What is the recommended place to check: ViewModel, Model or Controller?

Question

I have a registration page and you want to do some verification (in addition to StringLength and Required annotations on my ViewModel) to duplicate usernames and email addresses. I am currently doing this check in my controller when the registration form is sent back. I am not sure if this is the right place for this.

I cannot imagine that the ViewModel is the right place, since the ViewModel will need a link to my UserRepository. Does it make sense to have such validation in model classes?

If so, how to implement this on a model to check if the information is valid before I sent it to my repository?

Update

My controller action code:

if (ModelState.IsValid)
        {
            if (!_userRepository.Exists(registerViewModel.Username))
            {
                if (!_userRepository.EmailExists(registerViewModel.Email))
                {
                    _userRepository.Add(
                        new User
                            {
                                Created = DateTime.Now, 
                                Email = registerViewModel.Email, 
                                Password = registerViewModel.Password, 
                                Username = registerViewModel.Username
                            });

                    _userRepository.SaveChanges();
                    TempData["registrationDetails"] = registerViewModel;

                    return RedirectToAction("Confirm");
                }
                else
                {
                    ModelState.AddModelError(string.Empty, "This email address is already in use.");
                }
            }
            else
            {
                ModelState.AddModelError(string.Empty, "This username is already taken.");
            }
        }

        return View(registerViewModel);
    }

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for MVC3, you can add methods to the controller as actions using the Remote attribute in your property in the view model for instant results

Answer 5

You should check both on the view (on the client side) and on the controller (on the server side). If you have MVC 3, you can use the new RemoteAttribute . It uses jQuery for a server-side call where you can check for a user or email address.