DropDownList必需不使用ViewModel

Question

我在SO中提到了类似的问题，但无法对此进行排序。所以请不要认为是重复的。 Link1 Link2

ViewModel：

public class UserAssignRoleViewModel
    {

        [Required(ErrorMessage = "Please select a User")]
        public string Username { get; set; }
        [Required(ErrorMessage = "Please select a Role")]
        public string Role { get; set; }

        public IEnumerable<SelectListItem> Users { get; set; }

        public IEnumerable<SelectListItem> Roles { get; set; }

    }

Controller ActionMethod

 [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult RoleAddToUser(string UserName, string Role)
        {
            AppUser user = context.Users.Where(u => u.UserName.Equals(UserName, StringComparison.CurrentCultureIgnoreCase)).FirstOrDefault();
            var userManager = new UserManager<AppUser>(new UserStore<AppUser>(context));

            userManager.AddToRole(user.Id, Role);

            ViewBag.ResultMessage = "Role Assigned successfully !";


            var model = new UserAssignRoleViewModel();
            var users = context.Users.ToList();
            model.Users = users.Select(s => new SelectListItem
            {
                Value = s.UserName,
                Text = s.UserName
            });
            var roles = context.Roles.ToList();
            model.Roles = roles.Select(s => new SelectListItem
            {
                Value = s.Name,
                Text = s.Name
            });

            return View(model);
        }

查看

@model ERPLite.Models.UserAssignRoleViewModel
@using (Html.BeginForm("RoleAddToUser", "Role"))
{
    @Html.AntiForgeryToken()
    @Html.ValidationSummary(true)


    <p>

        @Html.ValidationMessageFor(model => model.Username)
        User Name: @Html.DropDownListFor(u =>u.Username , (IEnumerable<SelectListItem>)Model.Users, "--Select--")
        @Html.ValidationMessageFor(model => model.Role)
        Role Name: @Html.DropDownListFor(r =>r.Role, (IEnumerable<SelectListItem>)Model.Roles, "--Select-- ")

    </p>

    <input type="submit" value="Save" />
}

单击按钮时，验证永远不会触发。 感谢任何帮助，提前感谢。