C# 值不能为null。参数名称：项目MVC5（下拉列表）

我的模态

public class Employee
{
    [Key]
    public int ID { get; set; }

    [MaxLength(10,ErrorMessage ="Max Lenght 20")]
    public string Name { get; set; }
    [MaxLength(20,ErrorMessage ="Max Lenght 20")]
    public string Address { get; set; }
    public decimal Salary { get; set; }
    [DataType(DataType.DateTime)]
    [DisplayFormat(DataFormatString = "{0:dd/MM/yyyy}")]
    [Display(Name ="Date of Joining")]
    public DateTime Dateofjoin { get; set; }
    [DataType(DataType.EmailAddress)]
    public string Email { get; set; }
    public List<CityModal> City { get; set; }
    [Display(Name="City")]
    public int CityId { get; set; }
    public string CityName { get; set; }


}
public class CityModal {
    public string CityId { get; set; }
    public string CityName { get; set; }
}

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在这里，我检查了控制器查询是否有结果，并且包含CityId，但视图显示“值不能为null。参数名称：items”。我认为create方法遇到了一些我没有遇到的问题。需要帮助。

ViewBag.city

是在您的

Create

操作的GET版本中设置的，但我想您可能忽略了在POST版本中再次设置它。但是，您已经发布了相应的代码。此外，它也不会在

编辑

操作中设置。这可能与当前的问题无关，但很可能很快就会成为问题本身

---

通常，例外情况是告诉您

项

参数（即

选择列表

构造函数的第一个参数）为空。如果在您的操作中未设置

ViewBag.city

，则会出现这种情况。由于它在GET版本中，我只能假设当再次返回视图以允许用户进行必要的编辑时，您在验证错误时收到此异常。

ViewBag.city已填充，但下拉列表中没有值？这意味着

ViewBag.city

的值为

null

<div class="form-group">
                @Html.LabelFor(model => model.CityId, htmlAttributes: new { @class = "control-label col-md-2" })
                <div class="col-md-10">
                    @Html.DropDownListFor(model => model.CityId, new SelectList(ViewBag.city, "CityId", "CityName"), "Select")
                    @Html.ValidationMessageFor(model => model.CityId, "Please select the City", new { @class = "error" })
                </div>
            </div>

[HttpGet]
    public ActionResult Create()
    {
        using (RegMVCEntities obj = new RegMVCEntities())
        {
            ViewBag.city = obj.tblCities.Select(e => new CityModal
            {

                CityName = e.City,
                CityId = e.ID                    
            }).ToList();
        }
            return View();
    }
 [HttpGet]
    public ActionResult Edit(int id)
    {
        using (RegMVCEntities obj = new RegMVCEntities())
        {
            var query = (from reg in obj.tblRegistrations
                         join citi in obj.tblCities on reg.CityID equals citi.ID
                         where reg.ID == id
                         select new Employee
                         {
                             ID = reg.ID,
                             Name = reg.Name,
                             Address = reg.Address,
                             Salary = reg.Salary,
                             Dateofjoin = reg.DateOfJoining,
                             Email = reg.Email,
                             CityId = citi.ID,
                             CityName = citi.City
                         }).FirstOrDefault();

            /*var employee = obj.tblRegistrations.Where(m => m.ID == id).Select(e => new Employee {
                ID = e.ID,Name = e.Name, Address = e.Address, Dateofjoin = e.DateOfJoining, Salary = e.Salary, Email = e.Email
            }).FirstOrDefault();*/
            return View(query);
        }                
    }