c#调用C++的DLL时调用失败

本人C#小白。老大要求用C#调用C++的DLL.
C++的我写出来了，可以调用成功.
#include
#include
void main(){
HMODULE hModule = ::LoadLibrary(_T("C:\CT_InfoTerm.dll"));
typedef LONG (WINAPI pDLLProc)(int,char,int,int,int,char*,int);
pDLLProc dllfunc;
int iPort = 0;
char cExtendPort[1] = {'9'};
int iBaudRate = 9600;
int iVoiceType = 0;
int iTimeOut = 30;
char cPin[256];
memset(cPin, NULL, sizeof(cPin));
dllfunc = (pDLLProc)
::GetProcAddress((HMODULE)hModule, "CT_ReadPin");
int iReturn;
if (dllfunc != NULL){
iReturn = dllfunc(iPort, cExtendPort, iBaudRate, iVoiceType, iTimeOut, cPin, sizeof(cPin));
}
}

现在改为C#的
[DllImport("CT_InfoTerm.dll", EntryPoint = "CT_ClosePinPad" ,CallingConvention=CallingConvention.Cdecl)]
public unsafe static extern int CT_ReadPin(int iPortNo, char extendPort, int iBaudRate, int iVoiceType, int iTimeOut, StringBuilder Pin, int iPinSize);

        public unsafe void CT_ReadPin(WebCommand command) 
    {
        int nRet = -3;//打开串口失败(默认)
        if (_canRead)
        {
            int iPortNo = 0;
            char extendPort = '9';
            int iBaudRate = 9600;
            int iVoiceType = Convert.ToInt16(command.Parameter);
            int iTimeOut =30;
           //var Pin=new byte[100];
            StringBuilder Pin = new StringBuilder(1024);
           // string Pin = new string(Convert.ToChar(" "), 256);
           // char Pin = new char();
            int iPinSize =256;
            nRet = Centerm.CT_ReadPin(iPortNo,extendPort,iBaudRate,iVoiceType,iTimeOut,Pin,iPinSize);
            checkresult(command, nRet);
        }
     }
            pin是返回值
            注释的地方是我尝试的方法，并不报任何错误，但是C#的调用不成功，可能是pin这个参数有问题.
            各位大神能不能提供一些思路，供我参考一下，多谢