DropDownListFor抛出ArgumentNullException

Question

不知道这是怎么回事。

我的控制器方法看起来像这样：

[HttpGet]
public ActionResult RequestAppointment()
{
    var appointmentRequest = new AppointmentRequest
    {
        Stylists = _repository.Stylists // <-- Debugging shows that Stylists IS being populated here
    };

    return View(appointmentRequest);
}

[HttpPost]
public ActionResult RequestAppointment(AppointmentRequest appointmentRequest)
{
    if(ModelState.IsValid)
    {
        // Process...
        return RedirectToAction("Confirmation");
    }
    return View(appointmentRequest);
}

表单如下所示：

@model MyDomain.Models.AppointmentRequest

@using(Html.BeginForm("RequestAppointment", "Appointment" FormMethod.Post))
{
    // This following line throws the exception:
    @Html.DropDownListFor(x => x.Stylist,
        Model.Stylists.Select(x => new SelectListItem{ Text = x.Name, Value = x.Name })))

    <input type="submit" value="Make Request" />
}

下拉列表中填充了正确的文本和值。但是当表单提交时，就会抛出异常。到底怎么回事？

堆栈跟踪：

[ArgumentNullException: Value cannot be null.
Parameter name: source]
   System.Linq.Enumerable.Select(IEnumerable`1 source, Func`2 selector) +6396316
   ASP._Page_Views_Appointment_RequestAppointment_cshtml.Execute() in c:\Projects\OasisSalon\OasisSalon.Mvc\Views\Appointment\RequestAppointment.cshtml:9
   System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +280
   System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +104
   System.Web.WebPages.StartPage.ExecutePageHierarchy() +143
   System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) +157
   System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +384
   System.Web.Mvc.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() +33
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) +825460
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +265
   System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +825488
   System.Web.Mvc.Controller.ExecuteCore() +159
   System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +335
   System.Web.Mvc.<>c__DisplayClassb.<BeginProcessRequest>b__5() +62
   System.Web.Mvc.Async.<>c__DisplayClass1.<MakeVoidDelegate>b__0() +20
   System.Web.Mvc.<>c__DisplayClasse.<EndProcessRequest>b__d() +54
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +469
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +375

Answer 1

如果要重新显示相同的视图，则必须在POST操作中初始化Stylists属性：

[HttpPost]
public ActionResult RequestAppointment(AppointmentRequest appointmentRequest)
{
    if(ModelState.IsValid)
    {
        // Process...
        return RedirectToAction("Confirmation");
    }

    appointmentRequest.Stylists = _repository.Stylists;
    return View(appointmentRequest);
}

您会得到一个NRE，因为您试图在视图中使用Model.Stylists呈现视图中的DropDownList，但是很明显，在执行POST操作之后，此属性为null，因为您从未对其赋值，并且它的值不会自动填充，因为发送到POST请求的所有内容都是下拉列表( Stylist属性)的选定值。

Answer 2

我的心理调试技能告诉我，您在没有创建模型的情况下从POST操作返回View()。

Answer 3

ASP.NET MVC没有ViewState。因此，它不能像ASP.ENT Webforms那样跨回发保留DropDown列表的值。所以我猜，在HttpPost操作方法中，您将样式返回给视图，但是appointmentRequest属性为空。在将appointmentRequest返回到视图之前，您可能需要重新加载它。