C# MVC下拉列表项目问题

我是MVC的新手。我正在创建用户管理模块，用于验证和授权使用该应用程序的用户。因此，我创建了两个模型，一个用于用户角色，另一个用于用户详细信息

我创建了两个这样的模型：

public class RolesModels
{
    public RolesModels()
    {
        this.Users = new HashSet<UserModels>();
    }

    [Key]
    [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
    public int RoleId { get; set; }

    [Required]
    [DataType(DataType.Text)]
    [StringLength(20, ErrorMessage = "The {0} must be at least 6 characters long.", MinimumLength = 6)]
    [Display(Name = "Caption")]
    public string Caption { get; set; }

    [Display(Name = "Can Create")]
    public bool createRole { get; set; }

    [Display(Name = "Can View")]
    public bool viewRole { get; set; }

    [Display(Name = "Can Modify")]
    public bool modifyRole { get; set; }

    [Display(Name = "Can Delete")]
    public bool deleteRole { get; set; }

    public virtual ICollection<UserModels> Users { get; set; }
}

public class UserModels
{
    [Key]
    [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]

    public int user_id { get; set; }

    [Required]
    [DataType(DataType.Text)]
    [Display(Name = "User Name")]
    [StringLength(20, ErrorMessage = "The {0} must be at least 3 characters long.", MinimumLength = 3)]
    public string user_name { get; set; }

    [Required]
    [DataType(DataType.Password)]
    [Display(Name = "Password")]
    [StringLength(10, ErrorMessage = "The {0} must be at least 4 characters long.", MinimumLength = 4)]
    public string user_pass { get; set; }

    [DataType(DataType.EmailAddress)]
    [Display(Name = "Email")]
    [StringLength(50, ErrorMessage = "The {0} must be at least 6 characters long.", MinimumLength = 6)]
    public string UserEmail { get; set; }

    [Display(Name = "Registeration Date")]
    [DataType(DataType.Date)]
    [DisplayFormat(DataFormatString = "{0:dd-MM-yyyy}", ApplyFormatInEditMode = true)]
    public DateTime RegDate { get; set; }

    [Display(Name = "Enable")]
    public bool status { get; set; }

    [Required]
    public int RoleId { get; set; }

    [Display(Name = "Roles")]
    public virtual RolesModels Roles { get; set; }
}

[HttpGet]
public ActionResult registration()
{
   // var md = from m in db.RolesModels
   // orderby m.Caption descending
   // select m;
   // Html.DropDownList("Roles", new SelectList(ViewBag.Roles), new { @id = "dropdown" });
   ViewBag.Roles = db.RolesModels.Select(r => r.Caption);
   ViewBag.RolesId = db.RolesModels.Select(r => r.RoleId);

   return View();
}

@Html.DropDownList("RoleId", new SelectList(ViewBag.Roles), new { @id = "dropdown" })

我使用dropdownlist创建了一个注册视图，如下所示：

public class RolesModels
{
    public RolesModels()
    {
        this.Users = new HashSet<UserModels>();
    }

    [Key]
    [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
    public int RoleId { get; set; }

    [Required]
    [DataType(DataType.Text)]
    [StringLength(20, ErrorMessage = "The {0} must be at least 6 characters long.", MinimumLength = 6)]
    [Display(Name = "Caption")]
    public string Caption { get; set; }

    [Display(Name = "Can Create")]
    public bool createRole { get; set; }

    [Display(Name = "Can View")]
    public bool viewRole { get; set; }

    [Display(Name = "Can Modify")]
    public bool modifyRole { get; set; }

    [Display(Name = "Can Delete")]
    public bool deleteRole { get; set; }

    public virtual ICollection<UserModels> Users { get; set; }
}

public class UserModels
{
    [Key]
    [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]

    public int user_id { get; set; }

    [Required]
    [DataType(DataType.Text)]
    [Display(Name = "User Name")]
    [StringLength(20, ErrorMessage = "The {0} must be at least 3 characters long.", MinimumLength = 3)]
    public string user_name { get; set; }

    [Required]
    [DataType(DataType.Password)]
    [Display(Name = "Password")]
    [StringLength(10, ErrorMessage = "The {0} must be at least 4 characters long.", MinimumLength = 4)]
    public string user_pass { get; set; }

    [DataType(DataType.EmailAddress)]
    [Display(Name = "Email")]
    [StringLength(50, ErrorMessage = "The {0} must be at least 6 characters long.", MinimumLength = 6)]
    public string UserEmail { get; set; }

    [Display(Name = "Registeration Date")]
    [DataType(DataType.Date)]
    [DisplayFormat(DataFormatString = "{0:dd-MM-yyyy}", ApplyFormatInEditMode = true)]
    public DateTime RegDate { get; set; }

    [Display(Name = "Enable")]
    public bool status { get; set; }

    [Required]
    public int RoleId { get; set; }

    [Display(Name = "Roles")]
    public virtual RolesModels Roles { get; set; }
}

[HttpGet]
public ActionResult registration()
{
   // var md = from m in db.RolesModels
   // orderby m.Caption descending
   // select m;
   // Html.DropDownList("Roles", new SelectList(ViewBag.Roles), new { @id = "dropdown" });
   ViewBag.Roles = db.RolesModels.Select(r => r.Caption);
   ViewBag.RolesId = db.RolesModels.Select(r => r.RoleId);

   return View();
}

@Html.DropDownList("RoleId", new SelectList(ViewBag.Roles), new { @id = "dropdown" })

我的HTTPPost在这里

  [HttpPost]
    public ActionResult registration(Solnet_HotelSuite.Models.UserModels user)
    {
        if (ModelState.IsValid)
        {
            using (var db = new DBEntity()) 
            {
                var sysUser = db.UserModels.Create();
                sysUser.user_name = user.user_name;
                sysUser.user_pass = user.user_pass;
                sysUser.UserEmail = user.UserEmail;
                sysUser.RegDate = user.RegDate;
                sysUser.RoleId = user.RoleId;
                //sysUser.Roles = user.Roles;
                sysUser.status = user.status;
                db.UserModels.Add(sysUser);
                db.SaveChanges();
            }
        }

        return View(user);
    }

但是，每当我单击Register user时，就会出现错误：

值不能为null。参数名称：items

---

我做错了什么？

回发后，您放在viewbag中的项目丢失了。尝试将HttpPost代码的结尾更改为：

ViewBag.Roles = db.RolesModels.Select(r => r.Caption);
ViewBag.RolesId = db.RolesModels.Select(r => r.RoleId);

return View(user);

---

您可能需要调整此代码，因为此时

db

对象可能超出范围。

注册用户是按钮吗？您的列表是否已填充？是的，列表已填充，“注册用户”是一个按钮。我还注意到，当我检查生成的html时，select标记在选项中没有值，只有文本属性。所以，你放了Get控制器，但你说问题在于Post控制器…你的Post在哪里？哦！我刚刚添加了HTTPPost操作，请更清楚地说明这一点。在将代码调整为suite后，我得到了“ObjectContext实例已被释放，不能再用于需要连接的操作”。我该怎么办？