LINQ to SQL 中的序列化

本文介绍 LINQ to SQL 序列化功能。 后面的段落提供有关如何在设计时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。

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 命令行上，添加 /serialization 选项。 有关详细信息，请参阅 SqlMetal.exe（代码生成工具）。

LINQ to SQL 生成的代码默认提供延迟加载功能。 延迟加载在中间层上非常方便，以便按需透明加载数据。 但是，序列化存在问题，因为序列化程序会触发延迟加载，无论是否打算延迟加载。 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 序列化功能主要通过两种机制解决此问题：

• 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。 此方面（包括序列化下延迟加载类的行为）是本主题的主要主题。

• DataContract 序列化程序：.NET Framework 3.0 或更高版本的 Windows 通信框架 （WCF） 组件使用的默认序列化程序。

• 单向序列化：仅包含单向关联属性的类的序列化版本（以避免循环）。 按照约定，主键-外键关系的父级端的属性标记为序列化。 双向关联的另一端未序列化。

  单向序列化是 LINQ to SQL 支持的唯一序列化类型。

以下代码使用 Northwind 示例数据库中的传统 Customer 类和 Order 类，并演示如何使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

Order对于以下示例中的类，为了简洁起见，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

可以在上一部分所示的代码中序列化实体，如下所示：

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()

自我递归关系遵循相同的模式。 与外键对应的关联属性没有属性 DataMemberAttribute ，而父属性则具有该属性。

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)

• 背景信息
• SqlMetal.exe （代码生成工具）
• 如何：使实体可序列化