MetaDataException未被用户处理

Question

这是我的代码，我得到MetadataException未被用户处理。

  

“System.Data.Entity.Core.MetadataException”类型的异常   发生在EntityFramework.dll中但未在用户代码中处理

希望有人可以帮助我，谢谢！

    [HttpPost]
    public JsonResult UserRegistration(User u)
    {
       string message = "";
            //Here we will save data to the database
            if (ModelState.IsValid)
            {
                using (barometerDBEntities1 dc = new barometerDBEntities1())
                {
                    //check username available
                    var user = dc.Users.Where(a => a.Username.Equals(u.Username)).FirstOrDefault();
                    if (user == null)
                    {
                        //Save here
                        dc.Users.Add(u);
                        dc.SaveChanges();
                        message = "Success";
                    }
                    else
                    {
                        message = "Username not available!";
                    }
                }
            }
            else
            {
                message = "Failed!";
            }
            return new JsonResult { Data = message,  JsonRequestBehavior = JsonRequestBehavior.AllowGet };
        }
    }
}