我C#程序里面调用我自己用Delphi写的DLL,调用InitReader这个函数正常，但调用RWCom这个函数后就报“尝试读取或写入受保护的内存”错误，请各位朋友指教！代码如下：
[DllImport("ExportMeter.dll")]
public static extern bool InitReader(string ComName, int Rate, int ParityBit, int iIntervalTimeout, int iDelay);
[DllImport("ExportMeter.dll")]
public static extern string RWCom(string ComData, bool bAns);

private string SendReceivedData(string strCmd, string strMsg, bool bBcc)
{
    string strReceivedData = "";
    string strReturnData = "";
    byte[] ASendData = GetSendByteArray(strCmd, strMsg, bBcc);
    string strSendData = pf.ByteArrayToHexString(ASendData, false);
    //向串口发送数据
    bool bRevDataOK = false;
    byte[] AReceivedData = null;
    if (ASendData[0] == 6) //发送数据后立即关闭串口更改波特率再打开串口接收数据
    {
        strReturnData = RWComUpdBaudrate(strSendData, SerialParam[0], int.Parse(SerialParam[0]), 2, 100, 3000);
        if (!strReturnData.Equals(""))
        {
            AReceivedData = pf.HexStringToByteArray(strReturnData);
            bRevDataOK = true;
         }
     }
     else
     {
         strReturnData = RWCom(strSendData, true);
         if (!strReturnData.Equals(""))
         {
             AReceivedData = pf.HexStringToByteArray(strReturnData);
             bRevDataOK = true;
         }
      }
      if (bRevDataOK) //读取数据成功
      {
          strReceivedData = Encoding.ASCII.GetString(AReceivedData);
          strReceivedData = strReceivedData.ToUpper();
          ShowReceiveData(strReceivedData);
      }
      return strReceivedData;
}
执行strReturnData = RWCom(strSendData, true);这行时就报“尝试读取或写入受保护的内存”错误
DLL中RWCom这个函数的定义是function RWCom(ComData: String; bAns: Boolean): String; stdcall;且这个函数没有任何问题，为何会报这个错，我都快疯了。调用InitReader这个函数正常。跪请各位大侠帮忙看一下，谢谢！

Dll源码没有任何问题，我把DLL中RWCom这个函数写成如下，也报同样的错误。
function RWCom(ComData: String; bAns: Boolean): String; stdcall;
begin
  Result := ComData;
end;

怎么没有朋友回呀，急呀！