設置自引用類

Question

我有一個遞歸數據結構，它有一個（空）父和一個（空）子集合。

我想將該數據結構保存到windows phone上的本地數據庫中。 我設置了一個DataContext，它工作的很好，但是當我想插入一個新的對象（它沒有父母和子女）時，我得到了「對象引用未設置爲對象的實例」。錯誤。

搜索後，我發現此線程LINQ to SQL: Self-referencing entity - ParentID/ChildID association，它聲明，由於父對象和子對象爲空，所以引發此異常。 但是，因爲這是一個允許的狀態，這不應該阻止我的代碼正常運行。

所以問題是：如何設置一個自引用類。

我迄今所做的：

using System; 
using System.Collections.Generic; 
using System.Collections.ObjectModel; 
using System.ComponentModel; 
using System.Data.Linq; 
using System.Data.Linq.Mapping; 
using System.Linq; 
using System.Text; 
using System.Threading.Tasks; 
using System.Runtime.CompilerServices; 

namespace Notepad.Codes.Models 
{ 

    [Table] 
    public class DirectoryItem 
    { 
     [Column(IsVersion=true)] 
     private Binary version; 

     [Column(IsPrimaryKey = true, IsDbGenerated = true, DbType = "INT NOT NULL Identity", CanBeNull = false, AutoSync = AutoSync.OnInsert)] 
     public int primaryKey; 

     private DirectoryItem parent; 
     [Association(Storage = "parent", ThisKey = "primaryKey", OtherKey = "primaryKey")] 
     public DirectoryItem Parent 
     { 
      get 
      { 
       return parent; 
      } 
      set 
      { 
       this.SetProperty(ref this.parent, value); 
      } 
     } 

     private EntitySet<DirectoryItem > children; 
     [Association(Storage = "children", OtherKey = "primaryKey")] 
     public EntitySet<DirectoryItem > Children 
     { 
      get 
      { 
       if(children == null) 
        children = new EntitySet<DirectoryItem >(); 
       return children; 
      } 
      set 
      { 
       if (this.PropertyChanging != null) 
        PropertyChanging(this, new PropertyChangingEventArgs("Children")); 
       this.children.Assign(value); 
       if (this.PropertyChanged != null) 
        PropertyChanged(this, new PropertyChangedEventArgs("Children")); 
      } 
     } 
    } 
} 

可能有人請告訴我，我怎麼也得使用或更改Associoaton -Attribute，這樣我可以插入null的父母和孩子到我的數據庫？

Answer 1

我解決了我與Northwind的問題。 我按照這些步驟（我一一列舉了，因爲MSDN是一個嚴重改變的地方:)

1. 去http://www.microsoft.com/en-us/download/details.aspx?id=23654
2. 下載並運行安裝程序
3. 打開Visual Studio（< 2012）
4. 開放服務器瀏覽器，打開快捷菜單數據連接並選擇添加連接
5. 選擇微軟SQL服務器數據庫文件
6. 選擇瀏覽並瀏覽到安裝的數據庫（對我來說它是根據安裝C:\Sample Database（或類似）
7. 使用Windows身份驗證並單擊確定*
8. 從northwind數據庫中選擇所有表格，並將它們拖放到對象關係設計器中
9. 現在您可以切換到文件後面的代碼並查看所有邏輯和屬性

而對於我來說，解決現在的樣子：

[Table] 
public class DirectoryItem 
{ 
    [Column(IsVersion=true)] 
    private Binary version; 

    [Column(IsPrimaryKey = true, IsDbGenerated = true, DbType = "INT NOT NULL Identity", CanBeNull = false, AutoSync = AutoSync.OnInsert)] 
    public int primaryKey; 

    private System.Nullable<int> parentId; 
    [Column(Storage = "parentId", DbType="Int")] 
    public System.Nullable<int> ParentId 
    { 
     get 
     { 
      return this.parentId; 
     } 
     set 
     { 
      this.SetProperty(ref this.parentId, value); 
     } 
    } 

    private EntityRef<DirectoryItem > parent; 
    [Association(Name = "DirectoryItem_parent", Storage = "parent", ThisKey = "ParentId", OtherKey = "primaryKey", IsForeignKey = true)] 
    public DirectoryItem Parent 
    { 
     get 
     { 
      return parent.Entity; 
     } 
     set 
     { 
      if (this.PropertyChanging != null) 
       PropertyChanging(this, new PropertyChangingEventArgs("Parent")); 

      parent.Entity = value; 

      if (this.PropertyChanged != null) 
       PropertyChanged(this, new PropertyChangedEventArgs("Parent")); 
     } 
    } 

    private EntitySet<DirectoryItem > children; 
    [Association(Name = "DirectoryItem_DirectoryItem", Storage = "Children", ThisKey = "primaryKey", OtherKey = "ParentId")] 
    public EntitySet<DirectoryItem > Children 
    { 
     get 
     { 
      if (children == null) 
       children = new EntitySet<DirectoryItem >(); 
      return children; 
     } 
     set 
     { 
      if (this.PropertyChanging != null) 
       PropertyChanging(this, new PropertyChangingEventArgs("Children")); 

      if (children == null) 
       children = new EntitySet<DirectoryItem >(); 
      this.children.Assign(value); 

      if (this.PropertyChanged != null) 
       PropertyChanged(this, new PropertyChangedEventArgs("Children")); 
     } 
    } 
} 

- 附錄：

1. instrucitons安裝示例數據庫：http://msdn.microsoft.com/en-us/library/vstudio/8b6y4c7s.aspx

Answer 2

有一個在Children setter方法存在問題：

if (this.PropertyChanging != null) 
    PropertyChanging(this, new PropertyChangingEventArgs("Children")); 

this.children.Assign(value); 

if (this.PropertyChanged != null) 
    PropertyChanged(this, new PropertyChangedEventArgs("Children")); 

你打電話this.children.Assign即使children可能未初始化。使用與您的吸氣劑相同的支票來防止此問題：

if(children == null) 
    children = new EntitySet<DirectoryItem >(); 

this.children.Assign(value);