public ActionResult Delete(int id)
        {
            using (TaskAllocationDBContext ctx = new TaskAllocationDBContext())
            {
                var user = ctx.Users.Where(u => u.Id == id).FirstOrDefault();
                if (user != null)
                {
                    ctx.Entry<User>(user).State = EntityState.Deleted;
                }
                else
                {
                    throw new Exception("user id is not exist:" + id);
                }

                ctx.SaveChanges();
            }

            return SuccessMessage(BasicMessage.DeleteSuccess);
        }
        private static void SaveUser(UserSaveObject userDTO)
        {
            if (string.IsNullOrWhiteSpace(userDTO.Name))
            {
                throw new ArgumentNullException("用户名为空。");
            }

            using (TaskAllocationDBContext ctx = new TaskAllocationDBContext())
            {
                if (userDTO.Id.HasValue)
                {
                    var user = ctx.Users.Where(u => u.Id == userDTO.Id).FirstOrDefault();
                    if (user != null)
                    {
                        user.Name = userDTO.Name;
                    }
                    else
                    {
                        throw new ArgumentException("user id is not exist:" + userDTO.Id);
                    }
                }
                else
                {
                    var existCount = ctx.Users.Where(u => u.Name == userDTO.Name).Count();
                    if (existCount > 0)
                    {
                        throw new ArgumentException("用户名" + userDTO.Name + "已存在。");
                    }

                    ctx.Users.Add(new User() { Name = userDTO.Name });
                }

                ctx.SaveChanges();
            }
        }