Creating custom data annotation validation in MVC 3

For instance, I have an Employee view model. When creating an employee, I want to validate the username to make sure it doesn’t exist.

public class EmployeeViewModel
{
    [ScaffoldColumn(false)]
    public int EmployeeId { get; set; }

    [ValidateDuplicate(ErrorMessage = "That username already exists")]
    [Required(ErrorMessage = "Username is required")]
    [DisplayName("Username")]
    public string Username { get; set; }
}

And then have my ValidateDuplicate function somewhere with the code to check for a duplicate.

Is this possible?

Answers:

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Method 1

I would suggest looking at remote validation. The example even matches your case.

Basically, add the remote attribute to your viewmodel property that points to a controller action

[Remote("IsUserExists", "Account", ErrorMessage = "Can't add what already exists!")]
[Required(ErrorMessage = "Username is required")]
[DisplayName("Username")]
public string Username { get; set; }

which does your work

public ActionResult IsUserExists(string userName) 
{
 if (!UserService.UserNameExists(userName) || (CurrentUser.UserName == userName))
 {
      return "Ok.";
 }
}

Method 2

Yes, it’s possible. You’ll need to write your own validation attribute.

Method 3

You can write your own custom validation as explained here. I’ve modified the code to add validation in the model as I prefer rails active record’s validation style in the model.

public class EmployeeViewModel
{

    [CustomValidation(typeof(EmployeeViewModel), "ValidateDuplicate")]
    [Required(ErrorMessage = "Username is required")]
    [DisplayName("Username")]
    public string Username { get; set; }

    public static ValidationResult ValidateDuplicate(string username)
    {
      bool isValid;

      using(var db = new YourContextName) {
        if(db.EmployeeViewModel.Where(e => e.Username.Equals(username)).Count() > 0)
       {
          isValid = false;
       } else {
          isValid = true;
       }
      }

      if (isValid)
      {
        return ValidationResult.Success;
      }
      else
      {
        return new ValidationResult("Username already exists");
      }

    }
}

Method 4

You can do that by extending the Attribute class calling it ValidateDuplicate. I’d avoid doing it this way because its just another place where you potentially have to make a call to a database.

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