在用户注册视图中添加其他模型的项目的下拉视图（ASP.NET MVC5）

Question

I'm creating a system in ASP.NET that allows users to register, choosing (among other things) a cell phone number and a cell carrier. I am using Individual User Accounts in the MVC Template using MVC 5. I have another model called "Carriers," that keeps track of all carriers as well as their Email to SMS gateway. The model specification is in it's own file, CarrierModels.cs:

public class Carrier
{
    public string name { get; set; }
    public int id { get; set; }
    public string gateway { get; set; }
}

I am trying to get a drop-down in the user registration that will let users select from the set of carriers (There is another controller handling creation and modification of carriers). I tried modifying the RegisterViewModel class in AccountViewModels and using LINQ to pull items from a list, but that continuously gives me a Null Reference Exception. How should I go about getting this drop down menu into the registration page?

Edit 2: Showing more code (In it's updated form):

Here is the View Code overall, the DropDownListFor line is what causes NullReferenceExceptions.

<div class="form-group">
     @Html.LabelFor(m => m.carrierListIndex, new { @class = "col-md-2 control-label" })  
    <div class="col-md-10">
        @Html.DropDownListFor(m=>m.carrierListIndex, Model.carrierList.Select(Text = x.name, Value = x.Id.ToString(), Selected = Model.CarrierListIndex == x.Id), "Please Select...",null)
          </div>
</div>

This is the portion of RegisterViewModel that corresponds to carriers:

[Display(Name = "Wireless Carrier")]
public int carrierListIndex { get; set; }
public static  db db = new db();
public List<Carrier> carrierList = db.Carriers.ToList();

My two Register functions in AccountController.cs:

    [AllowAnonymous]
    public ActionResult Register()
    {
        return View();
    }

    //
    // POST: /Account/Register
    [HttpPost]
    [AllowAnonymous]
    [ValidateAntiForgeryToken]
    public async Task<ActionResult> Register(RegisterViewModel model)
    {
        if (ModelState.IsValid)
        {
            ApplicationUser user = new ApplicationUser { UserName = model.Email, Email = model.Email, studentID = model.studentID, phone=model.phone, carrierList=model.carrierList };
            var result = await UserManager.CreateAsync(user, model.Password);

            var context = new ApplicationDbContext();
            var roleStore = new RoleStore<IdentityRole>(context);
            var roleManager = new RoleManager<IdentityRole>(roleStore);

            var userStore = new UserStore<ApplicationUser>(context);
            var userManager = new UserManager<ApplicationUser>(userStore);



            if (result.Succeeded)
            {
                userManager.AddToRole(user.Id, "User");
                await SignInManager.SignInAsync(user, isPersistent:false, rememberBrowser:false);

                // For more information on how to enable account confirmation and password reset please visit http://go.microsoft.com/fwlink/?LinkID=320771
                // Send an email with this link
                // string code = await UserManager.GenerateEmailConfirmationTokenAsync(user.Id);
                // var callbackUrl = Url.Action("ConfirmEmail", "Account", new { userId = user.Id, code = code }, protocol: Request.Url.Scheme);
                // await UserManager.SendEmailAsync(user.Id, "Confirm your account", "Please confirm your account by clicking <a href=\"" + callbackUrl + "\">here</a>");

                return RedirectToAction("Index", "Home");
            }
            AddErrors(result);
        }

        // If we got this far, something failed, redisplay form
        return View(model);
    }

Answer 1

The 2nd argument of DropDownListFor() is Ienumerable<SelectListItem> but you just passing an anonymous object. Change the view model to

public class RegisterViewModel
{
    ....
    [Display(Name = "Wireless Carrier")]
    [Required(ErrorMessage = "Please select a carrier")]
    public int SelectedCarrier { get; set; }
    public IEnumerable<SelectListItem> CarrierList { get; set; }
}

and pass the model to the view in the GET method

[AllowAnonymous]
public ActionResult Register()
{
    RegisterViewModel model = new RegisterViewModel();
    ConfigureViewModel(model);
    return View(model);
}

[HttpPost]
[AllowAnonymous]
[ValidateAntiForgeryToken]
public async Task<ActionResult> Register(RegisterViewModel model)
{
    if (!ModelState.IsValid)
    {
        ConfigureViewModel(model); // must reassign the SelectList
        return View(model);
    }
    .... // save and redirect
}

private ConfigureViewModel(RegisterViewModel model)
{
    model.carrierList = new SelectList(db.Carriers, "Id", "name");
}

and in the view

@model RegisterViewModel
....
@Html.DropDownListFor(m => m.SelectedCarrier, Model.CarrierList,"Please select")
@Html.ValidationMessageFor(m => m.SelectedCarrier)