C＃调用C＋＋的dll，返回空字符串，求助

[hanfaye]

C++的Dll
extern "C" __declspec(dllexport)
bool __cdecl GetResult(int value1,unsigned char * value2)
{
if (value1)
FuncA(value2);
else
FuncB(value2);

return true;
}

void FuncA(unsinged char* value)
{
....
}

void FuncB(unsigned char* value)
{
...
}

C#里调用
string strValue;
int iCount;
GetResult(iCount,ref strValue);//不用ref也是返回空值
得到的strValue为空值，请问，Dll里应该怎样定义传入的参数，才能返回字符串给C#?

[wuyi8808]

在函数声明的 ref 前面加上 [in] [out] 属性试试:

[DllImport("xxx.dll", CharSet = CharSet.Auto)]
static extern bool GetResult(int iCount, [in] [out] ref string strValue);

[lovevsnet]

[DllImport("xxx.dll", CharSet = CharSet.Auto)]
static extern bool GetResult(int iCount, [In,Out]string strValue);
-------------------------------------------------------------------
[DllImport("xxx.dll", CharSet = CharSet.Auto)]
static extern bool GetResult(int iCount,[MarshalAs(UnmanagedType.LPWStr)] string strValue);
------------------------------------------------------------------------
试试行不行？
不行改用,[MarshalAs(UnmanagedType.LPWStr)] ref string

[暗石绿]

static extern bool GetResult(int value1, byte[] value2);

如果这个不行的话，就试试

static extern bool GetResult(int value1,ref byte[] value2);

[真相重于对错]

ref stringbuilder//注意大小写

[hanfaye]

问题还没有解决。。。

昨天没有把每个函数都描述得很清楚。

其中，FuncA的定义是
FuncA( CONNECTION_HANDLE hConnection,
const char* pchSendBuffer, // command to send
unsigned long ulSendBufferLen,// command length
unsigned char uchDeviceID,// device id
int iCmdDataMode, // command data mode
unsigned char* pchRcvBuffer, // buffer to store response
//这里的pchRcvBuffer就是我想返回给C＃的值
unsigned long ulRcvBufferSize,// size of buffer to store response
unsigned long ulTimeout, // timeout in milliseconds
unsigned long* pulNumBytesRcved,// out param - number of bytes received
unsigned long* pulStatusCode // out param - device status code
);

当C＃调用Dll中的GetResult时（参数定义上面已说明），
bool bSend=SendCommand(
CONNECTION_HANDLE,
command,
out receiveBuffer,
//这里无论用out/ref string/StringBuilder,当FuncA里的pchRcvBuffer有返回值时，
//C＃就会出现一个NullReferenceExcpetion.
//当pchRcvBuffer没有返回值时，就不会出错。
ref bytesReceived,
ref statusCode);