Vin frame number check

     public static bool CheckVin(string vin)
        {
            if (vin == null || vin.Length != 17) return false;
            int symbol, weight, verify, sum = 0;
            bool isRepeat = true;
            if (vin[8] >= 48 && vin[8] <= 57) verify = vin[8] - 48; //0到9
            else if (vin[8] == 88) verify = 10; //X
            else return false;
            for (int i = 0; i < vin.Length; i++)
            {
                if (i < 7) weight = 8 - i;
                else if (i > 8) weight = 18 - i;
                else if (i == 7) weight = 10;
                else continue;
                if (vin[i] >= 48 && vin[i] <= 57) symbol = vin[i] - 48;//0-9
                else if (vin[i] >= 65 && vin[i] <= 72) symbol = vin[i] - 64;//A-H
                else if (vin[i] >= 74 && vin[i] <= 82 && vin[i] != 79 && vin[i] != 81) symbol = vin[i] - 73;//J-R不含O,Q
                else if (vin[i] >= 83 && vin[i] <= 90) symbol = vin[i] - 81;//S-Z
                else return false;
                sum += symbol * weight;
                if (isRepeat && i > 0) isRepeat = vin[i] == vin[i - (i != 9 ? 1 : 2)];
            }

            if (isRepeat) return false;
            return verify == sum % 11;
        }