LINQ to SQL中的对象引用被错误地保存或加载

Question

我试图存储和重新加载对象和它们之间的引用。问题是当我从数据库重新加载数据时，引用设置不正确。

下面是描述问题的示例代码。我在代码中评论输出的已确定状态和实际状态。

[Table(Name = "ClassA")] 
public class ClassA 
{ 
    public ClassA() 
    { 
     LinksToClassB = new EntitySet<ClassB>(); 
    } 

    [Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert, DbType = "Int NOT NULL IDENTITY")] 
    public int ID { get; set; } 

    [Association] 
    public EntitySet<ClassB> LinksToClassB { get; set; } //=> 1 to n cardinality 

    [Association] 
    public ClassB OneLinkToClassB { get; set; }//=> 1 to 1 cardinality 
} 


[Table(Name = "ClassB")] 
public class ClassB 
{ 
    [Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert, DbType = "Int NOT NULL IDENTITY")] 
    public int ID { get; set; } 

    [Column(CanBeNull = true)] 
    public string Name { get; set; } 
} 

public class DatabaseContext : DataContext 
{ 
    public Table<ClassA> ClassATable; 
    public Table<ClassB> ClassBTable; 

    public DatabaseContext(string connection) : base(connection) { } 
} 



[TestClass] 
public class Test 
{ 
    string path = @"F:\Temp\Testspace - Forum Database\database.mdf";//path to database 

    [TestMethod] 
    public void TestMethod() 
    { 
     //creates Database 
     DatabaseContext context = new DatabaseContext(path); 

     if (context.DatabaseExists())//Delete if exists 
     { 
      context.DeleteDatabase(); 
     } 

     context.CreateDatabase(); 

     ClassB b1 = new ClassB(); b1.Name = "name 1"; 
     ClassB b2 = new ClassB(); b2.Name = "name 2"; 
     ClassB b3 = new ClassB(); b3.Name = "name 3"; 

     ClassA a = new ClassA(); 

     //now the references will be added to the object a 

     //in 1-n references 
     a.LinksToClassB.Add(b1); 
     a.LinksToClassB.Add(b2); 
     a.LinksToClassB.Add(b3); 

     //and the has-one reference (OneLinkToClassB) 
     a.OneLinkToClassB = b1; 


     context.ClassATable.InsertOnSubmit(a); 

     context.SubmitChanges(); //store in database 


     //now the database will be reloaded 
     context = new DatabaseContext(path); 


     //Check if all ClassB objects were correctly stored and reloaded 
     foreach (ClassB x in context.ClassBTable) 
     { 
      Console.WriteLine(x.ID + "; " + x.Name); 
      /*  
       -> expected output: 
        1; name 1 
        2; name 2 
        3; name 3 
       -> real output 
        1; name 1 
        2; name 2 
        3; name 3 

       -> check! 
      */ 
     } 

     //check if all ClassA objects were correctly stored and reloaded 
     foreach (ClassA x in context.ClassATable)//context.ClassATable has only one entry 
     { 
      Console.WriteLine("check of entitys set"); 

      //check of 1-n references 
      foreach (ClassB b in x.LinksToClassB) 
      { 
       Console.WriteLine(x.ID + " has a link to " + b.ID + ", " + b.Name); 
       /* 
        -> expected output: 
         1 has a link to 1, name 1 
         1 has a link to 2, name 2 
         1 has a link to 3, name 3 

        -> real output 
         1 has a link to 1, name 1 

        -> doesn't match... 
       */ 
      } 


      Console.WriteLine("check of single link"); 

      //check of 1-1 reference 
      Console.WriteLine(x.ID + " has a link to " + x.OneLinkToClassB.ID + ", " + x.OneLinkToClassB.Name); 
      /* 
        -> expected output: 
         1 has a link to 1, name 1 

        -> real output 
         this line throws an NullReferenceException 
      */ 
     } 
    } 
} 

我希望任何人都可以给我一个提示如何解决这个bug在我的代码:)

Answer 1

正如@Nick所说的“父母/子女关联（s）缺失”。这里现在完整更改的代码：

[Table(Name = "ClassA")] 
public class ClassA 
{ 
    public ClassA() 
    { 
     LinksToClassB = new EntitySet<ClassB>(); 
    } 

    [Column(IsPrimaryKey = true, IsDbGenerated = true)] 
    private int ID; 

    [Association(ThisKey = "ID", OtherKey = "ClassAId")] 
    private EntitySet<ClassB> LinksToClassB;//=> 1 to n cardinality 

    public void addLink(ClassB x) 
    { 
     LinksToClassB.Add(x); 
     x.setLink(this); 
    } 

    public HashSet<ClassB> getLinks() 
    { 
     HashSet<ClassB> collection = new HashSet<ClassB>(); 
     foreach(ClassB x in LinksToClassB) 
     { 
      collection.Add(x); 
     } 
     return collection; 
    } 
} 


