Model Binding: Safe cast returns null

Question

I have the following snippet.

public async Task<ActionResult> Register(RegisterViewModel model)
{
    if (!ModelState.IsValid) return View(model as LocalRegisterViewModel);
        var user = new User
    {
        UserId = model.Username,
        Password = null,
        Email = model.Email,
        AccType = model.AccountType
    };

    var modelAsLocalRegisterViewModel = model as LocalRegisterViewModel;
    if (modelAsLocalRegisterViewModel != null)
        user.Password = modelAsLocalRegisterViewModel.Password;
    //...
}

The classes looks as follows.

public class RegisterViewModel
{
    public string Username { get; set; }
    public string Email { get; set; }
    public int AccountType { get; set; }
}

public interface IInternalPassword
{
    string Password { get; set; }
    string ConfirmPassword { get; set; }
}

public class LocalRegisterViewModel : RegisterViewModel, IInternalPassword
{
    public string Password { get; set; }
    public string ConfirmPassword { get; set; }
}

The LocalRegisterViewModel is passed to the controller as follows from a cshtml page.

@model  LocalRegisterViewModel

@{
    ViewBag.Title = "Register";
    Layout = "~/Views/Shared/_LayoutAnonymous.cshtml";
}

<h2>@ViewBag.Title.</h2>

@using (Html.BeginForm("Register", "Account", FormMethod.Post, new { @class = "form-horizontal", role = "form" }))

My problem is that, modelAsLocalRegisterViewModel is null after the safe cast.

    var modelAsLocalRegisterViewModel = model as LocalRegisterViewModel;
    if (modelAsLocalRegisterViewModel != null)
        user.Password = modelAsLocalRegisterViewModel.Password;

Can someone look into this and tell me why?

EDIT

Seems like my questioning style is bad. So let me clarify my exact intention as well. The Register action I have written is intended to serve multiple ViewModels, each having some additional info. So what I have done is writing a parent which carries the common attributes and extending that to get the added attributes. For an instance, I pass an instance of a LocalRegisterViewModel to the controller, so that it will first execute the common functionality and if the instance passed is of type LocalRegisterViewModel if will carry out the extended functionality. That's why I need to check the passed RegisteredViewModel is also of type LocalRegisterViewModel.

EDIT 2 This is not trying to assign a base class instance to a derived class reference. It's a fact that following is completely valid in C#.

class Program
{
    static void Test(Parent p)
    {
        var c = p as Child;
        Console.WriteLine(c == null ? "Can't do it!" : "Can do it!");
        Console.WriteLine(c.GetType().ToString());
    }

    static void Main(string[] args)
    {
        var c = new Child();
        Test(c);
    }
}

public class Parent
{
}

public class Child : Parent
{
}

Answer 1

Right; so if:

var modelAsLocalRegisterViewModel = model as LocalRegisterViewModel;

gives null, then there are exactly 2 options:

• model is null
• model is something, but something other than LocalRegisterViewModel

So: you'll need to look at model and find out what it is. We can't tell you that: it isn't in the code shown. But string typeName = model?.GetType()?.Name; should tell you which; it'll return either null or the name of the type that model is.

---

With the recent edit, we can see that model is a RegisterViewModel; but: it sounds like it isn't a LocalRegisterViewModel. Since there is an inheritance tree, it sounds like model is either the base-type (RegisterViewModel) or a different sub-type unrelated to LocalRegisterViewModel.

Answer 2

Your confusion I think comes from thinking that you are "calling" controller method Register() from cshtml page, and "passing" your model there. It's not exactly true.

When you submit your form, it will post all inputs to server, to the specified url. Those inputs might include properties of LocalRegisterViewModel, such as Password. Request body might look like this:

{"email": "my@email.com", "password": "bla" }

When request comes to server, ASP.NET looks for controller action matching given url. It sees that matching action is Register() and this action accepts parameter of type RegisterViewModel. Now it tries to bind that model (fill its properties from http request). It has absolutely no idea that there are additional values, such as Password, in incoming request.

So asp.net will create instance of RegisterViewModel and fill its properties, ignoring all the rest (such as password), because there is no information in request itself about which C# type it should be parsed into.