RadixConvert.cs

public class RadixConvert
{
    private const int Radix = 62;
    private const string Table = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

    public static string Encode(long number)
    {
        const int length = 16;

        int index = length;
        var buffer = new char[length];

        do
        {
            buffer[--index] = Table[(int)(number % Radix)];

            number = number / Radix;
        } while (number != 0);

        return new string(buffer, index, length - index);
    }

    public static long Decode(string str)
    {
        long pow = 1;
        long result = 0;

        for (int i = str.Length - 1; i >= 0; i--)
        {
            int digit;

            if ((byte)(str[i] - '0') < 10)
            {
                digit = str[i] - '0';
            }
            else if ((byte)(str[i] - 'a') < 26)
            {
                digit = str[i] - 'a' + 10;
            }
            else if ((byte)(str[i] - 'A') < 26)
            {
                digit = str[i] - 'A' + 36;
            }
            else
            {
                return -1;
            }

            result = result + (digit * pow);

            pow = pow * Radix;
        }

        return result;
    }
}