[英]Entity Framework Null Reference Exception
我正在嘗試先掌握EF Code,但仍然不知道如何從另一個類訪問被引用的對象(由於缺乏足夠的知識,我什至無法提出問題)。
這是我的簡單代碼:
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);
}
}
}
我在嘗試將目標名稱寫入控制台的那一行上收到NullReferenceException。
提前致謝。
嘗試使用navigation properties
:
首先將Destination
虛擬化
public virtual Destination Destination { get; set; }
然后使用Include
方法
foreach(var l in context.Lodgings.Include(x => x.Destination))
只需在Lodging
類virtual
設置Destination
屬性。 這表明,EF在需要時自動加載Destination
(延遲加載)。 因此,您的Lodging
課程應類似於:
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; }
}
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