实现 Finalize 和 Dispose 以清理非托管资源

项目
08/12/2011

注意
关于最后确定和处理使用 c + + 的资源的信息，请参阅Destructors and Finalizers in Visual C++。

类实例经常封装对不受运行时管理的资源（如窗口句柄 (HWND)、数据库连接等）的控制。因此，应该既提供显式方法也提供隐式方法来释放这些资源。通过在对象上实现受保护的 Finalize（在 C# 和 C++ 中为析构函数语法）可提供隐式控制。当不再有任何有效的对象引用后，垃圾回收器在某个时间调用此方法。

在有些情况下，您可能想为使用该对象的程序员提供显式释放这些外部资源的能力，以便在垃圾回收器释放该对象之前释放这些资源。当外部资源稀少或者昂贵时，如果程序员在资源不再使用时显式释放它们，则可以获得更好的性能。若要提供显式控制，需实现 IDisposable 提供的 Dispose。在完成使用该对象的操作时，该对象的使用者应调用此方法。即使对对象的其他引用是活动的，也可以调用 Dispose。

注意，即使在通过 Dispose 提供显式控制时，也应该使用 Finalize 方法提供隐式清理。 Finalize 提供了候补手段，可防止在程序员未能调用 Dispose 时造成资源永久泄漏。

有关如何实现 Finalize 和 Dispose 以清理非托管理资源的更多信息，请参见垃圾回收。下面的示例阐释的基本设计模式，实施处置。此示例需要 System 命名空间。

' Design pattern for a base class.

Public Class Base
   Implements IDisposable
    ' Field to handle multiple calls to Dispose gracefully.
    Dim disposed as Boolean = false

   ' Implement IDisposable.
   Public Overloads Sub Dispose() Implements IDisposable.Dispose
      Dispose(True)
      GC.SuppressFinalize(Me)
   End Sub

   Protected Overloads Overridable Sub Dispose(disposing As Boolean)
      If disposed = False Then
          If disposing Then
             ' Free other state (managed objects).
             disposed = True
          End If
          ' Free your own state (unmanaged objects).
          ' Set large fields to null.
      End If
   End Sub

   Protected Overrides Sub Finalize()
      ' Simply call Dispose(False).
      Dispose (False)
   End Sub
End Class

' Design pattern for a derived class.
Public Class Derived
   Inherits Base

    ' Field to handle multiple calls to Dispose gracefully.
    Dim disposed as Boolean = false

   Protected Overloads Overrides Sub Dispose(disposing As Boolean)
      If disposed = False Then
          If disposing Then
             ' Release managed resources.
          End If
          ' Release unmanaged resources.
          ' Set large fields to null.
          disposed = True
      End If
      ' Call Dispose on your base class.
      Mybase.Dispose(disposing)
   End Sub
   ' The derived class does not have a Finalize method
   ' or a Dispose method without parameters because it inherits
   ' them from the base class.
End Class

// Design pattern for a base class.
public class Base: IDisposable
{
    private bool disposed = false;

    //Implement IDisposable.
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (!disposed)
        {
            if (disposing)
            {
                // Free other state (managed objects).
            }
            // Free your own state (unmanaged objects).
            // Set large fields to null.
            disposed = true;
        }
    }

    // Use C# destructor syntax for finalization code.
    ~Base()
    {
        // Simply call Dispose(false).
        Dispose (false);
    }
}

// Design pattern for a derived class.
public class Derived: Base
{
    private bool disposed = false;

    protected override void Dispose(bool disposing)
    {
        if (!disposed)
        {
            if (disposing)
            {
                // Release managed resources.
            }
            // Release unmanaged resources.
            // Set large fields to null.
           // Call Dispose on your base class.
            disposed = true;
        }
        base.Dispose(disposing);
    }
    // The derived class does not have a Finalize method
    // or a Dispose method without parameters because it inherits
    // them from the base class.
}

下面的代码将展开前面的示例中显示的不同方式处置调用时 Finalize调用。处理模式的阶段跟踪输出到控制台。在派生类中处理的分配和释放非托管资源。

Imports System
Imports System.Collections.Generic
Imports System.Runtime.InteropServices

' Design pattern for a base class.
Public MustInherit Class Base
    Implements IDisposable

    Private disposed as Boolean = false
    Private instName As String
    Private trackingList As List(Of Object)

    Public Sub New(instanceName As String, tracking As List(Of Object))
        MyClass.instName = instanceName
        trackingList = tracking
        trackingList.Add(Me)
    End Sub

    Public ReadOnly Property InstanceName() As String
        Get
            Return instName
        End Get
    End Property

