Question

I try to test my controller

[TestMethod]
    public void Index()
    {
        AdminController controller = new AdminController();
        ViewResult result = controller.Index() as ViewResult;
        Assert.IsNotNull(result);
    }

Here is Index() code:

public ActionResult Index()
        {
            var repository = new PostsRepository();
            var posts = repository.GetAllPosts();

            return View(posts);
        }

Repository:

public class PostsRepository : IPostsRepository
    {
        PostsDataContext _dataContext = new PostsDataContext();

        public IQueryable<Post> GetAllPosts()
        {
            var posts = from t in _dataContext.Posts select t;
            return posts;
        }

        public Post GetPostById(int id)
        {
            var post = from t in _dataContext.Posts
                       where t.id == id
                       select t;
            return post.First();
        }
    }

But my Index() unit test fails with error:

Test method MvcBlog.Tests.Controllers.AdminControllerTest.Index threw exception: 
System.NullReferenceException: Object reference not set to an instance of an object.

Stack Trace:

MvcBlog.Models.PostsDataContext..ctor() in C:\Users\cL1Nk3r\Documents\Visual Studio 2010\Projects\MvcBlog\MvcBlog\Models\Posts.designer.cs: line 38
MvcBlog.Repository.PostsRepository..ctor() in C:\Users\cL1Nk3r\Documents\Visual Studio 2010\Projects\MvcBlog\MvcBlog\Repository\PostsRepository.cs: line 11
MvcBlog.Controllers.AdminController.Index() in C:\Users\cL1Nk3r\Documents\Visual Studio 2010\Projects\MvcBlog\MvcBlog\Controllers\AdminController.cs: line 19
MvcBlog.Tests.Controllers.AdminControllerTest.Index() in C:\Users\cL1Nk3r\Documents\Visual Studio 2010\Projects\MvcBlog\MvcBlog.Tests\Controllers\AdminControllerTest.cs: line 19

Why it is an error?

If i just run my application, it works correctly.

Source code download: http://dl.dropbox.com/u/14053604/MvcBlog.rar

Answer 1

The problem is the following line in your controller action:

var repository = new PostsRepository();

Here you are tying your controller to a particular implementation of the repository making it very difficult to unit test in isolation. In order to achieve weaker coupling between your controller and data access layer I would recommend you using constructor injection:

public class PostsController: Controller
{
    private readonly IPostsRepository _repository;
    public PostsController(IPostsRepository repository)
    {
        _repository = repository;
    }

    public ActionResult Index()
    {
        var posts = _repository.GetAllPosts();
        return View(posts);
    }
}

Now your controller is completely decoupled from a particular implementation of a repository which might depend on a database, etc. Now you could use a mocking framework such as Rhino Mocks or Moq to provide a dummy implementation of this repository for the unit test.

Personally I like very much MVCContrib TestHelper which works with Rhino Mocks and allows for very elegant unit tests of controller actions. So once you’ve decoupled your controller from a specific implementation of the repository as I showed you could have the following unit test:

[TestMethod]
public void PostsController_Index_Action_Should_Fetch_All_Posts_From_Repository()
{
    // arrange
    var postsRepositoryStub = MockRepository.GenerateStub<IPostsRepository>();
    var sut = new PostsController(postsRepositoryStub);
    var expectedPosts = new Post[0];
    postsRepositoryStub.Stub(x => x.GetAllPosts).Return(expectedPosts);

    // act
    var actual = sut.Index();

    // assert
    actual
        .AssertViewRendered()
        .WithViewData<IEnumerable<Post>>()
        .ShouldBe(expectedPosts);
}