LINQ to SQL 中的序列化
本文将介绍 LINQ to SQL 的序列化功能。 下面几段提供了有关在设计时如何在代码生成期间添加序列化以及 LINQ to SQL 类的运行时序列化行为的信息。
您可以通过以下任一方法在设计时添加序列化代码：
在对象关系设计器中，将“序列化模式”属性更改为“单向”。
在 SQLMetal 命令行中，添加 /serialization 选项。 有关详细信息，请参阅 SqlMetal.exe（代码生成工具）。
概述
默认情况下，由 LINQ to SQL 生成的代码提供延迟加载功能。 延迟加载对在中间层以透明方式根据需要加载数据而言非常方便。 但是，它会给序列化过程带来问题，原因是不论是否需要进行延迟加载，序列化程序都会触发延迟加载。 实际上，对对象进行序列化时，会对其在所有出站延迟加载引用下的可传递闭包进行序列化。
LINQ to SQL 的序列化功能主要通过以下两种机制来解决此问题：
用于关闭延迟加载的 DataContext 模式 (ObjectTrackingEnabled)。 有关详细信息，请参阅 DataContext。
用于在生成的实体中生成 System.Runtime.Serialization.DataContractAttribute 和 System.Runtime.Serialization.DataMemberAttribute 属性的代码生成开关。 这方面（包括处于序列化过程中的延迟加载类的行为）是本主题的主要介绍对象。
定义
DataContract 序列化程序：.NET Framework 3.0 或更高版本的 Windows Communication Framework (WCF) 组件使用的默认序列化程序。
单向序列化：只包含单向关联属性（为避免出现循环）的类的已序列化版本。 按照约定，主键-外键关系的父级端的属性标记为序列化。 双向关联中的另一端不进行序列化。
单向序列化是 LINQ to SQL 支持的唯一序列化类型。
代码示例
下面的代码使用 Northwind 示例数据库中的传统 Customer 和 Order 类，并演示了如何用序列化属性修饰这些类。
// The class is decorated with the DataContract attribute.
[Table(Name="dbo.Customers")]
[DataContract()]
public partial class Customer : INotifyPropertyChanging, INotifyPropertyChanged
{
' The class is decorated with the DataContract attribute.
<Table(Name:="dbo.Customers"), _
DataContract()> _
Partial Public Class Customer
Implements System.ComponentModel.INotifyPropertyChanging, System.ComponentModel.INotifyPropertyChanged
// Private fields are not decorated with any attributes, and are
// elided.
private string _CustomerID;
// Public properties are decorated with the DataMember
// attribute and the Order property specifying the serial
// number. See the Order class later in this topic for
// exceptions.
public Customer()
{
this.Initialize();
}
[Column(Storage="_CustomerID", DbType="NChar(5) NOT NULL", CanBeNull=false, IsPrimaryKey=true)]
[DataMember(Order=1)]
public string CustomerID
{
get
{
return this._CustomerID;
}
set
{
if ((this._CustomerID != value))
{
this.OnCustomerIDChanging(value);
this.SendPropertyChanging();
this._CustomerID = value;
this.SendPropertyChanged("CustomerID");
this.OnCustomerIDChanged();
}
}
}
' Private fields are not decorated with any attributes,
' and are elided.
Private _CustomerID As String
' Public properties are decorated with the DataMember
' attribute and the Order property specifying the
' serial number. See the Order class later in this topic
' for exceptions
<Column(Storage:="_CustomerID", DbType:="NChar(5) NOT NULL", CanBeNull:=false, IsPrimaryKey:=true), _
DataMember(Order:=1)> _
Public Property CustomerID() As String
Get
Return Me._CustomerID
End Get
Set
If ((Me._CustomerID = value) _
= false) Then
Me.OnCustomerIDChanging(value)
Me.SendPropertyChanging
Me._CustomerID = value
Me.SendPropertyChanged("CustomerID")
Me.OnCustomerIDChanged
End If
End Set
End Property
// The following Association property is decorated with
// DataMember because it is the parent side of the
// relationship. The reverse property in the Order class
// does not have a DataMember attribute. This factor
// prevents a 'cycle.'
