MVC错误:未将对象引用设置为对象的实例

Question

今天我已经接近放弃了这个mvc应用!

我正在关注Mvc音乐商店教程,我被困在第54页.

这是我得到的错误:

System.NullReferenceException:未将对象引用设置为对象的实例.

错误发生在以下代码中的第三个段落(下拉列表)中:

<%@ 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>

此ascx文件包含在Edit.aspx文件中:

<%@ 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>

我意识到那里有很多代码,但是如果有人能看到一些明显我做错的事情,我会非常感激.

编辑

StoreManagerController.cs(编辑)

 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; }
    }
}

我再次意识到我称之为MvcMovies1,这是一个错字,但所有内容都相应标记.

Answer 1

在那行中你是否Album有一个ArtistId调用Model.ArtistId,如果Album没有该属性,你将获得一个空引用异常.这是因为模型是对您的视图强类型的对象的简写,恰好Album在您的情况下.

您在上面的代码中没有设置ViewData ["Artists"]的位置.您是否在任何地方设置,因为这也可能是您的问题.

编辑

在操作中设置ViewData,它应该工作:

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

首先,您需要在视图模型中添加属性以保留选定的艺术家和所选类型:

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; }
}

然后在您的Edit.aspx视图中而不是:

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

你可以简单地说:

<%: Html.EditorForModel() %>

并在您的编辑器模板中~/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>

并在您的编辑器模板中~/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>