Why am I getting the following error: "System.NullReferenceException: Object reference not set to an instance of an object." when I run a Unit Test?

Question

I have never run Unit testing before and I am just trying to run an example I have found on the net over and over in regards to the view name.

My Test code is:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Admin.Web.API.Controllers;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Web.Mvc;
namespace Admin.Web.API.Controllers.Tests
{
    [TestClass()]
    public class HomeControllerTests
    {
        [TestMethod()]
        public void IndexTest()
        {
            HomeController controller = new HomeController();
            var result = controller.Index() as ViewResult;
            Assert.AreEqual("Index", result.ViewName);
        }
    }
}

The error I am getting is System.NullReferenceException: Object reference not set to an instance of an object. on the Line that sets the view result.

What do I need to do to get this working? Is there anything out there that is more descriptive as to Unit testing examples?

Edit one

Controller Code for Index

public ActionResult Index()
{
    if (this.Session["UserID"] == null)
    {
        return View("Login");
    }
    else
    {
        ViewBag.Title = "Index";
        ViewBag.SiteID = this.Session["SiteID"];
        ViewBag.AssemblyVersion = this.Session["AssemblyVersion"];
        ViewBag.UserFirstName = this.Session["FirstName"];
        GoogleAnalytics _oGoogleAnalytics = new GoogleAnalytics();
        ViewBag.GoogleAnalytics = _oGoogleAnalytics.GetGoogleAnalytics(
            this.Session["GoogleAnalyticsAccountCode"].ToString(),
            Convert.ToBoolean(this.Session["UseGoogleAnalytics"]));
            return View("Index");
    }
}

Answer 1

controller.Index() is not returning a ViewResult and thus result is null.

result.ViewName is then failing as you are dereferencing null.

You need to look at the definition of Index and determine what type of object it is returning. Alternatively you can create a variable for controller.Index() and see what type it has.

EDIT:

Taking a deeper look at Index() it appears that you are relying on some data points that might not be available. The controller is relying on its framework (in this case your test framework) to provide certain pieces, IIRC the Session and ViewBag are two of those pieces.

Answer 2

You're using the controllers Session property, which will be null because you haven't supplied the controller with the information it needs to create it. This information is normally supplied automatically when running under the Asp.Net pipeline. You can verify this by debugging and stepping into (F11) the Index action method and hovering over Session.

You need to set the ControllerContext property of the controller. Even better would be to use the Authorize attribute on your action method / controller. This is a good post about how to do Forms authentication in MVC.

The simplest way to get your test going though is to use the Controller's User property. You also do this by creating an instance of ControllerContext and setting its HttpContext property, probably by Moq'ing HttpContextBase so that you can return whatever IPrincipal you want.

So this is what you'd need to add after you new up your controller (I'm showing you with the Moq framework, but the VS UnitTest tools might provide its own way to do mocks. Either is fine):

var principalMock = new Mock<IPrincipal>(); // Mock<T> is from the Moq framework
principalMock.Setup(x => x.IsAuthenticated).Returns(true); // Or false, depending on what you're testing

var httpContextMock = new Mock<HttpContextBase>();
httpContextMock.Setup(x => x.User).Returns(principalMock.Object);

var controllerContext = new ControllerContext { HttpContext = contextMock.Object };
conrollerContext.Controller = controller;
controller.ControllerContext = controllerContext;

After you have all that setup, then you can safely call the action method you're testing.