    'Implement IDisposable.
    Public Overloads Sub Dispose() Implements IDisposable.Dispose
        Console.WriteLine(vbNewLine + "[{0}].Base.Dispose()", instName)
        Dispose(true)
        GC.SuppressFinalize(Me)
    End Sub

    Protected Overloads Overridable Sub Dispose(disposing As Boolean)
        If disposed = False Then
            If disposing Then
                ' Free other state (managed objects).
                Console.WriteLine("[{0}].Base.Dispose(true)", instName)
                trackingList.Remove(Me)
                Console.WriteLine("[{0}] Removed from tracking list: {1:x16}",
                    instanceName, MyClass.GetHashCode())
            Else
                Console.WriteLine("[{0}].Base.Dispose(false)", instName)
            End If
            disposed = True
        End If
    End Sub

    Protected Overrides Sub Finalize()
        ' Simply call Dispose(False).
        Console.WriteLine(vbNewLine + "[{0}].Base.Finalize()", instName)
        Dispose(False)
    End Sub
End Class

' Design pattern for a derived class.
Public Class Derived
   Inherits Base

    Private disposed as Boolean = false
    Private umResource As IntPtr

    Public Sub New(instanceName As String, tracking As List(Of Object))
        MyBase.New(instanceName, tracking)
        ' Save the instance name as an unmanaged resource
        umResource = Marshal.StringToCoTaskMemAuto(instanceName)
    End Sub

    Protected Overloads Overrides Sub Dispose(disposing As Boolean)
        If disposed = False Then
            If disposing Then
                Console.WriteLine("[{0}].Derived.Dispose(true)", InstanceName)
                ' Release managed resources.
            Else
                Console.WriteLine("[{0}].Derived.Dispose(false)", InstanceName)
            End If
           ' Release unmanaged resources.
            If umResource <> IntPtr.Zero
                Marshal.FreeCoTaskMem(umResource)
                Console.WriteLine("[{0}] Unmanaged memory freed at {1:x16}", _
                    InstanceName, umResource.ToInt64())
                umResource = IntPtr.Zero
            End If
            disposed = True
        End If
        ' Call Dispose in the base class.
        MyBase.Dispose(disposing)
    End Sub
    ' The derived class does not have a Finalize method
    ' or a Dispose method without parameters because it inherits
    ' them from the base class.
End Class

Public Class TestDisposal
    Public Shared Sub Main()
        Dim tracking As New List(Of Object)()

        ' Dispose is not called, Finalize will be called later.
        Using Nothing
            Console.WriteLine(vbNewLine + "Disposal Scenario: #1" + vbNewLine)
            Dim d3 As New Derived("d1", tracking)
        End Using

        ' Dispose is implicitly called in the scope of the using statement.
        Using d1 As New Derived("d2", tracking)
            Console.WriteLine(vbNewLine + "Disposal Scenario: #2" + vbNewLine)
        End Using

        ' Dispose is explicitly called.
        Using Nothing
            Console.WriteLine(vbNewLine + "Disposal Scenario: #3" + vbNewLine)
            Dim d2 As New Derived("d3", tracking)
            d2.Dispose()
        End Using

        ' Again, Dispose is not called, Finalize will be called later.
        Using Nothing
            Console.WriteLine(vbNewLine + "Disposal Scenario: #4" + vbNewLine)
            Dim d4 As New Derived("d4", tracking)
        End Using

        ' List the objects remaining to dispose.
        Console.WriteLine(vbNewLine + "Objects remaining to dispose = {0:d}", tracking.Count)
        For Each dd As Derived in tracking
            Console.WriteLine("    Reference Object: {0:s}, {1:x16}",
                dd.InstanceName, dd.GetHashCode())
        Next dd
        ' Queued finalizers will be exeucted when Main() goes out of scope.
        Console.WriteLine(vbNewLine + "Dequeueing finalizers...")
    End Sub
End Class

' The program will display output similar to the following:
'
' Disposal Scenario: #1
'
'
' Disposal Scenario: #2
'
'
' [d2].Base.Dispose()
' [d2].Derived.Dispose(true)
' [d2] Unmanaged memory freed at 00000000001ce420
' [d2].Base.Dispose(true)
' [d2] Removed from tracking list: 0000000002bf8098
'
' Disposal Scenario: #3
'
'
' [d3].Base.Dispose()
' [d3].Derived.Dispose(true)
' [d3] Unmanaged memory freed at 00000000001ce420
' [d3].Base.Dispose(true)
' [d3] Removed from tracking list: 0000000000bb8560
'
' Disposal Scenario: #4
'
'
' Objects remaining to dispose = 2
'     Reference Object: d1, 000000000297b065
'     Reference Object: d4, 0000000003553390
'
' Dequeueing finalizers...
'
' [d4].Base.Finalize()
' [d4].Derived.Dispose(false)
' [d4] Unmanaged memory freed at 00000000001ce420
' [d4].Base.Dispose(false)
'
' [d1].Base.Finalize()
' [d1].Derived.Dispose(false)
' [d1] Unmanaged memory freed at 00000000001ce3f0
' [d1].Base.Dispose(false)