[Table(Name = "ClassB")] 
public class ClassB 
{ 
    private EntityRef<ClassA> _classA = default(EntityRef<ClassA>); 

    [Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert, DbType = "Int NOT NULL IDENTITY")] 
    public int ID { get; set; } 

    [Column(CanBeNull = true)] 
    public string Name { get; set; } 

    [Column(CanBeNull = false)] 
    private int ClassAId { get; set; } 

    [Association(ThisKey = "ClassAId", OtherKey = "ID", IsForeignKey = true)] 
    private ClassA ClassA 
    { 
     get 
     { 
      return _classA.Entity; 
     } 
     set 
     { 
      _classA.Entity = value; 
     } 
    } 

    private ClassA getLink() 
    { 
     return _classA.Entity; 
    } 

    public void setLink(ClassA x) 
    { 
     _classA.Entity = x; 
    } 
} 

public class DatabaseContext : DataContext 
{ 
    public Table<ClassA> ClassATable; 
    public Table<ClassB> ClassBTable; 

    public DatabaseContext(string connection) : base(connection) { } 
} 



[TestClass] 
public class Test 
{ 
    string path = @"F:\Temp\Testspace - Forum Database\database.mdf";//path to database 

    [TestMethod] 
    public void TestMethod() 
    { 
     //creates Database 
     DatabaseContext context = new DatabaseContext(path); 

     if (context.DatabaseExists())//Delete if exists 
     { 
      context.DeleteDatabase(); 
     } 

     context.CreateDatabase(); 

     ClassB b1 = new ClassB(); b1.Name = "name 1"; 
     ClassB b2 = new ClassB(); b2.Name = "name 2"; 
     ClassB b3 = new ClassB(); b3.Name = "name 3"; 

     ClassA a = new ClassA(); 

     //now the references will be added to the object a 

     //in 1-n references 
     a.addLink(b1); 
     a.addLink(b2); 
     a.addLink(b3); 

     context.ClassATable.InsertOnSubmit(a); 

     context.SubmitChanges(); //store in database 


     //now the database will be reloaded 
     context = new DatabaseContext(path); 


     //Check if all ClassB objects were correctly stored and reloaded 


     foreach (ClassB x in context.ClassBTable) 
     { 
      Console.WriteLine(x.ID + "; " + x.Name); 
      /*  
       -> expected output: 
        1; name 1 
        2; name 2 
        3; name 3 
       -> real output 
        1; name 1 
        2; name 2 
        3; name 3 

       -> check! 
      */ 
     } 

     //check if all ClassA objects were correctly stored and reloaded 
     foreach (ClassA x in context.ClassATable)//context.ClassATable has only one entry 
     { 
      Console.WriteLine("check of entitys set"); 

      //check of 1-n references 
      foreach (ClassB b in x.getLinks()) 
      { 
       Console.WriteLine(b.ID + " has a link to " + b.ID + ", " + b.Name); 
       /* 
        -> expected output: 
         1 has a link to 1, name 1 
         1 has a link to 2, name 2 
         1 has a link to 3, name 3 

        -> real output 
         1 has a link to 1, name 1 
         1 has a link to 2, name 2 
         1 has a link to 3, name 3 

        -> check! 
       */ 
      } 
     } 
    } 
} 

Answer 2

除非省略，它看起来像你的父/子关联（一个或多个）丢失。

[Table(Name = "ClassA")] 
public class ClassA 
{ 
    public ClassA() 
    { 
     LinksToClassB = new EntitySet<ClassB>(); 
    } 

    [Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert, DbType = "Int NOT NULL IDENTITY")] 
    public int ID { get; set; } 

    [Association(ThisKey="ID", OtherKey="ClassAId")] 
    public EntitySet<ClassB> LinksToClassB { get; set; } //=> 1 to n cardinality 

    [Association] 
    public ClassB OneLinkToClassB { get; set; }//=> 1 to 1 cardinality 
} 


[Table(Name = "ClassB")] 
public class ClassB 
{ 
    private EntityRef<ClassA> _classA = default(EntityRef<ClassA>); 

    [Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert, DbType = "Int NOT NULL IDENTITY")] 
    public int ID { get; set; } 

    [Column(CanBeNull = true)] 
    public string Name { get; set; } 

    [Column(CanBeNull = false)] 
    public int ClassAId { get; set; } 

    [Association(ThisKey = "ClassAId", OtherKey = "ID", IsForeignKey = true)] 
    public ClassA ClassA { 
     get 
     { 
      return _classA.Entity; 
     } 
     set { 
      // property changed implementation omitted for brevity... 
      _classA.Entity = value; 
     } 
    } 
}