• 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();
          }
      }
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  • 原文地址:https://www.cnblogs.com/onsuccessway/p/3619269.html
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