18位的身份证验证

private string CheckCidInfo(string cid)
  {
   string[] aCity = new string[]{null,null,null,null,null,null,null,null,null,null,null,"北京","天津","河北","山西","内蒙古",null,null,null,null,null,"辽宁","吉林","黑龙江",null,null,null,null,null,null,null,"上海","江苏","浙江","安微","福建","江西","山东",null,null,null,"河南","湖北","湖南","广东","广西","海南",null,null,null,"重庆","四川","贵州","云南","西藏",null,null,null,null,null,null,"陕西","甘肃","青海","宁夏","新疆",null,null,null,null,null,"台湾",null,null,null,null,null,null,null,null,null,"香港","澳门",null,null,null,null,null,null,null,null,"国外"};
   double iSum=0;
   string info="";
   System.Text.RegularExpressions.Regex rg = new System.Text.RegularExpressions.Regex(@"^\d{17}(\d|x)$");
   System.Text.RegularExpressions.Match mc = rg.Match(cid);
   if(!mc.Success)
   {
    return "";
   }  
   cid = cid.ToLower();
   cid = cid.Replace("x","a");
   if(aCity[int.Parse(cid.Substring(0,2))]==null)
   {
    return "非法地区";
   }
   try
   {
    DateTime.Parse(cid.Substring(6,4)+"-"+cid.Substring(10,2)+"-"+cid.Substring(12,2));
   }
   catch
   {
    return "非法生日";
   }
   for(int i=17;i>=0;i--)
   {   
    iSum +=(System.Math.Pow(2,i)%11)*int.Parse(cid[17-i].ToString(),System.Globalization.NumberStyles.HexNumber);
   }
   if(iSum%11!=1)
    return("非法证号");
  
   return(aCity[int.Parse(cid.Substring(0,2))]+","+cid.Substring(6,4)+"-"+cid.Substring(10,2)+"-"+cid.Substring(12,2)+","+(int.Parse(cid.Substring(16,1))%2==1¡"男":"女"));
 
  }