GCHandler的使用

众所周知，我们在使用c#托管代码时，内存地址和GC回收那不是我们关心的，CLR已经给我们暗箱操作。

但是如果我们在c#中调用了一个非托管代码，比如vc的DLL,而且他有个回调函数，需要引用c#中的某个对象并操作，
这时候你就得要小心了。
要是非托管代码中用到得托管代码那个对象被GC给回收了，这时候就会报内存错误。
所以我们就要把那个对象“钉”住(pin)，让它的内存地址固定，而不被垃圾回收掉，然后最后我们自己管理，自己释放内存,这时候就需要GCHandle,来看个msdn上的例子:

 public delegate bool CallBack(int handle, IntPtr param);
         public class LibWrap
         {
             [DllImport("user32.dll")]
          public static extern bool EnumWindows(CallBack cb, IntPtr param);
      }
  
  class Program
      {
          static void Main(string[] args)
          {
  
  TextWriter tw = System.Console.Out;
              GCHandle gch = GCHandle.Alloc(tw);
              CallBack cewp = new CallBack(CaptureEnumWindowsProc);
              LibWrap.EnumWindows(cewp, (IntPtr)gch);
              gch.Free();
              Console.Read();
  
  }
  private static bool CaptureEnumWindowsProc(int handle, IntPtr param)
          {
              GCHandle gch = (GCHandle)param;
              TextWriter tw = (TextWriter)gch.Target;
  
  tw.WriteLine(handle);
              return true;
          }
  
  }

对上面的代码，略加解释：gch 会钉住(pin)tw这个对象，使其不受GC管理，告诉它，以后你崩管我，我也不用给你上税,其实管理权已经给gch，通过free来释放内存。
这种情况主要用在托管和非托管代码交互的时候，防止内存泄露来使用GCHandle。

另也可以使用GC.KeepAlive 方法(引用msdn)
KeepAlive 方法的目的是确保对对象的引用存在，该对象有被垃圾回收器过早回收的危险。这种现象可能发生的一种常见情形是，当在托管代码或数据中已没有对该对象的引用，但该对象仍然在非托管代码（如 Win32 API、非托管 DLL 或使用 COM 的方法）中使用。
下面是例子:

using System;
  using System.Threading;
  using System.Runtime.InteropServices;
  
  // A simple class that exposes two static Win32 functions.
  // One is a delegate type and the other is an enumerated type.
  public class MyWin32
  {
      // Declare the SetConsoleCtrlHandler function 
      // as external and receiving a delegate.   
      [DllImport("Kernel32")]
      public static extern Boolean SetConsoleCtrlHandler(HandlerRoutine Handler,
          Boolean Add);
  
      // A delegate type to be used as the handler routine 
      // for SetConsoleCtrlHandler.
      public delegate Boolean HandlerRoutine(CtrlTypes CtrlType);
  
      // An enumerated type for the control messages 
      // sent to the handler routine.
      public enum CtrlTypes
      {
          CTRL_C_EVENT = 0,
          CTRL_BREAK_EVENT,
          CTRL_CLOSE_EVENT,
          CTRL_LOGOFF_EVENT = 5,
          CTRL_SHUTDOWN_EVENT
      }
  }
  
  public class MyApp
  {
      // A private static handler function in the MyApp class.
      static Boolean Handler(MyWin32.CtrlTypes CtrlType)
      {
          String message = "This message should never be seen!";
   // A switch to handle the event type.
          switch (CtrlType)
          {
              case MyWin32.CtrlTypes.CTRL_C_EVENT:
                  message = "A CTRL_C_EVENT was raised by the user.";
                  break;
              case MyWin32.CtrlTypes.CTRL_BREAK_EVENT:
                  message = "A CTRL_BREAK_EVENT was raised by the user.";
                  break;
              case MyWin32.CtrlTypes.CTRL_CLOSE_EVENT:
                  message = "A CTRL_CLOSE_EVENT was raised by the user.";
                  break;
              case MyWin32.CtrlTypes.CTRL_LOGOFF_EVENT:
                  message = "A CTRL_LOGOFF_EVENT was raised by the user.";
                  break;
              case MyWin32.CtrlTypes.CTRL_SHUTDOWN_EVENT:
                  message = "A CTRL_SHUTDOWN_EVENT was raised by the user.";
                  break;
          }
  
          // Use interop to display a message for the type of event.
          Console.WriteLine(message);
  
          return true;
      }
  
      public static void Main()
      {
  
          // Use interop to set a console control handler.
          MyWin32.HandlerRoutine hr = new MyWin32.HandlerRoutine(Handler);
          MyWin32.SetConsoleCtrlHandler(hr, true);
  
          // Give the user some time to raise a few events.
          Console.WriteLine("Waiting 30 seconds for console ctrl events");
  
          // The object hr is not referred to again.
 // The garbage collector can detect that the object has no
          // more managed references and might clean it up here while
          // the unmanaged SetConsoleCtrlHandler method is still using it.      
  
          // Force a garbage collection to demonstrate how the hr
          // object will be handled.
          GC.Collect();
          GC.WaitForPendingFinalizers();
          GC.Collect();
  
          Thread.Sleep(10000);
  
          // Display a message to the console when the unmanaged method
          // has finished its work.
          Console.WriteLine("Finished!");
  
          // Call GC.KeepAlive(hr) at this point to maintain a reference to hr. 
          // This will prevent the garbage collector from collecting the 
          // object during the execution of the SetConsoleCtrlHandler method.
          GC.KeepAlive(hr);
          Console.Read();
      }
  }