Follow these steps to implement a couple of tests on the GetQuestion method:

1. Let's add the following test in the QuestionsControllerTests class to verify that we get the correct result when the question isn't found:

[Fact]
public async void GetQuestion_WhenQuestionNotFound_Returns404()
{
  var mockDataRepository = new Mock<IDataRepository>();
  mockDataRepository
    .Setup(repo => repo.GetQuestion(1))
    .Returns(() => Task.FromResult(default(QuestionGetSingleResponse)));

  var mockQuestionCache = new Mock<IQuestionCache>();
  mockQuestionCache
    .Setup(cache => cache.Get(1))
    .Returns(() => null);

  var mockConfigurationRoot = new Mock<IConfigurationRoot>();
  mockConfigurationRoot.SetupGet(config =>
    config[It.IsAny<string>()]).Returns("some setting");

  var questionsController = new QuestionsController(
    mockDataRepository.Object, null, mockQuestionCache.Object, 
    null, mockConfigurationRoot.Object);

  var result = await questionsController.GetQuestion(1);
  
  var actionResult = 
    Assert.IsType<
      ActionResult<QuestionGetSingleResponse>
    >(result);
  Assert.IsType<NotFoundResult>(actionResult.Result);
}

We need to mock the cache in this test because this is used in the GetQuestion method.

Here, we check that the result is of the NotFoundResult type.

2. Let's add another test to verify a question is returned when the one that's requested does exist:

[Fact]
public async void GetQuestion_WhenQuestionIsFound_ReturnsQuestion()
{
  var mockQuestion = new QuestionGetSingleResponse
  {
    QuestionId = 1,
    Title = "test"
  };

  var mockDataRepository = new Mock<IDataRepository>();
  mockDataRepository
    .Setup(repo => repo.GetQuestion(1))
    .Returns(() => Task.FromResult(mockQuestion));

  var mockQuestionCache = new Mock<IQuestionCache>();
  mockQuestionCache
   .Setup(cache => cache.Get(1))
   .Returns(() => mockQuestion);

  var mockConfigurationRoot = new Mock<IConfigurationRoot>();
  mockConfigurationRoot.SetupGet(config =>
    config[It.IsAny<string>()]).Returns("some setting");

  var questionsController = new QuestionsController(
    mockDataRepository.Object, null, mockQuestionCache.Object, 
    null, mockConfigurationRoot.Object);

  var result = await questionsController.GetQuestion(1);

  var actionResult = 
    Assert.IsType<
      ActionResult<QuestionGetSingleResponse>
    >(result);
  var questionResult = 
    Assert.IsType<QuestionGetSingleResponse>(actionResult.Value);
  Assert.Equal(1, questionResult.QuestionId);
}

This time, we check that the result is of the QuestionGetSingleResponse type and that the correct question is returned by checking the question ID.

That completes the tests we are going to perform on our GetQuestion action method.

The same approach and pattern can be used to add tests for controller logic we haven't covered yet. We can do this using Moq, which mocks out any dependencies that the method relies on. In the next section, we'll start to implement tests on the frontend.