object reference not set to instance of an object issues in unit testing of asp.net mvc - asp.net

Question

I am the beginner of writing unit tests for asp.net. I created a simple project and try to start my testing journey. However, I met two errors with the same issue:"object reference not set to instance of an object" The first place is in the home controller as below:
public class HomeController : Controller
{
private readonly ILogger<HomeController> _logger;
public HomeController(ILogger<HomeController> logger)
{
_logger = logger;
}
[ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
public IActionResult Error()
{
return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
}
Here is my test method:
public class HomeControllerUnitTests
{
ILogger<HomeController> _logger;
[Fact]
public void Error_ActionExecutes_ReturnsAViewResult()
{
// Arrange
var homeController = new HomeController(_logger);
// Act
var result = homeController.Error() as ViewResult;
// Assert
Assert.Null(result.ViewData.Model);
}
}
The second place is in the Movie Controller:
public class MoviesController : Controller
{
private readonly MvcMovieContext _context;
public MoviesController(MvcMovieContext context)
{
_context = context;
}
// GET: Movies
public async Task<IActionResult> Index()
{
return View(await _context.Movie.ToListAsync());
}
}
My test method is as below:
public class MoviesControllerUnitTests
{
private Mock<MvcMovieContext> _mock;
[Fact]
public async Task Index_ActionExecutes_ReturnsAViewResult()
{
// Arrange
MoviesController controller = new MoviesController(_mock.Object);
// Act
var result = await controller.Index() as Task<ViewResult>;
// Assert
Assert.IsType<ViewResult>(result);
}
}
Please help me and thanks in advance.

Answer 1

Below the Object reference not set to an instance of an object line there should be an indication about which file and line the error occurred, which helps you to determine which variables are null (but you could also use the debugger).
For the MoviesControllerUnitTests this probably is the _mock variable, so be sure to initialize it as shown in the docs, e.g.:
private Mock<MvcMovieContext> _mock = new Mock<MvcMovieContext>();
For the HomeControllerUnitTests you might need to mock the Activity or set a HttpContext (see e.g. this question).