asp.net-mvc - ASP.NET MVC 验证针对空列表框抛出 NullReferenceException

Question

我有两个列表框，一个用于可用服务器，一个用于选定服务器。用户将项目从第一个框移动到第二个框以指示他们的选择。

<table id="ServerLists" style="width: 100%;">
  <tr>
    <td style="width: 40%;">
      <%=Html.ListBox("AvailableServerList", Model.AvailableServerList, New With {.size = 8, .ondblclick = "addServer();"})%>
    </td>
    <td style="width: 20%;">
      <button type="button" onclick="addAll();">Add All &gt;&gt;</button>
      <button type="button" onclick="addServer();">Add &gt;</button>
      <button type="button" onclick="removeServer();">&lt; Remove</button>
      <button type="button" onclick="removeAll();">&lt;&lt; Remove All</button>
    </td>
    <td style="width: 40%;">
      <%=Html.ListBox("SelectedServerList", Model.SelectedServerList, New With {.size = 8, .ondblclick = "removeServer();"})%>
    </td>
  </tr>
</table>

如果没有服务器添加到第二个列表框，我会向 ModelState 添加错误。

问题是，如果列表框中没有条目，则在渲染带有验证警告的 View 时，MVC 会抛出 NullReferenceException。

[NullReferenceException: Object reference not set to an instance of an object.]
   System.Web.Mvc.HtmlHelper.GetModelStateValue(String key, Type destinationType) +63  
   System.Web.Mvc.Html.SelectExtensions.SelectInternal(HtmlHelper htmlHelper, String optionLabel, String name, IEnumerable`1 selectList, Boolean allowMultiple, IDictionary`2 htmlAttributes) +155  
   System.Web.Mvc.Html.SelectExtensions.ListBox(HtmlHelper htmlHelper, String name, IEnumerable`1 selectList, Object htmlAttributes) +62  
   ASP.views_serveraccess_create_aspx.__RendercMain(HtmlTextWriter __w, Control parameterContainer) in C:\Documents and Settings\nfoster\My Documents\Visual Studio Projects\Client Portal 3\Trunk\src\Stargate3.Web\Views\ServerAccess\Create.aspx:148  
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +256  
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19  
   System.Web.UI.Control.Render(HtmlTextWriter writer) +10  
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +134
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
   System.Web.UI.Control.Render(HtmlTextWriter writer) +10
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   ASP.views_shared_site_master.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in C:\Documents and Settings\nfoster\My Documents\Visual Studio Projects\Client Portal 3\Trunk\src\Stargate3.Web\Views\Shared\Site.Master:97
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +256
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
   System.Web.UI.Control.Render(HtmlTextWriter writer) +10
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +134
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
   System.Web.UI.Control.Render(HtmlTextWriter writer) +10
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +134
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
   System.Web.UI.Page.Render(HtmlTextWriter writer) +29
   System.Web.Mvc.ViewPage.Render(HtmlTextWriter writer) +57
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1266

还有其他人遇到过这个问题吗？有什么我可以尝试的解决方法吗？如果没有做出任何选择，我可能必须在其中插入一个[请选择服务器]条目，但如果能修复此问题，我们将不胜感激。

编辑:根据要求添加 Controller 操作

<AcceptVerbs(HttpVerbs.Post)> _
Function Create(ByVal collection As FormCollection) As ActionResult
  Dim acceptedFields() As String = {"StartTime", "EndTime", "ReasonForAccess", "WorkToBeDone", "RegressionPlan", "Servers", "AccessAccount", "Password"}
  Dim accessRequest As New ServerAccessRequest
  UpdateModel(Of ServerAccessRequest)(accessRequest, acceptedFields)
  If accessRequest.IsValid Then
    'TODO: Do database stuff here
    Return View("AccessRequestConfirmation")
  Else
    For Each violation In accessRequest.GetRuleViolations
      ModelState.AddModelError(violation.PropertyName, violation.ErrorMessage)
    Next
  End If
  Return View(New ServerAccessViewModel(accessRequest))
End Function

Answer 1

请参阅release notes适用于 ASP.NET MVC 2 预览版 1；显然，这个(将 null 传递到模型状态时出现 NullReferenceException)是 ASP.NET MVC 中的一个已知错误。