asp.net-mvc-3 – 数据注释MVC3必需属性

我有下面的模型(用户),我用它来添加新用户和更新现有用户.
当我添加新用户时,需要输入用户名和密码,当我更新时,只需要输入用户名,因为不允许更改密码.这是问题,添加一个新用户一切正常,因为我输入了名称和密码值,因此ModelState.IsValid返回true,但更新用户时没有输入密码,所以它始终具有null值是什么使得ModelState.IsValid始终返回false.
有没有办法使用相同的模型,即使在添加视图中需要密码,在更新视图中它不是？
拜托,有什么建议吗？

public class User {

        public int ID { get; set; }

        [Display(Name = "Nome do Usuário")]
        [Required(ErrorMessage = "Digite o Nome do Usuário.")]
        public string name { get; set; }

        [Display(Name = "Senha")]
        [Required(ErrorMessage = "Digite a Senha.")]
        public string password { get; set; }

    }



public ActionResult Add()
        {
            return View();
        }

      [HttpPost]
            public ActionResult Add(User user)
            {
                UsuariosViewModel viewModel = new UsuariosViewModel();


                if (ModelState.IsValid)
                {
                    viewModel.Msg = new AdmUsuarios().CadastraUsuario(user);
                }
                return View(viewModel);
            }



    public ActionResult Update(int id)
            {
                UsuariosViewModel viewModel = new UsuariosViewModel();

            using (var dataContext = new DBEntities()) 
            {
                Repository<User> repository = new Repository<User>(dataContext);
                viewModel.User = repository.GetById(id);
            }

                return View(viewModel);
            }

        [HttpPost]
        public ActionResult Update(User user,int id)
        {          
            UsuariosViewModel viewModel = new UsuariosViewModel();

            if (ModelState.IsValid)
            {
                viewModel.Msg = new AdmUsuarios().AlteraUsuario(user,id);
            }
            return View();
        }

public class CreateViewModel
{
  public int ID { get; set; }

  [Display(Name = "Nome do Usuário")]
  [Required(ErrorMessage = "Digite o Nome do Usuário.")]
  public string name { get; set; }

  [Display(Name = "Senha")]
  [Required(ErrorMessage = "Digite a Senha.")]
  public string password { get; set; }
}

public class EditViewModel
    {
      public int ID { get; set; }

      [Display(Name = "Nome do Usuário")]
      [Required(ErrorMessage = "Digite o Nome do Usuário.")]
      public string name { get; set; }

      //perhaps you don't need the password at all in the edit view
    }