ASP.NET MVC Problem with validation

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?

pgb pgb · Accepted Answer · 2008-11-06T18:55:29

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.

ASP.NET MVC Problem with validation

3 Answers