MVC MusicStore Artist.Name对象引用不设置到对象的实例 [英] MVC MusicStore Artist.Name Object reference not set to an instance of an object
问题描述
破解我的头，同时通过MVC MusicStore学习MVC。得到这个错误的Model.Artist.Name的详细信息查看页面。
我的Storecontroller详细方法应该罚款。
公众的ActionResult详细信息（INT ID）
{
//返回和专辑从ID搜索
VAR专辑= storeDB.Albums.Find（ID）;
返回查看（专辑）;
}
这是我怎么输出的看法
＆LT;李＆GT;价格：＆LT;％= Model.Price％＆GT;＆LT; /李＆GT;
＆LT;李＆GT;艺术家：＆LT;％= Model.Artist.Name％＆GT;＆LT; /李＆GT;
工作正常的价格，而且只显示错误Model.Genre.Name和Artist.Name。我怀疑在声明中sampledata.cs这些特性导致出现此问题
艺术家= artists.Single（A =＆GT; a.Name ==柯普兰和放大器;伦敦交响乐团）
但我这方面的知识太薄弱了，补什么。请帮忙 。
好了，该值是从获取类文件看起来像这样
保护覆盖无效种子（MusicStoreEntities上下文）
{
VAR艺术家=新的名单，其中，艺术家＆GT;
{
新人{名称=柯普兰和放大器;伦敦交响乐团}，
etcetc
}
新名单，其中，相册和GT;
{
新专辑{标题=ABC，艺术家= artists.Single（G =＆GT; g.Name ==测试）}
}
}
以怎样的价值被转让的通知，我可以访问Model.Title罚款（其中型号为短手），但Model.Artist.Name使我的错误。
解决
好了，把它加入到工作的虚拟关键字在Album类的艺术家和流派声明。但我仍然不知道发生了什么，并希望有任何人照顾摆脱一些灯。
在我的专辑类，它解决之前，它看起来像这样
公共类专辑
{
公众诠释ALBUMID {获得;组; }
公众诠释GenreId {获得;组; }
公众诠释ArtistId {获得;组; }
公共字符串名称{获取;组; }
公共十进制价格{获得;组; }
公共字符串AlbumArtUrl {获得;组; }
公共体裁类型{获取;组; }
公共艺术家艺术家{获得;组; }
}
引起的流派和艺术家的错误是通过添加虚拟关键字解决
public虚拟艺术家艺术家{获得;组; }
不知怎的，我仍然不能证明发生了什么，并希望了解更多信息。有人在乎解释？
它是这样的，Album.cs
命名空间MvcMusicStore.Models
{
公共类专辑
{
公众诠释ALBUMID {获得;组; }
公众诠释GenreId {获得;组; }
公众诠释ArtistId {获得;组; }
公共字符串名称{获取;组; }
公共虚拟体裁类型{获取;组; }
公共虚拟艺术家艺术家{获得;组; }
}
}
使用EF MusicStore.cs
命名空间MvcMusicStore.Models
{
//重新present实体框架，处理创建，读取，更新和删除OPS
公共类MusicStoreEntities：的DbContext
{
公共DbSet＆LT;相册和GT;专辑{获得;组; }
公共DbSet＆LT;类型＆GT;流派{获得;组; }
公共DbSet＆LT;艺术家＆GT;艺术家{获得;组; }
}
}
和在StoreController.cs实施
MusicStoreEntities storeDB =新MusicStoreEntities（）;
公众的ActionResult详细信息（INT ID）
{
//返回和专辑从ID搜索
VAR专辑= storeDB.Albums.Find（ID）;
返回查看（专辑）;
}
最后的样本数据。
保护覆盖无效种子（MusicStoreEntities上下文）
{
VAR艺术家=新的名单，其中，艺术家＆GT;
{
新人{名称=柯普兰和放大器;伦敦交响乐团}，
etcetc
}
新名单，其中，相册和GT;
{
新专辑{标题=ABC，艺术家= artists.Single（G =＆GT; g.Name ==测试）}
}
}
好了，把它加入虚拟关键字在Album类的艺术家和流派申报工作。但我仍然不知道发生了什么，并希望有任何人照顾摆脱一些灯。
见上面。
Cracking my head while learning MVC through MVC MusicStore . Got this error for Model.Artist.Name for Details View page .
My Storecontroller Details method should be fine .
public ActionResult Details(int id)
{
//returns and albums searched from the id
var albums = storeDB.Albums.Find(id);
return View(albums);
}
And this is how I output the view
<li>Price : <%=Model.Price %></li>
<li>Artist : <%=Model.Artist.Name%></li>
Works fine for the price , and it only show error for Model.Genre.Name and Artist.Name . I am suspecting the declaration for these properties in sampledata.cs caused this issue
Artist = artists.Single(a => a.Name == "Aaron Copland & London Symphony Orchestra")
But my knowledge of this is too weak to make up anything . Please help .
Alright , the value is fetched from a class file looking like this
protected override void Seed(MusicStoreEntities context)
{
var artists = new List<Artist>
{
new Artist { Name = "Aaron Copland & London Symphony Orchestra" },
etcetc
}
new List<Album>
{
new Album { Title = "ABC", Artist= artists.Single(g => g.Name == "Test")}
}
}
Take notice on how the value are being assigned , I can access Model.Title fine (where Model as the short hand) , but Model.Artist.Name caused me that error .
Solved
Alright , got it to work by adding virtual keyword to the Artist and Genre declaration in Album class . But I am still not sure what happens and would like to have anyone care to shed some lights .
In my Album class , before it is solved it looks like this
public class Album
{
public int AlbumId { get; set; }
public int GenreId { get; set; }
public int ArtistId { get; set; }
public string Title { get; set; }
public decimal Price { get; set; }
public string AlbumArtUrl { get; set; }
public Genre Genre { get; set; }
public Artist Artist { get; set; }
}
The error caused by Genre and Artist is solved by adding Virtual keyword
public virtual Artist Artist { get; set; }
Somehow , I still couldn't justify what happens and wish to learn more . Anyone care to explain ?
It goes like this , Album.cs
namespace MvcMusicStore.Models
{
public class Album
{
public int AlbumId { get; set; }
public int GenreId { get; set; }
public int ArtistId { get; set; }
public string Title { get; set; }
public virtual Genre Genre { get; set; }
public virtual Artist Artist { get; set; }
}
}
using EF MusicStore.cs
namespace MvcMusicStore.Models
{
//represent entity framework , handle create ,read , update and del ops
public class MusicStoreEntities : DbContext
{
public DbSet<Album> Albums { get; set; }
public DbSet<Genre> Genres { get; set; }
public DbSet<Artist> Artists { get; set; }
}
}
and implemented in StoreController.cs
MusicStoreEntities storeDB = new MusicStoreEntities();
public ActionResult Details(int id)
{
//returns and albums searched from the id
var albums = storeDB.Albums.Find(id);
return View(albums);
}
Finally the sample data
protected override void Seed(MusicStoreEntities context)
{
var artists = new List<Artist>
{
new Artist { Name = "Aaron Copland & London Symphony Orchestra" },
etcetc
}
new List<Album>
{
new Album { Title = "ABC", Artist= artists.Single(g => g.Name == "Test")}
}
}
Alright , got it to work by adding virtual keyword to the Artist and Genre declaration in Album class . But I am still not sure what happens and would like to have anyone care to shed some lights .
See above .
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