C#调用C++ DLL传参byte[]数组字节值大于127时会变为0x3f的问题解决

最近做了一个网络编程的DLL给C#调用，DLL中封装了一个TCP Client的函数接口，如下所示

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//C++ TCP报文发送接口
int TcpClient_send(unsigned char* buffSend, unsigned int nLen)
{
	unsigned char buff[1024];
	int len = StringToHex(buffSend, buff);
	int nRet = 0;
	if (Connect())
	{
		int sendRes = send(m_hTcpClientSocket, (char*)buff, nLen, 0);
		if (sendRes == SOCKET_ERROR)
		{
			nRet = -2;
			closesocket(m_hTcpClientSocket);
			m_hTcpClientSocket = NULL;
		}
		else
		{
			nRet = sendRes;
		}
	}
	else
	{
		nRet = -1;
	}
	return nRet;
}

char ConvertHexChar(char ch)
{
	if ((ch >= '0') && (ch <= '9'))
		return   ch - 0x30;
	else   if ((ch >= 'A') && (ch <= 'F'))
		return   ch - 'A' + 10;
	else   if ((ch >= 'a') && (ch <= 'f'))
		return   ch - 'a' + 10;
	else   return   (-1);

}

int StringToHex(CString str, unsigned char* senddata)
{
	int hexdata, lowhexdata;
	int hexdatalen = 0;
	int len = str.GetLength();
	for (int i = 0; i < len;)
	{
		char lstr, hstr = str[i];
		if (hstr == ' ')
		{
			i++;
			continue;
		}
		i++;
		if (i >= len)
			break;
		lstr = str[i];
		hexdata = ConvertHexChar(hstr);
		lowhexdata = ConvertHexChar(lstr);
		if ((hexdata == 16) || (lowhexdata == 16))
			break;
		else
			hexdata = hexdata * 16 + lowhexdata;
		i++;
		senddata[hexdatalen] = (char)hexdata;
		hexdatalen++;
	}
	return hexdatalen;
}

在C#中调用该接口的时候，刚开始我是通过string down = System.Text.Encoding.Default.GetString()或者System.Text.Encoding.UTF8.GetString()的方式将字节数组转换成string然后传递给TcpClientSyn_send函数，但是这两种方法传递的字节数组到达DLL中时大于127的字节都会变为0x3f也就是63。后来果断抛弃该种方法，转而通过自定义函数BytesToHexString()将字节数组变为类似于"AA AA A0 01"这样的字符串，然后再DLL中通过另一个自定义函数StringToHex()来再次转换成字节数组。

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//C#调用DLL中的TcpClient_send函数
private static void SendThread(object n)
{
    //发送
    byte[] buffSend = { 0xAA, 0xAA, 0x70, 0x01, 0x00, 0x00, 0x00, 0x01, 0x00, 0x80, 0x17, 0xC5, 0x51, 0xE2, 0xEE, 0xEE };
    //string down = System.Text.Encoding.Default.GetString(buffSend);
    //string down = System.Text.Encoding.UTF8.GetString(buffSend);
    string HexString_down = BytesToHexString(buffSend);
    int sendLen=TcpClient_send(HexString_down, 16);
    if (sendLen> 0)
    {
    }
    
    //接收     
    uint nRecvLen;
    nRecvLen = 0;
    StringBuilder buffRecv = new StringBuilder(1024);
    buffRecv.Clear();
    int recvLen=TcpClientSyn_recv(buffRecv,ref nRecvLen);
     if (recvLen> 0)
     {
     }

}


private static string BytesToHexString(byte[] byteDatas)
{
    StringBuilder builder = new StringBuilder();
    for (int i = 0; i < byteDatas.Length; i++)
    {
        builder.Append(string.Format("{0:X2} ", byteDatas[i]));
    }
    return builder.ToString().Trim();
}