驗證身份證字號的正確性

public string IDChk(string vid)

{

    List<string> FirstEng = new List<string> { "A", "B", "C", "D", "E", "F", "G", "H", "J", "K", "L", "M", "N", "P", "Q", "R", "S", "T", "U", "V", "X", "Y", "W", "Z", "I", "O" };

    string aa = vid.ToUpper();

    bool chackFirstEnd = false;

    if (aa.Trim().Length == 10)

    {

        byte firstNo = Convert.ToByte(aa.Trim().Substring(1, 1));

        if (firstNo > 2 || firstNo < 1)

        {

            return "2";

        }

        else

        {

            int x;

            for (x = 0; x < FirstEng.Count; x++)

            {

                if (aa.Substring(0, 1) == FirstEng[x])

                {

                    aa = string.Format("{0}{1}", x + 10, aa.Substring(1, 9));

                    chackFirstEnd = true;

                    break;

                }

  

            }

            if (!chackFirstEnd)

                return "3";

  

            int i = 1;

            int ss = int.Parse(aa.Substring(0, 1));

            while (aa.Length > i)

            {

                ss = ss + (int.Parse(aa.Substring(i, 1)) * (10 - i));

                i++;

            }

            aa = ss.ToString();

            if (vid.Substring(9, 1) == "0")

            {

                if (aa.Substring(aa.Length - 1, 1) == "0")

                {

                    return "0";

                }

                else

                {

                    return "4";

                }

            }

            else

            {

                if (vid.Substring(9, 1) == (10 - int.Parse(aa.Substring(aa.Length - 1, 1))).ToString())

                {

  

                    return "0";

                }

                else

                {

                    return "4";

                }

            }

        }

    }

    else

    {

  

        return "1";

    }

}

//回傳1 代表字數不到10  

//回傳2代表第二碼非1,2  

//回傳3 代表首碼有誤  

//回傳4代表檢查碼不對