當沒有將模型類的實例傳遞給視圖時，如何避免Create.cshtml中的EditorFor（x = x.Prop）生成的null異常？

Question

約束：

1. 我不想使用Nullable類型。
2. 我不想在HttpGet Create操作方法中實例化一個模型類。

型號類：

namespace MvcTest.Models
{
    public enum Gender { Male, Female }
    public class Person
    {
        public Gender Gender { get; set; }
    }
}

模板編輯助手（Gender.cshtml）：

@using MvcTest.Models
@model Gender
@{
    var genders = from Gender g in Enum.GetValues(typeof(Gender))
                  select new { Value = g, Text = g.ToString() };

    var sl = new SelectList(genders, "Value", "Text", Model);
}
@Html.DropDownList(string.Empty, sl, "--Select--")

控制器：

using System.Web.Mvc;
using MvcTest.Models;

namespace MvcTest.Controllers
{
    public class HomeController : Controller
    {
        public ActionResult Create()
        {
            //Person p = new Person();
            //return View(p);

            return View();
        }

    }
}

視圖：

@model MvcTest.Models.Person
@using (Html.BeginForm())
{

    @Html.EditorFor(x => x.Gender)
    <p>
        <input type="submit" value="Create" />
    </p>
}

錯誤：

Server Error in '/' Application.

The model item passed into the dictionary is null, but this dictionary requires a non-null model item of type 'MvcTest.Models.Gender'.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.InvalidOperationException: The model item passed into the dictionary is null, but this dictionary requires a non-null model item of type 'MvcTest.Models.Gender'.

Source Error: 


Line 3:  {
Line 4:      
Line 5:      @Html.EditorFor(x => x.Gender)
Line 6:      <p>
Line 7:          <input type="submit" value="Create" />

Source File: e:\MvcTest\MvcTest\Views\Home\Create.cshtml    Line: 5 

Stack Trace: 


[InvalidOperationException: The model item passed into the dictionary is null, but this dictionary requires a non-null model item of type 'MvcTest.Models.Gender'.]
   System.Web.Mvc.ViewDataDictionary`1.SetModel(Object value) +143
   System.Web.Mvc.ViewDataDictionary..ctor(ViewDataDictionary dictionary) +377
   System.Web.Mvc.WebViewPage`1.SetViewData(ViewDataDictionary viewData) +27
   System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +99
   System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +115
   System.Web.Mvc.Html.TemplateHelpers.ExecuteTemplate(HtmlHelper html, ViewDataDictionary viewData, String templateName, DataBoundControlMode mode, GetViewNamesDelegate getViewNames, GetDefaultActionsDelegate getDefaultActions) +584
   System.Web.Mvc.Html.TemplateHelpers.TemplateHelper(HtmlHelper html, ModelMetadata metadata, String htmlFieldName, String templateName, DataBoundControlMode mode, Object additionalViewData, ExecuteTemplateDelegate executeTemplate) +1027
   System.Web.Mvc.Html.TemplateHelpers.TemplateHelper(HtmlHelper html, ModelMetadata metadata, String htmlFieldName, String templateName, DataBoundControlMode mode, Object additionalViewData) +66
   System.Web.Mvc.Html.TemplateHelpers.TemplateFor(HtmlHelper`1 html, Expression`1 expression, String templateName, String htmlFieldName, DataBoundControlMode mode, Object additionalViewData, TemplateHelperDelegate templateHelper) +118
   System.Web.Mvc.Html.TemplateHelpers.TemplateFor(HtmlHelper`1 html, Expression`1 expression, String templateName, String htmlFieldName, DataBoundControlMode mode, Object additionalViewData) +100
   System.Web.Mvc.Html.EditorExtensions.EditorFor(HtmlHelper`1 html, Expression`1 expression) +57
   ASP._Page_Views_Home_Create_cshtml.Execute() in e:\MvcTest\MvcTest\Views\Home\Create.cshtml:5
   System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +207
   System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +81
   System.Web.WebPages.StartPage.RunPage() +19
   System.Web.WebPages.StartPage.ExecutePageHierarchy() +65
   System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) +76
   System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +220
   System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +115
   System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +303
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) +13
   System.Web.Mvc.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() +23
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) +260
   System.Web.Mvc.<>c__DisplayClass1e.<InvokeActionResultWithFilters>b__1b() +19
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +177
   System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +343
   System.Web.Mvc.Controller.ExecuteCore() +116
   System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +97
   System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) +10
   System.Web.Mvc.<>c__DisplayClassb.<BeginProcessRequest>b__5() +37
   System.Web.Mvc.Async.<>c__DisplayClass1.<MakeVoidDelegate>b__0() +21
   System.Web.Mvc.Async.<>c__DisplayClass8`1.<BeginSynchronous>b__7(IAsyncResult _) +12
   System.Web.Mvc.Async.WrappedAsyncResult`1.End() +62
   System.Web.Mvc.<>c__DisplayClasse.<EndProcessRequest>b__d() +50
   System.Web.Mvc.SecurityUtil.<GetCallInAppTrustThunk>b__0(Action f) +7
   System.Web.Mvc.SecurityUtil.ProcessInApplicationTrust(Action action) +22
   System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +60
   System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +8862381
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +184

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.225

問題：如果沒有將模型類的實例傳遞給視圖，如何避免Create.cshtml中的EditorFor（x = x.Prop）生成的null異常？

Answer 1

AFAIK你需要使用可空的枚舉來實現這一目標。 它也有意義，因為當未選擇值（ --SELECT-- ）時，枚舉必須為空。 這是一篇關於有用擴展方法的好文章 。