Setup
Here's a brief feature description: When a user story is saved, if it's new then it's assigned the latest sequence number + 1.
How might this be developed:
- Using Test-Driven Development (TDD)
- ...in a web application
- ...that calls a service?
Puzzle 1: The Controller
In truth, this article isn't specific to ASP.NET Core. But it's what I was working on at the time, and I found the answer via a Core-specific article.
To illustrate where we can have mental hiccups, let's start with the controller and work backwards.
[HttpPost]
public IActionResult SaveStory(UserStory model)
{
if (!ModelState.IsValid)
{
return View(model);
}
//Save
//The service takes care of setting the Sequence property on new models.
model = _storyService.SaveStory(model);
return RedirectToAction(actionName: nameof(Index));
}
This is reasonable code. The controller passes the model to the service, and the service implements the business rule ofincrementing the Sequence property.
Take a minute and ask yourself: What would your controller's unit test...test?
Done? Now ask yourself: If I hadn't written any code yet, what would I test for?
Maybe your first answer started off something like this in your imagination.
[Fact]
public void SaveStory_increments_UserStory_Sequence_by_one()
{
//arrange
var service = MockUserStoryService();
service.MaxSequence = 10;
UserStory userStory = new UserStory(service)
{
// set some fields
};
var controller = new HomeController();
//act
var result = controller.SaveStory(model);
//assert
result.Should().Be()...uh...um....
}
And there's the trap. What I, and I'm sure others, find hard about unit testing and TDD is being clear on the dependencies of what's being testing.
In TDD, ask yourself, "What is this unit going to do or change by itself?"
My first thought would be, "Well, the Sequence is going to change. That's the feature, after all." But that isn't what the controller is doing.
Assuming no errors, the only thing the controller does is pass the model to the service's SaveStory method.
The service is a dependency, and we don't test a dependency's behavior. Let me call that one out, because it's crucial.
In unit testing, don't test a dependency's behavior.
You always control the dependency's state, and always return a value you've determined. What you test is what the unit is supposed to do with that value. This is why we mock dependencies.
OK, what's the unit test for the controller? I admit, I was puzzled until I read Steve Smith's article, Test controller logic in ASP.NET Core | Microsoft Docs.
I should ensure that the service's SaveStory method was called. I don't need to test that something was saved, only that it should be. He's using Moq's Validate feature for this. I can implement a similar feature in a self-created mock.
// https://stackoverflow.com/a/11296961/1628707
// This is one of those cases where it's simpler to inherit Collection<T>
// and add a couple of needed methods.
public class CalledMethods : Collection<CalledMethod>
{
public CalledMethod this[string name]
{
get { return this.SingleOrDefault(a => a.Name == name); }
}
private CalledMethod AddAndReturn(string name)
{
if (this[name] == null) Add(new CalledMethod(name,0));
return this[name];
}
/// <summary>
/// Adds a <see cref="CalledMethod"/> if necessary and increments its <see cref="CalledMethod.Count"/>
/// </summary>
/// <param name="name"></param>
public void Increment(string name)
{
var entry = this[name] ?? AddAndReturn(name);
entry.Count++;
}
}
public class CalledMethod
{
public string Name { get; set; }
public int Count { get; set; }
public CalledMethod() { }
public CalledMethod(string name, int count = 0)
{
Name = name;
Count = count;
}
}
Calling from the Mock class method.
public UserStory SaveStory(UserStory story)
{
CalledMethods.Increment(System.Reflection.MethodBase.GetCurrentMethod().Name);
CheckException();
return UserStory;
}
And using in the test.
// assume arrange and act before this, then
_storyService.CalledMethods["SaveStory"].Count.Should().Be(1);
Puzzle 2: The Service
We still haven't implemented the feature. In fact, arguably we shouldn't have written the controller or its test at all; the controller doesn't save the story, the service does.
Regardless, let's write the test first this time:
[Fact]
public void SaveStory_sets_new_UserStory_Sequence_to_Max_plus_one()
{
var service = new UserStoryService();
var userStory = new UserStory()
{
//set needed fields. Sequence is null or 0.
};
userStory = service.SaveStory(userStory);
userStory.Sequence.Should().Be(???);
}
Yeah. We run into a question of how to setup the Max Sequence. But writing the test is helping us. We need to answer
- Does the service depend on something else to get the MaxSequence?
- If so, mock it
- If not, it will be a functional test
Let's assume our service depends on a data service, and finish the unit test.
[Fact]
public void SaveStory_sets_new_UserStory_Sequence_to_Max_plus_one()
{
var dataService = new MockDataService();
dataService.MaxSequence = 15;
var service = new UserStoryService(dataService);
var userStory = new UserStory()
{
//set needed fields. Sequence is null or 0.
};
userStory = service.SaveStory(userStory);
userStory.Sequence.Should().Be(16);
}
For you to figure out: What if SaveStory were a void method?
Functional Testing: The Proof of the Pudding Is In the Tasting
At some point, some piece of code is actually persisting data. There's no way to unit test that. If your service depends on an ORM such as Entity Framework (EF), then you can mock EF. But if you want to test that that your concrete UnitOfWork/Repository/DbContext/Whatever works as expected, you have to use a real database and check the values. Another example: if you at some point write to file, you'll need to write functional tests for that, and verify that what was written is what you expected.
Bonus: how might the functional tests look? Remember, these will be slower and likely run as part of a separate project, just like your integration tests.
public class DataServiceTests {
Db _db = new Db();
public DataServiceTests()
{
//In xUnit.Net, the constructor is used to reset the environment
//to a known state.
//There could be a lot of actions to take, so this is simplistic.
_db.Reset();
}
[Fact]
public void GetMaxSequence_returns_expected_value()
{
var service = new DataService(_db);
//Our known starting point for MaxSequence is 10.
service.GetMaxSequence().Should().Be(10);
}
[Fact]
public void IncrementMaxSequence_sets_expected_value()
{
var service = new DataService(_db);
// _db is reset before every test, so MaxSequence is 10 again.
service.IncrementMaxSequence(1);
service.GetMaxSequence().Should().Be(11);
}
}
Wrap Up
TDD isn't nearly so much what to do, as how to think. Especially, I find it forces thinking about how to decouple code and make it testable. The tricky part, requiring practice, is seeing what are dependencies and what aren't. Knowing what your unit is responsible for.
I think learning from the simplest cases is great, because it teaches the principles to apply.
