Required validator always returns false - asp.net-mvc-3

Question

I have an EF Code First model which I'm editing via an MVC page, with a particular field that always returns false with the [Required] data annotation. If I hard set a value right before validating, it still fails.
It's for a User object, of which I can configure if I'm using a username or email address as the 'username' property.
The model:
public class User {
[DisplayName("User Id")]
public int Id { get; set; }
[Required(ErrorMessage="Username is required")]
public string Username { get; set; }
[UIHint("EmailAddress")]
[Required(ErrorMessage = "Email is required")]
[EmailAddress]
public string Email { get; set; }
}
In my view, I'm only drawing the Username editor if it's required:
#if (#ViewBag.LoginMethod == "username") {
#Html.LabelFor(m => m.Username)
#Html.TextBoxFor(m => m.Username, new { autocomplete = "off" })
#Html.ValidationMessageFor(m => m.Username)
}
#Html.LabelFor(m => m.Email)
#Html.TextBoxFor(m => m.Email, new { autocomplete = "off" })
#Html.ValidationMessageFor(m => m.Email)
My controller:
[HttpPost]
public ActionResult Create(UserModel viewModel) {
ViewBag.LoginMethod = this.loginMethod.ToString();
var user = new User();
if (this.loginMethod == LoginMethods.Username)
user.Username = viewModel.User.Username;
else
user.Username = viewModel.User.Email;
user.Email = viewModel.User.Email;
user.FirstName = viewModel.User.FirstName;
user.LastName = viewModel.User.LastName;
user.Username = "TEST";
if (TryValidateModel(user) == false) {
this.FlashError("Validation Errors!");
return View(viewModel);
}
throw new Exception("here");
}
As you can see, I'm setting the User.Username property, based on the login method. For the sake of testing, I'm setting it to "TEST", right before validation.
The Username Required validation returns false, and I end up back in my view. I never get to the exception.
I have managed to make it work correctly, by rendering the Username editor on the page no matter what. As I have client side validation enabled, I can't submit the form without entering a value, and it works - even though the Username value is still "TEST" once validated.
I'm beginning to think TryValidateModel isn't the right function. Using ModelState.IsValid yields the same result - an incorrect Required fail.

Answer 1

First, I think you can use EditFor now, to avoid creating the trio of controls every time (label, textbox, validator).
Another thing that strikes me as weird is that your Model seems to be of type User (m.Email), but your action takes a UserModel (which contains a User). I am not sure why your code still works. Normally you could take directly a User as a parameter (so you won't have to copy the values by hand).
It's normal if ModelState.IsValid doesn't work. If your model is of type User, it will try to validate all POSTED properties, regardless of whether they are on the view. On the other hand, TryValidateModel SHOULD have worked in your scenario. It seems there's an added security feature there, which considers empty the properties for which there was no Edit control.
A workaround would be to create your custom model binder for the User object yourself (it's not that hard, you inherit IModelBinder, you override BindModel and call the base method. Afterwards, if the UserName is empty, then you add "TEST" or the correct value. Of course, you cannot leave it empty). Search about custom model binders in Asp.net MVC.
I hope that helps!