How to unit test whether a Core MVC controller action calls ControllerBase.Problem()

Question

We have a controller that derives from ControllerBase with an action like this:

public async Task Get(int id)
{
  try
  {
           


        

Answer 1

In your tests, if you first create a ControllerContext, then ProblemDetails should be created as expected while executing controller code.

...
MyController controller;

[Setup]
public void Setup()
{
    controller = new MyController();
    controller.ControllerContext = new ControllerContext
    {
        HttpContext = new DefaultHttpContext
        {
            // add other mocks or fakes 
        }
    };
}
...

Answer 2

The null exception is because of a missing ProblemDetailsFactory

In this case the controller needs to be able to create ProblemDetails instance via

[NonAction]
public virtual ObjectResult Problem(
    string detail = null,
    string instance = null,
    int? statusCode = null,
    string title = null,
    string type = null)
{
    var problemDetails = ProblemDetailsFactory.CreateProblemDetails(
        HttpContext,
        statusCode: statusCode ?? 500,
        title: title,
        type: type,
        detail: detail,
        instance: instance);

    return new ObjectResult(problemDetails)
    {
        StatusCode = problemDetails.Status
    };
}

Source

ProblemDetailsFactory is a settable property

public ProblemDetailsFactory ProblemDetailsFactory
{
    get
    {
        if (_problemDetailsFactory == null)
        {
            _problemDetailsFactory = HttpContext?.RequestServices?.GetRequiredService<ProblemDetailsFactory>();
        }

        return _problemDetailsFactory;
    }
    set
    {
        if (value == null)
        {
            throw new ArgumentNullException(nameof(value));
        }

        _problemDetailsFactory = value;
    }
}

Source

that could be mocked and populated when testing in isolation.

[TestMethod]
public async Task GetCallsProblemOnInvalidId() {
    //Arrange
    var problemDetails = new ProblemDetails() {
        //...populate as needed
    };
    var mock = new Mock<ProblemDetailsFactory>();
    mock
        .Setup(_ => _.CreateProblemDetails(
            It.IsAny<HttpContext>(),
            It.IsAny<int?>(),
            It.IsAny<string>(),
            It.IsAny<string>(),
            It.IsAny<string>(),
            It.IsAny<string>())
        )
        .Returns(problemDetails)
        .Verifyable();

    var sut = new MyController(...);
    sut.ProblemDetailsFactory = mock.Object;

    //...

    //Act
    var result = await sut.Get(someInvalidId);

    //Assert
    mock.Verify();//verify setup(s) invoked as expected

    //...other assertions
}