C#调用VC DLL 输出参数 急！！！

被调dll函数
ykt_readopencardno_local(char * pRegion, char * pDeptid, char * pDeptKey, char * pCardtype, char * pCardno, char * pWarnmsg, char * pErrmsg);

C#调用代码:
  [DllImport("kernel32.dll")]
  private extern static IntPtr LoadLibrary(String path);
  [DllImport("kernel32.dll")]
  private extern static IntPtr GetProcAddress(IntPtr lib, String funcName);
  [DllImport("kernel32.dll")]
  private extern static bool FreeLibrary(IntPtr lib);

  public static IntPtr hLib;

  public Integrated(String DLLPath)
  {

  hLib = LoadLibrary(DLLPath);
  }

  //将要执行的函数转换为委托 
  public Delegate Invoke(String APIName, Type t)
  {
  IntPtr api = GetProcAddress(hLib, APIName);
  return (Delegate)Marshal.GetDelegateForFunctionPointer(api, t);
  }
  public delegate int ykt_readopencardno_local(byte[] pRegion, byte[] pDeptid, byte[] pDeptKey, ref char pCardtype, ref StringBuilder pCardno, ref long pWarnmsg, ref long pErrmsg);//编译 

问题：输出参数pCardno用StringBuilder类型报错：试图读取或写入受保护内存，这通常是指示其他内存已损坏！
求高手指点。

pCardno Char(20) 卡号 可包含数字和字母

　ykt_readopencardno_local(char * pRegion, char * pDeptid, char * pDeptKey, char * pCardtype, char * pCardno, char * pWarnmsg, char * pErrmsg);


-----------------------------------------------

c++ c#
char * ====>string