asp.net-mvc-4 – ASP.NET MVC 4 Razor视图无法识别Dropdownlist

@using System;
@model SampleApp.Models.ServiceRequestModel

@{
  ViewBag.Title = "ServiceRequest";
}

@Html.DropDownListFor(m=>m.CategoryID,Model.Categories)

模型如下：

public class ServiceRequestModel
{
    public int ID { get; set; }
    public int CategoryID { get; set; }

    public SelectList Category { get; set; }
}

它始终在CSHTML文件中的intellisense中显示错误：

System.Web.WebPages.Html.HtmlHelper’ does not contain a definition for ‘DropDownListFor’ and no extension method ‘DropDownListFor’ accepting a first argument of type ‘System.Web.WebPages.Html.HtmlHelper’ could be found (are you missing a using directive or an assembly reference?)

它也给出了以下错误：

Error 3 The name ‘model’ does not exist in the current context

我检查了View文件夹中的web.config：

<system.web.webPages.razor>
<host factoryType="System.Web.Mvc.MvcWebRazorHostFactory,System.Web.Mvc,Version=3.0.0.0,Culture=neutral,PublicKeyToken=31BF3856AD364E35" />
<pages pageBaseType="System.Web.Mvc.WebViewPage">
  <namespaces>
    <add namespace="System.Web.Mvc" />
    <add namespace="System.Web.Mvc.Ajax" />
    <add namespace="System.Web.Mvc.Html" />
    <add namespace="System.Web.Routing" />
  </namespaces>
</pages>