《你不常用的c#之二》: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) { 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(); } }