
externally
visible
behaviour

25. 7

26. Fail well

27. Expected: True
But was: False

28. Expected: “The Dog & Duck”
But was: “The Dog %26 Duck”

29. Expected: 1
But was: 2
The item should be unique

30. Show intent

31. void TestPasswordValidation()
{
// ...
Assert(...);
// ...
Assert(...);
// ...
Assert(...);
}

32. class PasswordValidation
{
void Less_than_8_chars_long_not_valid() { }
void Number_sequences_not_valid() { }
void Same_as_username_not_valid() { }
...
}

33. class PasswordValidation
{
[TestCase("FqPd19", "Less than 8 chars")]
[TestCase("FqPd1234", "Numeric sequence")]
[TestCase("password", "Easy to guess")]
void Invalid(string pw, string reason) { }
[TestCase("FqPd0581Q")]
void Valid(string pw) { }
}

34. public class TweetControllerTests
{
[Test]
void Returns_tweets_stored_in_database()
{
var stubDatabase = new StubDatabase();
stubDatabase.Add("OMG I love those meerkats! <3");
stubDatabase.Add("Sergei is my faveourite");
var controller = new TweetController(stubDatabase);
var tweets = controller.GetTweets();
Assert.That(tweets.Count, Is.EqualTo(2));
Assert.That(tweets[0], Is.EqualTo(
"OMG I love those meerkats! <3"));
Assert.That(tweets[1], Is.EqualTo(
"Sergei is my faveourite"));
}
}

35. public class TweetControllerTests
{
private StubDatabase stubDatabase;
[SetUp]
public void SetUp()
{
stubDatabase = new StubDatabase();
stubDatabase.Add("OMG I love those meerkats! <3");
stubDatabase.Add("Sergei is my favourite");
}
[Test]
public void Returns_tweets_stored_in_database()
{
...
}
}

36. public class TweetControllerTests : Context
{
[Test]
public void Returns_tweets_stored_in_database()
{
var controller = new TweetController(stubDatabase);
var tweets = controller.GetTweets();
Assert.That(tweets.Count, Is.EqualTo(2));
Assert.That(tweets[0], Is.EqualTo(
"OMG I love those meerkats! <3"));
Assert.That(tweets[1], Is.EqualTo(
"Sergei is my favourite"));
}
}

37. public class RetrievingTweets : Context
{
public RetrievingTweets()
{
Given_tweets_in_the_DB(new[]
{
"I love those meerkats! <3",
"Sergei is my favourite"
});
When_calling_the_controller();
}
[Test]
public void The_stored_tweets_are_returned()
{
Assert.That(retrievedTweets.Count, Is.EqualTo(2));
Assert.That(retrievedTweets[0], Is.EqualTo(
"I love those meerkats! <3"));
Assert.That(retrievedTweets[1], Is.EqualTo(
"Sergei is my favourite"));
}
}

38. TDD by default
Test just enough
Dev experience matters
Change should be easy
Communicate well

39. Programming with GUTs
Kevlin Henney
TDD, where did it all go wrong?
Ian Cooper