ASP.NET MVC Problem with validation - asp.net-mvc

Question

I have this simple controller:
public class OneController : Controller
{
[AcceptVerbs(HttpVerbs.Get)]
public ActionResult Create()
{
return View();
}
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Create(IList<TestModel> m)
{
return View(m);
}
}
And a very simple view with two objects of type TestModel, properly indexed.
When I submit the form with invalid data, I get the view with the errors highlighted.
However, when I re-submit it (without changing anything), I get this error:
[NullReferenceException: Object reference not set to an instance of an
object.]
System.Web.Mvc.DefaultModelBinder.UpdateCollection(ModelBindingContext
bindingContext, Type itemType) +612
System.Web.Mvc.DefaultModelBinder.BindModelCore(ModelBindingContext
bindingContext) +519
System.Web.Mvc.DefaultModelBinder.BindModel(ModelBindingContext
bindingContext) +829
System.Web.Mvc.ControllerActionInvoker.GetParameterValue(ParameterInfo
parameterInfo) +313
System.Web.Mvc.ControllerActionInvoker.GetParameterValues(MethodInfo
methodInfo) +399
System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext
controllerContext, String actionName)
+232
System.Web.Mvc.Controller.ExecuteCore()
+152
System.Web.Mvc.ControllerBase.Execute(RequestContext
requestContext) +86
System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext
requestContext) +28
System.Web.Mvc.MvcHandler.ProcessRequest(HttpContextBase
httpContext) +332
System.Web.Mvc.MvcHandler.ProcessRequest(HttpContext
httpContext) +55
System.Web.Mvc.MvcHandler.System.Web.IHttpHandler.ProcessRequest(HttpContext
httpContext) +28
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
+358
System.Web.HttpApplication.ExecuteStep(IExecutionStep
step, Boolean& completedSynchronously)
+64
Any idea on how can I debug this?

Answer 1

I was already looking at that article, and found the bug I was having (subtle, yet critical).
If you render the hidden field with the index using Html.Hidden, the helper will "accumulate" the previous values, so you'll end up with a hidden saying index=1, and the next saying index=1,2.
Changing the helper call to a manually coded hidden field fixed the issue.

Answer 2

Not sure I can answer without seeing more of the code and how your form is setup.
But, you could take a look at Phil Haack's blog entry about Model Binding To A List.Hope this helps.

Answer 3

Thanks that fixed it!
I replaced
<%= Html.Hidden("submitFormFields.index", controlID) %>
with
<input type="hidden" id="submitFormFields.index" name="submitFormFields.index" value="<%=controlID %>" />
Should we report this as a bug - It would be nice to have it fixed for ASP.Net MVC RC1