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

使用lambdas代替IDisposable。

我从来没有对使用/IDisposable这个想法感到兴奋。问题是它要求调用者:

知道他们必须使用IDisposable 记住使用“using”。

我新的首选方法是使用工厂方法和lambda代替

假设我想用SqlConnection(应该用using包装的东西)做一些事情。通常你会这么做

using (Var conn = Factory.MakeConnection())
{
     conn.Query(....);
}

新方法

Factory.DoWithConnection((conn)=>
{
    conn.Query(...);
}

在第一种情况下,调用者不能简单地使用using语法。在第二种情况下,用户没有选择。没有创建SqlConnection对象的方法,调用者必须调用DoWithConnection。

DoWithConnection看起来像这样

void DoWithConnection(Action<SqlConnection> action)
{
   using (var conn = MakeConnection())
   {
       action(conn);
   }
}

MakeConnection现在是私有的

其他回答

使用lambdas代替IDisposable。

我从来没有对使用/IDisposable这个想法感到兴奋。问题是它要求调用者:

知道他们必须使用IDisposable 记住使用“using”。

我新的首选方法是使用工厂方法和lambda代替

假设我想用SqlConnection(应该用using包装的东西)做一些事情。通常你会这么做

using (Var conn = Factory.MakeConnection())
{
     conn.Query(....);
}

新方法

Factory.DoWithConnection((conn)=>
{
    conn.Query(...);
}

在第一种情况下,调用者不能简单地使用using语法。在第二种情况下,用户没有选择。没有创建SqlConnection对象的方法,调用者必须调用DoWithConnection。

DoWithConnection看起来像这样

void DoWithConnection(Action<SqlConnection> action)
{
   using (var conn = MakeConnection())
   {
       action(conn);
   }
}

MakeConnection现在是私有的

using(NoGateway objNoGateway = new NoGateway())

等于

try
{
    NoGateway = new NoGateway();
}

finally
{
    NoGateway.Dispose();
}

在GC销毁对象时调用终结器。这可能与你离开方法时完全不同。Dispose of IDisposable在你离开using块后立即被调用。因此,这种模式通常是在您不再需要资源后立即使用using来释放它们。

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);
         }
      }
   }
}

另一个答案的某些方面有点不正确,原因有二:

首先,

using(NoGateway objNoGateway = new NoGateway())

Actually相当于:

try
{
    NoGateway = new NoGateway();
}
finally
{
    if(NoGateway != null)
    {
        NoGateway.Dispose();
    }
}

This may sound ridiculous since the 'new' operator should never return 'null' unless you have an OutOfMemory exception. But consider the following cases: 1. You call a FactoryClass that returns an IDisposable resource or 2. If you have a type that may or may not inherit from IDisposable depending on its implementation - remember that I've seen the IDisposable pattern implemented incorrectly many times at many clients where developers just add a Dispose() method without inheriting from IDisposable (bad, bad, bad). You could also have the case of an IDisposable resource being returned from a property or method (again bad, bad, bad - don't 'give away your IDisposable resources)

using(IDisposable objNoGateway = new NoGateway() as IDisposable)
{
    if (NoGateway != null)
    {
        ...

如果'as'操作符返回null(或返回资源的属性或方法),并且'using'块中的代码可以防止'null',那么在尝试对空对象调用Dispose时,您的代码不会因为'内置'空检查而崩溃。

你的回复不准确的第二个原因是由于以下stmt:

在GC销毁对象时调用终结器

First, Finalization (as well as GC itself) is non-deterministic. THe CLR determines when it will call a finalizer. i.e. the developer/code has no idea. If the IDisposable pattern is implemented correctly (as I've posted above) and GC.SuppressFinalize() has been called, the the Finalizer will NOT be called. This is one of the big reasons to properly implement the pattern correctly. Since there is only 1 Finalizer thread per managed process, regardless of the number of logical processors, you can easily degrade performance by backing up or even hanging the Finalizer thread by forgetting to call GC.SuppressFinalize().

我已经在我的博客上发布了Dispose模式的正确实现:如何正确地实现Dispose模式

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
  }

}