DateTime与yyyy-MM-dd HH:mm格式的字符串相互转换

  string curtime = DateTime.Now.ToString("yyyy-MM-dd HH:mm");
  string endtime = couponDefine.EndDateTime;
  DateTime dtcur = DateTime.ParseExact(curtime, "yyyy-MM-dd HH:mm", null);
  DateTime dtend = DateTime.ParseExact(endtime, "yyyy-MM-dd HH:mm", null);
  if (dtcur >= dtend)
  {                
      issuccess = false;
      return;
   }