C#调用C++DLL中的回调函数（异步的情况，DLL中会在事件触发时调用函数）

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();

}

}