Question

public class AccountController : Controller
{

    private LoginModel loginModel = null;

    #region Constructor
    public AccountController(LoginModel loginModel)
    {
        this.loginModel = loginModel;
    }

    public AccountController()
    {
    }
    #endregion

    #region Login
    //
    // GET: /Account/Login
    public ActionResult Login()
    {
        return View();
    }

    //
    // POST: /Account/Login
    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Login(string userName, string password, bool rememberMe, string returnUrl)
    {
        try
        {
            string displayFullName = null, token = null;

            LoginViewModel loginViewModel = new LoginViewModel();
            loginViewModel.UserName = userName;
            loginViewModel.Password = password;
            loginViewModel.RememberMe = rememberMe;

            ModelState.AddModelErrors(loginViewModel.ValidateLogIn());

            if (!ModelState.IsValid)
            {
                return View();
            }

            //login failed than return view. 
            if (!this.loginModel.LogIn(loginViewModel, ref displayFullName, ref token))
            {
                ModelState.AddModelError("_FORM", PortalErrors.IncorrectDataMsg);
                return View();
            }

            if (Url.IsLocalUrl(returnUrl) && returnUrl.Length > 1 && returnUrl.StartsWith("/")
                    && !returnUrl.StartsWith("//") && !returnUrl.StartsWith("/\\"))
            {
                return Redirect(returnUrl);
            }
            else
            {
                return RedirectToAction("Index", "Home");
            }

        }
        catch (Exception exc)
        {
            ModelState.AddModelError("_FORM", PortalErrors.CommonErrMsg);
            return View();
        }
    } 

I get the error on my above code on line if (!this.loginModel.LogIn(loginViewModel, ref displayFullName, ref token))

Can any one help with this to find what’s wrong in this one?

Answer 1

Surely it’s because the loginModel is supposed to be set in the constructor; but MVC will not fire the constructor that sets it; it’ll be firing the default constructor – so by that point it’s null; hence the NullReferenceException.

I’m going to take a guess that you’ve inherited this Controller from someone else; and that they have written unit tests for it – hence the additional constructor; unless, as Jon has mentioned, there is some DI framework active on the site (in which case it’s misconfigured).

If there’s a DI framework creating the controller – then check that it can resolve an instance of LoginModel. An easy way, and good practise, is to check for a null LoginModel being passed in that constructor and throw an exception (typically ArgumentNullException; the use of Code Contracts as mentioned by another answer here is a good plan) and then run the site again. Either you’ll get the same error (in which case there’s probably no DI involved), or you’ll get your ArgumentNullException – in which case there is and it’s badly configured.

If there isn’t, you need to modify the default constructor of the controller to create the default instance of LoginModel – presumably it has roots in a Database or something like that – ultimately you’ll need to look at the rest of the project’s code to figure that one out.