ASP.NET MVC Validation throws NullReferenceException against empty listbox.

Question

I have two listboxes, one for available servers and one for selected servers. A user moves items from the first box to the second to indicate their selection.

<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>

If no servers are added to the second listbox I add an error to the ModelState.

The problem is that if there are no entries in the listbox MVC throws a NullReferenceException when rendering the view with the validation warnings.

[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

Has anyone else had this issue? Are there any workarounds I can try? I'm probably going to have to insert a [please select a server] entry in there if no choices have been made, but a fix for this problem would be appreciated.

EDIT: Added controller action as requested

<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

Is your Model.SelectedServerList a null value? Model.AvailableServerList is probably a collection of objects, but I don't know if Model.SelectedServerList is not null.

If all else fails, you can do like you say and add a dummy "Please Select a Server" option to the second list.

EDIT: I'd recommend switching to a 'strongly typed method signature' (what I call it anyways) - aka:

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Create(ServerAccessViewModel data)
{
    // Do stuff

    return View(data);
}

Looking at the control execution, it seems that if the validation succeeds, you are returning a different view to the user, whereas when the validation fails, you are passing back a new instance of the strongly typed view. Are you having to instantiate any components of your view object on the initial view?

EDIT2: I haven't been able to reproduce your exact error message. In the mean time though, I'd say using a foreach loop instead of the ListBox would let you better control the output and prevent the exception.

Answer 2

See the release notes for ASP.NET MVC 2 Preview 1; apparently this (NullReferenceException when passing null into model state) is a know bug in ASP.NET MVC.

Answer 3

I think this post will help you out (it fixed my problem which was very similar to yours):

http://www.crankingoutcode.com/?aspxerrorpath=/2009/02/01/IssuesWithAddModelErrorSetModelValueWithMVCRC1.aspx

The take away is that for some scenarios (it appears that validation errors for Html.ListBox is one of them) you need to make a call to ModelState.SetModelValue()

ModelState.SetModelValue("EmailListDisplay", new ValueProviderResult(registrationValues.EmailListDisplay, "", null));

the ValueProviderResult takes care of the NullReference and you will get your validation messages on the page.

It's odd that the Html.TextBox entries don't seem to need this code.

Anyways I hope that helps you.

-Ed