略谈GCHandle

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)          {    <span style="white-space:pre"></span>      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;    <span style="white-space:pre"></span>      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();      }  }