MVC表单DropDownListFor

Question

在我看来，我正在构建一个表单，其中用户从国家/地区列表中选择一个国家/地区。我收到关于对象引用未设置为对象实例的错误信息。我是不是遗漏了什么？

                    @Html.DropDownListFor(
                            x => x.selectedCountryId,
                            new SelectList(Model.ListOfCountries, "Value", "Text"),
                            "-- please select a Country--",
                            new { id = "ddlCountry", @class = "form-control" }
                    )
                    @Html.ValidationMessageFor(x => x.selectedCountryId)

模型

 [Required]
        public int? selectedCountryId{ get; set; }

错误

System.NullReferenceException: 'Object reference not set to an instance of an object.'

System.Web.Mvc.WebViewPage<TModel>.Model.get returned null.

动作方法

 public ActionResult Create(RequestFormViewModel model)
        {
            if (!ModelState.IsValid)
            {
                
            }

            return View(TemplateName, "");

    public ActionResult Index()
    {
        RequestFormViewModel  result = new RequestFormViewModel ();

        try
        {
            var FormInfo = GetFormInfo();

            if (FormInfo != null)
            {
                result = FormInfo;


            }
        }
        catch (Exception e)
        {

        }

        return View(TemplateName, result);
    }

Answer 1

您的操作方法应该如下所示

[HttpGet]
    public IActionResult Create()
    {
        //Way 1
        RequestFormViewModel model = new RequestFormViewModel();
        model.ListOfCountries = //Select from DB or Whatever
        return View(model);

        //Way 2
        ViewBag.ListOfCountries = //Select from DB or Whatever
        return View();
    }

    [HttpPost]
    [ValidateAntiForgeryToken]
    public async Task<IActionResult> Create(RequestFormViewModel model)
    {
        if (ModelState.IsValid)
        {
        }
        return View(model);
    }

对于视图，如果您将使用方法1，因此需要将ListOfCountries作为NotMapped添加到模型中，或者为way2的此视图创建Dto，则需要从ViewBag绑定dropdown，而不是使用ListOfCountries作为属性