C # 2008

我已经在这方面工作了一段时间,但我仍然对在代码中使用finalize和dispose方法感到困惑。我的问题如下:

I know that we only need a finalizer while disposing unmanaged resources. However, if there are managed resources that make calls to unmanaged resources, would it still need to implement a finalizer? However, if I develop a class that doesn't use any unmanaged resource - directly or indirectly, should I implement the IDisposable to allow the clients of that class to use the 'using statement'? Would it be feasible to implement IDisposable just to enable clients of your class to use the using statement? using(myClass objClass = new myClass()) { // Do stuff here } I have developed this simple code below to demonstrate the Finalize/dispose use: public class NoGateway : IDisposable { private WebClient wc = null; public NoGateway() { wc = new WebClient(); wc.DownloadStringCompleted += wc_DownloadStringCompleted; } // Start the Async call to find if NoGateway is true or false public void NoGatewayStatus() { // Start the Async's download // Do other work here wc.DownloadStringAsync(new Uri(www.xxxx.xxx)); } private void wc_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e) { // Do work here } // Dispose of the NoGateway object public void Dispose() { wc.DownloadStringCompleted -= wc_DownloadStringCompleted; wc.Dispose(); GC.SuppressFinalize(this); } }

关于源代码的问题:

Here I have not added the finalizer, and normally the finalizer will be called by the GC, and the finalizer will call the Dispose. As I don't have the finalizer, when do I call the Dispose method? Is it the client of the class that has to call it? So my class in the example is called NoGateway and the client could use and dispose of the class like this: using(NoGateway objNoGateway = new NoGateway()) { // Do stuff here } Would the Dispose method be automatically called when execution reaches the end of the using block, or does the client have to manually call the dispose method? i.e. NoGateway objNoGateway = new NoGateway(); // Do stuff with object objNoGateway.Dispose(); // finished with it I am using the WebClient class in my NoGateway class. Because WebClient implements the IDisposable interface, does this mean that WebClient indirectly uses unmanaged resources? Is there a hard and fast rule to follow this? How do I know that a class uses unmanaged resources?


当前回答

没有人回答你是否应该实现IDisposable的问题,即使你不需要它。

简单的回答:不

长一点的回答:

这将允许你的类的消费者使用“using”。我想问的问题是,他们为什么要这么做?大多数开发者不会使用“使用”,除非他们知道他们必须这么做——他们是怎么知道的。要么

很明显,从经验来看(例如,一个套接字类) 它的记录 他们很谨慎,可以看到类实现了IDisposable

因此,通过实现IDisposable,你可以告诉开发人员(至少是一些开发人员)这个类封装了一些必须发布的东西。他们会使用“using”——但在其他情况下,使用是不可能的(对象的作用域不是本地的);他们将不得不开始担心在其他情况下对象的生命周期-我肯定会担心。但这是不必要的

你实现Idisposable,让他们使用使用,但他们不会使用使用,除非你告诉他们。

所以不要这么做

其他回答

推荐的IDisposable模式如下。当编程一个使用IDisposable的类时,通常你应该使用两种模式:

当实现一个不使用非托管资源的密封类时,你只需像普通接口实现一样实现Dispose方法:

public sealed class A : IDisposable
{
    public void Dispose()
    {
        // get rid of managed resources, call Dispose on member variables...
    }
}

当实现一个非密封类时,像这样做:

public class B : IDisposable
{    
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (disposing)
        {
            // get rid of managed resources
        }   
        // get rid of unmanaged resources
    }

    // only if you use unmanaged resources directly in B
    //~B()
    //{
    //    Dispose(false);
    //}
}

注意,我没有在B中声明终结式;只有在有实际的非托管资源要处理时,才应该实现终结器。CLR处理可终结对象与处理不可终结对象是不同的,即使调用了SuppressFinalize。

所以,除非必须,否则你不应该声明终结器,但是你可以给类的继承者一个钩子来调用你的Dispose,如果他们直接使用非托管资源,他们自己实现终结器:

public class C : B
{
    private IntPtr m_Handle;

    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            // get rid of managed resources
        }
        ReleaseHandle(m_Handle);

        base.Dispose(disposing);
    }

    ~C() {
        Dispose(false);
    }
}

如果您没有直接使用非托管资源(SafeHandle和friend不计算在内,因为它们声明了自己的终结器),那么就不要实现终结器,因为GC以不同的方式处理可终结类,即使您稍后抑制了终结器。还要注意,即使B没有终结器,它仍然调用SuppressFinalize来正确处理任何实现了终结器的子类。

