实体框架空引用异常

问题描述：

我想抓住EF代码第一，但我仍然没有得到如何从另一个类访问被引用的对象（由于缺乏足够的知识，我甚至不能制定问题）。实体框架空引用异常

这里是我的简单的代码是什么样子：

public class Destination 
{ 
    public int DestinationId { get; set; } 
    public string Name { get; set; } 
    public string Country { get; set; } 
    public string Description { get; set; } 

    public byte[] Photo { get; set; } 

    public List<Lodging> Lodgings { get; set; } 
} 
public class Lodging 
{ 
    public int LodgingId { get; set; } 

    public string Name { get; set; } 
    public string Owner { get; set; } 
    public bool IsResort { get; set; } 

    public Destination Destination { get; set; } 
} 
public class BreakAwayContext: DbContext 
{ 
    public DbSet<Destination> Destinations { get; set; } 
    public DbSet<Lodging> Lodgings { get; set; } 
} 
private static void InsertDestination() 
    { 
     var destination = new Destination 
     { 
      Country = "Indonesia", 
      Description = "EcoTourism at its best in exquisite Bali", 
      Name = "Bali" 
     }; 
     using(var context = new BreakAwayContext()) 
     { 
      context.Destinations.Add(destination); 
      context.SaveChanges(); 
     } 
    } 

    private static void InsertLodging() 
    { 
     var lodging = new Lodging() 
     { 
      Name = "x", 
      IsResort = false, 
      Owner = "asdasd" 
     }; 
     using(var context = new BreakAwayContext()) 
     { 
      var dest = context.Destinations.Find(1); 
      lodging.Destination = dest; 
      context.Lodgings.Add(lodging); 
      context.SaveChanges(); 
     } 
    } 
    private static void ShowLodgings() 
    { 
     using(var context = new BreakAwayContext()) 
     { 
      foreach(var l in context.Lodgings) 
      { 
       Console.WriteLine("{0} {1} {2}", l.Name, l.Owner, l.Destination.Name); 
      } 
     } 
    }

我得到我尝试目的地名称写入控制台行一个NullReferenceException。

在此先感谢。

你'没有加载Destination'。您需要启用延迟加载或使用Eager Loading。请参阅：http://msdn.microsoft.com/en-us/data/jj574232.aspx。您已经选择了正确的书籍来顺便学习Code First :-) – Dabblernl 2014-09-21 21:09:28

答

尝试使用navigation properties：

首先要Destination虚拟

public virtual Destination Destination { get; set; }

然后用Include方法

foreach(var l in context.Lodgings.Include(x => x.Destination))

从性能的角度来看，'Include'可能是最好的，但是在您将Destination设置为virtual后，它将被延迟加载。 – Dabblernl 2014-09-21 21:07:07

答

只是在你的Lodging类，virtual设置Destination属性。这告诉，EF在你需要时自动加载Destination（延迟加载）。所以你Lodging类应该是这样的：

public class Lodging 
{ 
    public int LodgingId { get; set; } 

    public string Name { get; set; } 
    public string Owner { get; set; } 
    public bool IsResort { get; set; } 

    public virtual Destination Destination { get; set; } 
}

实体框架空引用异常

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