Question

Possible Duplicate:
MVC3 Razor (Drop Down List)

I’ve been getting “Object reference not set to an instance of an object.”. Been trying to figure out this error for the past 12 hours. Hope someone can help.

This is causing the error.

@Html.DropDownListFor(c => c.CategoryID, Model.CategoryTypeList)

In SearchController

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using Demo.Models;

namespace Demo.Controllers
{
public class SearchController : Controller
{
    //
    // GET: /Search/

    public ActionResult DisplayCategory()
    {
        var model = new SearchModel();
        model.CategoryTypeList = GetCategory();

        return View(model);
    }


    private List<SelectListItem> GetCategory()
    {
        List<SelectListItem> items = new List<SelectListItem>();
        items.Add(new SelectListItem { Text = "1", Value = "1" });
        items.Add(new SelectListItem { Text = "2", Value = "2" });
        return items;
    }

}
}

In SearchModel

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Web.Mvc;
using Demo.Controllers;

namespace Demo.Models
{
public class SearchModel
{
    public List<SelectListItem> CategoryTypeList { get; set; }

    [Display(Name = "Category")]
    public string CategoryID { get; set; }

}
}

In CSHTML

@model Demo.Models.SearchModel
@{
    ViewBag.Title = "Search";
}

<h2>Search</h2>
@using (Html.BeginForm())
{
<table>
<tr>
<td>@Html.LabelFor(c => c.CategoryID)</td>
<td>@Html.DropDownListFor(c => c.CategoryID, Model.CategoryTypeList)</td>
</tr>

</table>
}

Stack Trace

[NullReferenceException: Object reference not set to an instance of an object.]
ASP._Page_Views_Home_Search_cshtml.Execute() in c:\Users\User_me\Documents\Visual Studio 2010\Projects\Demo\Demo\Views\Home\Search.cshtml:12
System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +272
System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +67
System.Web.WebPages.StartPage.RunPage() +58
System.Web.WebPages.StartPage.ExecutePageHierarchy() +94
System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) +172
System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +574
System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +360
System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +409
System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) +39
System.Web.Mvc.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() +60
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) +391
System.Web.Mvc.<>c__DisplayClass1e.<InvokeActionResultWithFilters>b__1b() +61
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +285
System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +830
System.Web.Mvc.Controller.ExecuteCore() +136
System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +232
System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) +39
System.Web.Mvc.<>c__DisplayClassb.<BeginProcessRequest>b__5() +68
System.Web.Mvc.Async.<>c__DisplayClass1.<MakeVoidDelegate>b__0() +44
System.Web.Mvc.Async.<>c__DisplayClass8`1.<BeginSynchronous>b__7(IAsyncResult _) +42
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +141
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +54
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40
System.Web.Mvc.<>c__DisplayClasse.<EndProcessRequest>b__d() +61
System.Web.Mvc.SecurityUtil.<GetCallInAppTrustThunk>b__0(Action f) +31
System.Web.Mvc.SecurityUtil.ProcessInApplicationTrust(Action action) +56
System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +110
System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +38
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +8970061
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +184

Answer 1

Per your comments above, if you want to call this as a seperate unit within another page, then you will need to either pass the model in using Html.RenderPartial, or call Html.RenderAction in order to execute the entire action and output the resulting HTML.

Using Html.RenderPartial

@*
   Assuming that you have already initialized some variable
   called 'mySearchModel'
*@

@Html.RenderPartial("DisplayCategory", mySearchModel)

This will ensure that the Model property on the view gets set.

Using Html.RenderAction

//Slight change to your action method to ensure it returns
// a partial view, and will only ever be called as a child
// action of another action.
[ChildActionOnly]
public ActionResult DisplayCategory()
{
    var model = new SearchModel();
    model.CategoryTypeList = GetCategory();

    return PartialView(model);
}


@Html.RenderAction("DisplayCategory")

Further reading:

http://haacked.com/archive/2009/11/17/aspnetmvc2-render-action.aspx
http://devlicio.us/blogs/derik_whittaker/archive/2008/11/24/renderpartial-vs-renderaction.aspx