当一个类实现IDisposable接口时,这意味着在某个地方有一些非托管资源,当您使用完该类时,这些资源应该被删除。实际资源封装在类中;您不需要显式地删除它们。只需调用Dispose()或将类包装在using(…){}中,就可以确保在必要时删除任何非托管资源。

据我所知,强烈建议不要使用终结器/析构函数:

public ~MyClass() {
  //dont use this
}

大多数情况下,这是因为不知道何时或是否会调用它。处置方法是更好的,特别是如果你使用或直接处置。

使用是好的。使用它:)

处理模式:

public abstract class DisposableObject : IDisposable
{
    public bool Disposed { get; private set;}      

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

    ~DisposableObject()
    {
        Dispose(false);
    }

    private void Dispose(bool disposing)
    {
        if (!Disposed)
        {
            if (disposing)
            {
                DisposeManagedResources();
            }

            DisposeUnmanagedResources();
            Disposed = true;
        }
    }

    protected virtual void DisposeManagedResources() { }
    protected virtual void DisposeUnmanagedResources() { }
}

继承的例子:

public class A : DisposableObject
{
    public Component components_a { get; set; }
    private IntPtr handle_a;

    protected override void DisposeManagedResources()
    {
        try
        {
          Console.WriteLine("A_DisposeManagedResources");
          components_a.Dispose();
          components_a = null;
        }
        finally
        { 
          base.DisposeManagedResources();
        }
    }

    protected override void DisposeUnmanagedResources()
    {
        try
        {
          Console.WriteLine("A_DisposeUnmanagedResources");
          CloseHandle(handle_a);
          handle_a = IntPtr.Zero;
        }
        finally
        { 
          base.DisposeUnmanagedResources();
        }
    }
}

public class B : A
{
    public Component components_b { get; set; }
    private IntPtr handle_b;

    protected override void DisposeManagedResources()
    {
        try
        {
          Console.WriteLine("B_DisposeManagedResources");
          components_b.Dispose();
          components_b = null;
        }
        finally
        { 
          base.DisposeManagedResources();
        }
    }

    protected override void DisposeUnmanagedResources()
    {
        try
        {
          Console.WriteLine("B_DisposeUnmanagedResources");
          CloseHandle(handle_b);
          handle_b = IntPtr.Zero;
        }
        finally
        { 
          base.DisposeUnmanagedResources();
        }
    }
}

实现IDisposable的官方模式很难理解。我认为这个更好:

public class BetterDisposableClass : IDisposable {

  public void Dispose() {
    CleanUpManagedResources();
    CleanUpNativeResources();
    GC.SuppressFinalize(this);
  }

  protected virtual void CleanUpManagedResources() { 
    // ...
  }
  protected virtual void CleanUpNativeResources() {
    // ...
  }

  ~BetterDisposableClass() {
    CleanUpNativeResources();
  }

}

一个更好的解决方案是有一个规则,你总是必须为你需要处理的任何非托管资源创建一个包装器类:

public class NativeDisposable : IDisposable {

  public void Dispose() {
    CleanUpNativeResource();
    GC.SuppressFinalize(this);
  }

  protected virtual void CleanUpNativeResource() {
    // ...
  }

  ~NativeDisposable() {
    CleanUpNativeResource();
  }

}

对于SafeHandle及其衍生品,这些类应该非常少。

对于不直接处理非托管资源(即使存在继承)的一次性类,其结果是强大的:它们不再需要关心非托管资源。它们很容易实现和理解:

public class ManagedDisposable : IDisposable {

  public virtual void Dispose() {
    // dispose of managed resources
  }

}

来自msdn的模式

public class BaseResource: IDisposable
{
   private IntPtr handle;
   private Component Components;
   private bool disposed = false;
   public BaseResource()
   {
   }
   public void Dispose()
   {
      Dispose(true);      
      GC.SuppressFinalize(this);
   }
   protected virtual void Dispose(bool disposing)
   {
      if(!this.disposed)
      {        
         if(disposing)
         {
            Components.Dispose();
         }         
         CloseHandle(handle);
         handle = IntPtr.Zero;
       }
      disposed = true;         
   }
   ~BaseResource()      
   {      Dispose(false);
   }
   public void DoSomething()
   {
      if(this.disposed)
      {
         throw new ObjectDisposedException();
      }
   }
}
public class MyResourceWrapper: BaseResource
{
   private ManagedResource addedManaged;
   private NativeResource addedNative;
   private bool disposed = false;
   public MyResourceWrapper()
   {
   }
   protected override void Dispose(bool disposing)
   {
      if(!this.disposed)
      {
         try
         {
            if(disposing)
            {             
               addedManaged.Dispose();         
            }
            CloseHandle(addedNative);
            this.disposed = true;
         }
         finally
         {
            base.Dispose(disposing);
         }
      }
   }
}