public JsonResult IsUID_Available(string Username) {

            if (!_repository.UserExists(Username))
                return Json(true, JsonRequestBehavior.AllowGet);

            string suggestedUID = String.Format(CultureInfo.InvariantCulture,
                "{0} is not available.", Username);

            for (int i = 1; i < 100; i++) {
                string altCandidate = Username + i.ToString();
                if (!_repository.UserExists(altCandidate)) {
                    suggestedUID = String.Format(CultureInfo.InvariantCulture,
                   "{0} is not available. Try {1}.", Username, altCandidate);
                    break;
                }
            }
            return Json(suggestedUID, JsonRequestBehavior.AllowGet);
        }

public class CreateUserModel : EditUserModel {
    [Required]
    [StringLength(6, MinimumLength = 3)]
    [Remote("IsUID_Available", "Validation")]
    [RegularExpression(@"(\S)+", ErrorMessage = "White space is not allowed.")]
    [Editable(true)]
    public override string UserName { get; set; }
}