c# - C# 中的 C 回调函数问题 - 如何将值传递给指针？

Question

我有一个 C 回调定义如下:

Int16 (CALLBACK *ProcessMessage)(Uint16 ServerId,
   const char PTR *RequestMsg, Uint32 RequestSize,
   char PTR **ResponseMsg, Uint32 PTR *ResponseSize,
   Int16 PTR *AppErrCode);

在 C 中使用此回调的示例:

Int16 CALLBACK ProcessMessage(Uint16 ServerId, const char PTR *RequestMsg, Uint32 RequestSize, char PTR **ResponseMsg, Uint32 PTR *ResponseSize, Int16 PTR *AppErrCode){    printf("ProcessMessage() -> ServerId=%u\n", ServerId);    //**** SET THE VALUE FOR RESPONSEMSG (POINTER), THAT'S WHAT I NEED TO DO IN C# ****           sprintf(resp,"(%05lu) REPLY TEST", ServerId);    *ResponseMsg = resp;    printf("ProcessMessage() -> atribuido %p(p) a *ResponseMsg\n", *ResponseMsg);    *ResponseSize = strlen(*ResponseMsg);    *AppErrCode = -1;     return SS_OK;}

Then I've this callback implemented in C#:

 [DllImport("Custom.dll", SetLastError = true)]
    static extern Int16 SS_Initialize(
        UInt16[] ServerIds,
        UInt16 ServerQty,
        [MarshalAs(UnmanagedType.LPStr)] string Binding,
        [MarshalAs(UnmanagedType.LPStr)] string LogPath,
        UInt16 LogDays,
        Int16 LogLevel,
        UInt16 MaxThreads,
        UInt16 MaxConThread,
        ProcessMessageCallback callback);  

回调定义:

public delegate Int16 ProcessMessageCallback(
        UInt16 ServerId,
        [MarshalAs(UnmanagedType.LPStr)] string RequestMsg,
        UInt32 RequestSize,
        [MarshalAs(UnmanagedType.LPStr)] ref string ResponseMsg,
        ref UInt32 ResponseSize,
        ref Int16 AppErrCode);

设置回调的方法:

public void Call_SS_Initialize(
        UInt16[] serverIds,
        string binding,
        string logPath,
        UInt16 logDays,
        Int16 logLevel,
        UInt16 maxThreads,
        UInt16 maxConThread
        )
    {
        Int16 ret;
        try
        {
            pmc = new ProcessMessageCallback(ProcessMessage);

            ret = SS_Initialize(
                serverIds,
                Convert.ToUInt16(serverIds.ToList().Count),
                binding,
                logPath,
                logDays,
                logLevel,
                maxThreads,
                maxConThread,
                pmc);
        }
    }

最后是回调方法，问题在于:

public Int16 ProcessMessage(
      UInt16 ServerId,
      string RequestMsg,
      UInt32 RequestSize,
      ref string ResponseMsg,
      ref UInt32 ResponseSize,
      ref Int16 AppErrCode)
    {
       //Implement return to ResponseMsg POINTER
    }

问题是，ResponseMsg实际上是C中的POINTER。所以在C#方法ProcesMessage中，我必须设置为ResponseMsg DLL 从中获取字符串的内存空间(指针)。

我不能简单地设置 ResponseMsg = "REPLY"，因为当该方法完成时，字符串所在的内存已经被销毁。

我怎样才能做到这一点？欢迎任何建议!!

谢谢!

Answer 1

这就是我为重新创建它所做的事情。也许我的实验会有所帮助。

C# 代码:

public delegate void ProcessMessageCallback(
        [MarshalAs(UnmanagedType.LPStr)] ref string ResponseMsg);

static class test
{
    [DllImport("test.dll")]
    static extern void TestCallback(ProcessMessageCallback callback);

    static public void Main(string[] args)
    {
        TestCallback(MyCallback);
    }

    static void MyCallback(ref string ResponseMsg)
    {
        ResponseMsg = "hi there";
    }
}

C 代码(在 DLL 中):

#include <windows.h>
#include "objbase.h"

__declspec(dllexport) void TestCallback(
    void (* CALLBACK managedCallback(char **)))
{
    char *test = NULL;
    managedCallback(&test);
    printf(test);
    CoTaskMemFree(test); // NB!
}

这成功打印出“hi There”。

注意:CoTaskMemFree 应该用于释放 P/Invoke 层分配的内存。如果您希望返回的字符串保留的时间比调用 C# 回调的方法的时间长，请考虑在释放返回的内存之前将其复制到另一个位置。