MVC Error: Object reference not set to an instance of an object.

Question

I'm close to giving up on this mvc app for today!!

I'm following the Mvc Music Store Tutorial and I'm stuck on page 54.

this is the error I'm getting:

System.NullReferenceException: Object reference not set to an instance of an object.

The error occurs in the third paragraph block (dropdownlist) in the following code:

<%@ Import Namespace ="MvcMovies1" %>
<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<MvcMovies1.Models.Album>" %>

<p>
    <%: Html.LabelFor(model => model.Title) %>
    <%: Html.TextAreaFor(model => model.Title) %>
    <%: Html.ValidationMessageFor(model => model.Title) %>
</p>

<p>
    <%: Html.LabelFor(model => model.Price) %>
    <%: Html.TextAreaFor(model => model.Price) %>
    <%: Html.ValidationMessageFor(model => model.Price) %>
</p>

<p>
    <%: Html.LabelFor(model => model.AlbumArtUrl) %>
    <%: Html.TextAreaFor(model => model.AlbumArtUrl) %>
    <%: Html.ValidationMessageFor(model => model.AlbumArtUrl) %>
</p>

<p>
    <%: Html.LabelFor(model => model.Artist) %>
    <%: Html.DropDownList("ArtistId", new SelectList(ViewData["Artists"] as IEnumerable, "ArtistId", "Name", Model.ArtistId)) %>
</p>

<p>
    <%: Html.LabelFor(model => model.Genre) %>
    <%: Html.DropDownList("GenreId", new SelectList(ViewData["Genres"] as IEnumerable, "GenreId", "Name", Model.GenreId)) %>
</p>

    <div>
        <%: Html.ActionLink("Back to List", "Index") %>
    </div>

This ascx file is contained within an Edit.aspx file:

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<MvcMovies1.ViewModels.StoreManagerViewModel>" %>
<asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
    Edit
</asp:Content>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
<form id="form1" runat="server">
    <h2>Edit</h2>
    <% using (Html.BeginForm())
       { %>
      <%: Html.ValidationSummary(true)%>

    <fieldset>
      <legend>Edit Album</legend>
      <%: Html.EditorFor(model => model.Album,
          new { Artists = Model.Artists, Genres = Model.Genres }) %>

          <p><input type="submit" value="Save" /></p>


    </fieldset>

      <% } %>
      </form>
</asp:Content>

I realise there's a lot of code there but if anyone can see something obvious that I am doing wrong I'd be very grateful.

EDIT

StoreManagerController.cs (Edit)

 public ActionResult Edit(int id)
    {
        var viewModel = new StoreManagerViewModel
        {
            Album = storeDB.Albums.SingleOrDefault(a => a.AlbumId == id),
            Genres = storeDB.Genres.ToList(),
            Artists = storeDB.Artists.ToList()
        };

        return View(viewModel);
    }

Andddd..StoreManagerViewModel.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using MvcMovies1.Models;

namespace MvcMovies1.ViewModels
{
    public class StoreManagerViewModel
    {
        public Album Album { get; set; }
        public List<Artist> Artists { get; set; }
        public List<Genre> Genres { get; set; }
    }
}

Again I realise i called it MvcMovies1, this was a typo but everything is marked up accordingly.

Answer 1

Does Album have an ArtistId since in that line you are calling Model.ArtistId and if Album doesn't have that property on it you will get a null reference exception. That's because the Model is a shorthand for the object that is strongly typed to your view, which happens to be Album in your case.

There is no where in your above code where you are setting the ViewData["Artists"]. Are you setting that anywhere since that could be your issue too.

EDIT

Set the ViewData in the action and it should work:

public ActionResult Edit(int id)
{
     var viewModel = new StoreManagerViewModel
     {
         Album = storeDB.Albums.SingleOrDefault(a => a.AlbumId == id),
         Genres = storeDB.Genres.ToList(),
         Artists = storeDB.Artists.ToList()
     };

     ViewData["Artists"] = storeDB.Artists.ToList();
     ViewData["Genres"] = storeDB.Genres.ToList();

     return View(viewModel);
 }

Answer 2

u r simply not setting ViewData["Artists"] and ViewData["genres"] as i can c from ur action method. make sure u r setting these values somewhere during lifetime of a request if u want to use them in ur view

Answer 3

First you need to add properties in your view model to hold selected artist and selected genre:

public class StoreManagerViewModel
{
    public Album Album { get; set; }
    public int? SelectedArtistId { get; set; }
    public List<Artist> Artists { get; set; }
    public int? SelectedGenreId { get; set; }
    public List<Genre> Genres { get; set; }
}

Then in your Edit.aspx view instead of:

<%: Html.EditorFor(model => model.Album,
    new { Artists = Model.Artists, Genres = Model.Genres }) %>

You could simply:

<%: Html.EditorForModel() %>

and in your editor template ~/Views/Home/EditorTemplates/Album.ascx:

<%@ Import Namespace ="MvcMovies1" %>
<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<MvcMovies1.Models.Album>" %>

<p>
    <%: Html.LabelFor(model => model.Title) %>
    <%: Html.TextAreaFor(model => model.Title) %>
    <%: Html.ValidationMessageFor(model => model.Title) %>
</p>

<p>
    <%: Html.LabelFor(model => model.Price) %>
    <%: Html.TextAreaFor(model => model.Price) %>
    <%: Html.ValidationMessageFor(model => model.Price) %>
</p>

<p>
    <%: Html.LabelFor(model => model.AlbumArtUrl) %>
    <%: Html.TextAreaFor(model => model.AlbumArtUrl) %>
    <%: Html.ValidationMessageFor(model => model.AlbumArtUrl) %>
</p>

and in your editor template ~/Views/Home/EditorTemplates/StoreManagerViewModel:

<%@ Import Namespace ="MvcMovies1" %>
<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<MvcMovies1.ViewModels.StoreManagerViewModel>" %>

<%: Html.EditorFor(model => model.Album) %>

<p>
    <%: Html.LabelFor(model => model.SelectedArtistId) %>
    <%: Html.DropDownListFor(model => model.SelectedArtistId, new SelectList(Model.Artists, "ArtistId", "Name")) %>
</p>

<p>
    <%: Html.LabelFor(model => model.SelectedGenreId) %>
    <%: Html.DropDownListFor(model => model.SelectedGenreId, new SelectList(Model.Genres, "GenreId", "Name")) %>
</p>

<div>
    <%: Html.ActionLink("Back to List", "Index") %>
</div>

Answer 4

Have you tried casting to List and List instead of IEnumerable?