[Association(Name="FK_Orders_Customers", Storage="_Orders", OtherKey="CustomerID", DeleteRule="NO ACTION")]
[DataMember(Order=13)]
public EntitySet<Order> Orders
{
get
{
return this._Orders;
}
set
{
this._Orders.Assign(value);
}
}
' The following Association property is decorated with
' DataMember because it is the parent side of the
' relationship. The reverse property in the Order
' class does not have a DataMember attribute. This
' factor prevents a 'cycle.'
<Association(Name:="FK_Orders_Customers", Storage:="_Orders", OtherKey:="CustomerID", DeleteRule:="NO ACTION"), _
DataMember(Order:=13)> _
Public Property Orders() As EntitySet(Of [Order])
Get
Return Me._Orders
End Get
Set(ByVal value As EntitySet(Of [Order]))
Me._Orders.Assign(Value)
End Set
End Property
对于下例中的 Order 类，为简洁起见，只显示与 Customer 类对应的反向关联属性。 它没有用来避免出现循环的 DataMemberAttribute 属性。
// The class for the Orders table is also decorated with the
// DataContract attribute.
[Table(Name="dbo.Orders")]
[DataContract()]
public partial class Order : INotifyPropertyChanging, INotifyPropertyChanged
' The class for the Orders table is also decorated with the
' DataContract attribute.
<Table(Name:="dbo.Orders"), _
DataContract()> _
Partial Public Class [Order]
Implements System.ComponentModel.INotifyPropertyChanging, System.ComponentModel.INotifyPropertyChanged
// Private fields for the Orders table are not decorated with
// any attributes, and are elided.
private int _OrderID;
// Public properties are decorated with the DataMember
// attribute.
// The reverse Association property on the side of the
// foreign key does not have the DataMember attribute.
[Association(Name = "FK_Orders_Customers", Storage = "_Customer", ThisKey = "CustomerID", IsForeignKey = true)]
public Customer Customer
' Private fields for the Orders table are not decorated with
' any attributes, and are elided.
Private _CustomerID As String
' Public properties are decorated with the DataMember
' attribute.
' The reverse Association property on the side of the
' foreign key does not have the DataMember attribute.
<Association(Name:="FK_Orders_Customers", Storage:="_Customer", ThisKey:="CustomerID", IsForeignKey:=true)> _
Public Property Customer() As Customer
如何序列化实体
你可以按如下方式序列化上一节中显示的代码中的实体：
Northwnd db = new Northwnd(@"c\northwnd.mdf");
Customer cust = db.Customers.Where(c => c.CustomerID ==
"ALFKI").Single();
DataContractSerializer dcs =
new DataContractSerializer(typeof(Customer));
StringBuilder sb = new StringBuilder();
XmlWriter writer = XmlWriter.Create(sb);
dcs.WriteObject(writer, cust);
writer.Close();
string xml = sb.ToString();
Dim db As New Northwnd("...")
Dim cust = (From c In db.Customers _
Where c.CustomerID = "ALFKI").Single
Dim dcs As New DataContractSerializer(GetType(Customer))
Dim sb As StringBuilder = New StringBuilder()
Dim writer As XmlWriter = XmlWriter.Create(sb)
dcs.WriteObject(writer, cust)
writer.Close()
Dim xml As String = sb.ToString()
自递归关系
自递归关系遵循相同的模式。 与外键对应的关联属性 (Property) 不具有 DataMemberAttribute 属性 (Attribute)，而父属性 (Property) 则具有。
请考虑以下具有两个自递归关系的类：Employee.Manager/Reports 和 Employee.Mentor/Mentees。
// No DataMember attribute.
public Employee Manager;
[DataMember(Order = 3)]
public EntitySet<Employee> Reports;
// No DataMember
public Employee Mentor;
[DataMember(Order = 5)]
public EntitySet<Employee> Mentees;
' No DataMember attribute
Public Manager As Employee
<DataMember(Order:=3)> _
Public Reports As EntitySet(Of Employee)
' No DataMember attribute
Public Mentor As Employee
<DataMember(Order:=5)> _
Public Mentees As EntitySet(Of Employee)