C#API获取COMMBOX的文本

我想获取别的软件上的COMMBOX里文本值，
  string s = "";
  string s1 = "";
  IntPtr myIp = (IntPtr)18094834;
  int j=SendMessage (myIp ,CB_SETCURSEL,1,0);
  SendMessage(myIp, CB_GETLBTEXT, 0, s);
  SendMessage(myIp, CB_GETLBTEXT, 0, s1);
  int k=SendMessage(myIp, CB_GETCOUNT, 0, 0);
  SendMessage(myIp, CB_ADDSTRING, 0, "我是江华");




  SendMessage(myIp, CB_GETLBTEXT, 0, s);
  SendMessage(myIp, CB_GETLBTEXT, 0, s1);
这2行会出错，提示说尝试读取或写入受保护的内存。这通常指示其他内存已损坏
求高手啊，郁闷的很

如果是接受字符串需要用StringBuilder

StringBuilder sb = new StringBuilder(c);

c一定要够大，要不还是这个错误