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

// Design pattern for a base class.
public abstract class Base : IDisposable
{
    private bool disposed = false;
    private string instanceName;
    private List<object> trackingList;

    public Base(string instanceName, List<object> tracking)
    {
        this.instanceName = instanceName;
         trackingList = tracking;
         trackingList.Add(this);
    }

    public string InstanceName
    {
        get
        {
            return instanceName;
        }
    }

    //Implement IDisposable.
    public void Dispose()
    {
        Console.WriteLine("\n[{0}].Base.Dispose()", instanceName);
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (!disposed)
        {
            if (disposing)
            {
                // Free other state (managed objects).
                Console.WriteLine("[{0}].Base.Dispose(true)", instanceName);
                trackingList.Remove(this);
                Console.WriteLine("[{0}] Removed from tracking list: {1:x16}",
                    instanceName, this.GetHashCode());
            }
            else
            {
                Console.WriteLine("[{0}].Base.Dispose(false)", instanceName);
            }
            disposed = true;
        }
    }

    // Use C# destructor syntax for finalization code.
    ~Base()
    {
        // Simply call Dispose(false).
        Console.WriteLine("\n[{0}].Base.Finalize()", instanceName);
        Dispose(false);
    }
}

// Design pattern for a derived class.
public class Derived : Base
{
    private bool disposed = false;
    private IntPtr umResource;

    public Derived(string instanceName, List<object> tracking) :
        base(instanceName, tracking)
    {
         // Save the instance name as an unmanaged resource
         umResource = Marshal.StringToCoTaskMemAuto(instanceName);
    }

    protected override void Dispose(bool disposing)
    {
        if (!disposed)
        {
            if (disposing)
            {
                Console.WriteLine("[{0}].Derived.Dispose(true)", InstanceName);
                // Release managed resources.
            }
            else
            {
                Console.WriteLine("[{0}].Derived.Dispose(false)", InstanceName);
            }
            // Release unmanaged resources.
            if (umResource != IntPtr.Zero)
            {
                Marshal.FreeCoTaskMem(umResource);
                Console.WriteLine("[{0}] Unmanaged memory freed at {1:x16}",
                    InstanceName, umResource.ToInt64());
                umResource = IntPtr.Zero;
            }
            disposed = true;
        }
        // Call Dispose in the base class.
        base.Dispose(disposing);
    }
    // The derived class does not have a Finalize method
    // or a Dispose method without parameters because it inherits
    // them from the base class.
}

public class TestDisposal
{
    public static void Main()
    {
        List<object> tracking = new List<object>();

        // Dispose is not called, Finalize will be called later.
        using (null)
        {
            Console.WriteLine("\nDisposal Scenario: #1\n");
            Derived d3 = new Derived("d1", tracking);
        }

        // Dispose is implicitly called in the scope of the using statement.
        using (Derived d1 = new Derived("d2", tracking))
        {
            Console.WriteLine("\nDisposal Scenario: #2\n");
        }

        // Dispose is explicitly called.
        using (null)
        {
            Console.WriteLine("\nDisposal Scenario: #3\n");
            Derived d2 = new Derived("d3", tracking);
            d2.Dispose();
        }

        // Again, Dispose is not called, Finalize will be called later.
        using (null)
        {
            Console.WriteLine("\nDisposal Scenario: #4\n");
            Derived d4 = new Derived("d4", tracking);
        }

        // List the objects remaining to dispose.
        Console.WriteLine("\nObjects remaining to dispose = {0:d}", tracking.Count);
        foreach (Derived dd in tracking)
        {
            Console.WriteLine("    Reference Object: {0:s}, {1:x16}",
                dd.InstanceName, dd.GetHashCode());
        }

        // Queued finalizers will be exeucted when Main() goes out of scope.
        Console.WriteLine("\nDequeueing finalizers...");
    }
}

// The program will display output similar to the following:
//
// Disposal Scenario: #1
//
//
// Disposal Scenario: #2
//
//
// [d2].Base.Dispose()
// [d2].Derived.Dispose(true)
// [d2] Unmanaged memory freed at 000000000034e420
// [d2].Base.Dispose(true)
// [d2] Removed from tracking list: 0000000002bf8098
//
// Disposal Scenario: #3
//
//
// [d3].Base.Dispose()
// [d3].Derived.Dispose(true)
// [d3] Unmanaged memory freed at 000000000034e420
// [d3].Base.Dispose(true)
// [d3] Removed from tracking list: 0000000000bb8560
//
// Disposal Scenario: #4
//
//
// Objects remaining to dispose = 2
//    Reference Object: d1, 000000000297b065
//    Reference Object: d4, 0000000003553390
//
// Dequeueing finalizers...
//
// [d4].Base.Finalize()
// [d4].Derived.Dispose(false)
// [d4] Unmanaged memory freed at 000000000034e420
// [d4].Base.Dispose(false)
//
// [d1].Base.Finalize()
// [d1].Derived.Dispose(false)
// [d1] Unmanaged memory freed at 000000000034e3f0
// [d1].Base.Dispose(false)

