当我在视图模式下使用DropDownListFor时，[Required（ErrorMessage =“”）]无效

Question

我发现如果我在视图模式下使用DropDownListFor，变量的[Required（ErrorMessage =“”）]将无法在表单中使用。

型号：

    [Key]
    [Required(ErrorMessage = "Pls input id")]
    public int StaffId { get; set; }

    [DisplayName("UserType")]
    public int UserTypeId { get; set; }

控制器：

    public ActionResult Create()
    {

        List<SelectListItem> listItem = new List<SelectListItem>();
        listItem.Add(new SelectListItem { Text = "A", Value = "1" });
        listItem.Add(new SelectListItem { Text = "B", Value = "2" });
        listItem.Add(new SelectListItem { Text = "C", Value = "3" });
        ViewData["List"] = new SelectList(listItem, "Value", "Text", "");

        return View();
    }

查看：

<div class="form-horizontal">
    <hr />
    @Html.ValidationSummary(true, "", new { @class = "text-danger" })
    <div class="form-group">
        @Html.LabelFor(model => model.StaffId, "StaffId", htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.StaffId, new { htmlAttributes = new { @class = "form-control" },  })
            @Html.ValidationMessageFor(model => model.StaffId, "", new { @class = "text-danger" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.UserTypeId, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.DropDownListFor(model => model.UserTypeId, ViewData["List"] as SelectList, "請選擇", htmlAttributes: new { @class = "form-control" })
            @*@Html.EditorFor(model => model.UserTypeId, new { htmlAttributes = new { @class = "form-control" } })*@
            @Html.ValidationMessageFor(model => model.UserTypeId, "", new { @class = "text-danger" })
        </div>
    </div>

如果StaffId为null，则它将在Microsoft Visual Studio中返回错误

Answer 1

  

如果StaffId为空.....

使用您显示的代码，视图模型的StaffId属性（假设您使用的对象作为操作方法的参数）将永远不会为null，因为它不可为空。如果用户没有选择除默认标签以外的任何内容，则它将为0（默认的int值）。

现在关于错误，这通常发生在您发布表单时，并且在httppost操作方法中您返回相同的视图而不重新加载填充ViewData下拉列表所需的相关集合。因此，请确保在返回视图之前重新加载集合。

[HttpPost]
public ActionResult Index(YourViewModel model)
{
  if(ModelState.IsValid)
  {
       // to do :Save and Redirect ( PRG pattern)
  }
  // Let's reload the collection needed to render the dropdown
  var listItem = new List<SelectListItem>(){
    new SelectListItem { Text = "A", Value = "1" },
    new SelectListItem { Text = "B", Value = "2" }
  };
  ViewData["List"] = new SelectList(listItem, "Value", "Text", "");
  return View(model);
}