Unique custom profile field asp.net identity MVC5

Question

I'm new to MVC and I am developing a self service application for our clients at work which they will login to using their clientcode and password, I used asp.net identity with DBContext and managed to create a custom profile field which is ClientCode and is working fine but I need to make this field unique such that only one ClientCode exists in the users database, I was thinking maybe to check this on registering a new account, not sure how I can implement that constraint;

My AccountController Register method

[HttpPost]
    [AllowAnonymous]
    [ValidateAntiForgeryToken]
    public async Task<ActionResult> Register(RegisterViewModel model)
    {
        if (ModelState.IsValid)
        {
            var user = new ApplicationUser { UserName = model.Email, Email = model.Email, ClientCode = model.ClientCode };

            var result = await UserManager.CreateAsync(user, model.Password);
                if (result.Succeeded)
                {
                    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("ClientsHome", "Home");
                }
                AddErrors(result);
        }
        // If we got this far, something failed, redisplay form
        return View(model);
    }

My AccountViewModel RegisterViewModel

   public class RegisterViewModel
{
    [Required]
    [Display (Name = "Client Code")]
    public long ClientCode { get; set; }

    [Required]
    [EmailAddress]
    [Display(Name = "Email")]
    public string Email { get; set; }

    [Required]
    [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
    [DataType(DataType.Password)]
    [Display(Name = "Password")]
    public string Password { get; set; }

    [DataType(DataType.Password)]
    [Display(Name = "Confirm password")]
    [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
    public string ConfirmPassword { get; set; }
}

Answer 1

You could do something as simple as:

public bool ClientCodeExists(long clientCode)
{
   //you will get values from your db. I've used this as a simple exmaple.
   List<long> clientCodesList = new List<long>();

   clientCodesList.Add(100);
   clientCodesList.Add(200);

   return clientCodesList.Any(a => a == clientCode);
}

Then in your controller you can call this as:

[HttpPost]
[AllowAnonymous]
[ValidateAntiForgeryToken]
public async Task<ActionResult> Register(RegisterViewModel model)
{
    if (ModelState.IsValid)
    {
       long clientCode = model.ClientCode;
       bool clientCodeExists = ClientCodeExists(clientCode);

       if(clientCodeExists)
       {
          //setup app user, create identity etc
       }
       else
       {
          ModelState.AddModelError("", "Error message you want to show to end user.");
       }
    }
}