说明的设计模式，实施额外的代码示例为Finalize和处置，请参阅实现 Dispose 方法。

自定义释放方法名称

有时特定于域的名称比 Dispose 更合适。例如，文件封装可能需要使用 Close 方法名称。在此情况下，可以通过专用方式实现 Dispose 并创建调用 Dispose 的公共 Close 方法。下面的代码示例阐释了这种模式。可以用适合您的域的方法名称替换 Close。此示例需要 System 命名空间。

' Do not make this method overridable.
' A derived class should not be allowed
' to override this method.
Public Sub Close()
   ' Call the Dispose method with no parameters.
   Dispose()
End Sub

// Do not make this method virtual.
// A derived class should not be allowed
// to override this method.
public void Close()
{
   // Call the Dispose method with no parameters.
   Dispose();
}

Finalize

下面的规则概括了 Finalize 方法的使用准则：

仅在要求终结的对象上实现 Finalize。存在与 Finalize 方法相关的性能开销。
如果需要 Finalize 方法，应考虑实现 IDisposable，以使类的用户可以避免因调用 Finalize 方法而带来的开销。
不要提高 Finalize 方法的可见性。该方法的可见性应该是 protected，而不是 public。
对象的 Finalize 方法应该释放该对象拥有的所有外部资源。此外，Finalize 方法应该仅释放由该对象控制的资源。 Finalize 方法不应该引用任何其他对象。
不要对不是对象的基类的对象直接调用 Finalize 方法。在 C# 编程语言中，这不是有效的操作。
应在对象的 Finalize 方法中调用基类的 Finalize 方法。

注意

基类的 Finalize 方法通过 C# 和 C++ 析构函数语法自动进行调用。

注意
基类的 Finalize 方法通过 C# 和 C++ 析构函数语法自动进行调用。

释放

下面的规则概括了 Dispose 方法的使用准则：

在封装明确需要释放的资源的类型上实现释放设计方案。用户可以通过调用公共 Dispose 方法释放外部资源。
在通常包含控制资源的派生类型的基类型上实现释放设计方案，即使基类型并不需要也如此。如果基类型有 Close 方法，这通常指示需要实现 Dispose。在这类情况下，不要在基类型上实现 Finalize 方法。应该在任何引入需要清理的资源的派生类型中实现 Finalize。
使用类型的 Dispose 方法释放该类型所拥有的所有可释放资源。
对实例调用了 Dispose 后，应通过调用 GC.SuppressFinalize 禁止 Finalize 方法运行。此规则的一个例外是当必须用 Finalize 完成 Dispose 没有完成的工作的情况，但这种情况很少见。
如果基类实现了 IDisposable，则应调用基类的 Dispose 方法。
不要假定 Dispose 将被调用。如果 Dispose 未被调用，也应该使用 Finalize 方法释放类型所拥有的非托管资源。
当资源已经释放时，在该类型上从实例方法（非 Dispose）引发一个 ObjectDisposedException。该规则不适用于 Dispose 方法，该方法应该可以在不引发异常的情况下被多次调用。
通过基类型的层次结构传播对 Dispose 的调用。 Dispose 方法应释放由此对象以及此对象所拥有的任何对象所控制的所有资源。例如，可以创建一个类似 TextReader 的对象来控制 Stream 和 Encoding，两者均在用户不知道的情况下由 TextReader 创建。另外，Stream 和 Encoding 都可以获取外部资源。当对 TextReader 调用 Dispose 方法时，它应继而对 Stream 和 Encoding 调用 Dispose，使它们释放其外部资源。
考虑在调用了某对象的 Dispose 方法后禁止对该对象的使用。重新创建已释放的对象是难以实现的方案。
允许 Dispose 方法被调用多次而不引发异常。此方法在首次调用后应该什么也不做。

设计指引的详细信息，请参阅"框架设计准则：公约、成语和可重复使用的模式。网络图书馆"书 Krzysztof Cwalina 和布拉德 · 艾布拉姆斯，2005年艾迪生－韦斯利，发表。

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实现 Finalize 和 Dispose 以清理非托管资源

自定义释放方法名称

Finalize

释放

请参见

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