DecTo <-> ToDec

        static string decTo(int dec, int to)
        {
            string chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
            if (to < 2 || to > 36)
                return "";
            int r;
            string result = "";
            while (dec >= to)
            {
                r = dec % to;
                result = chars[r] + result;
                dec = dec / to;
            }
            result = chars[dec] + result;
            return result;
        }
        static int toDec(string value, int to)
        {
            int dec = 0;
            for (int i = 0, j = value.Length - 1; i < value.Length; i++, j--)
            {
                if (value[i] == 'A') { dec += 10 * (int)Math.Pow(to, j); }
                else if (value[i] == 'B') { dec += 11 * (int)Math.Pow(to, j); }
                else if (value[i] == 'C') { dec += 12 * (int)Math.Pow(to, j); }
                else if (value[i] == 'D') { dec += 13 * (int)Math.Pow(to, j); }
                else if (value[i] == 'E') { dec += 14 * (int)Math.Pow(to, j); }
                else if (value[i] == 'F') { dec += 15 * (int)Math.Pow(to, j); }
                else if (value[i] == 'G') { dec += 16 * (int)Math.Pow(to, j); }
                else if (value[i] == 'H') { dec += 17 * (int)Math.Pow(to, j); }
                else if (value[i] == 'I') { dec += 18 * (int)Math.Pow(to, j); }
                else if (value[i] == 'J') { dec += 19 * (int)Math.Pow(to, j); }
                else if (value[i] == 'K') { dec += 20 * (int)Math.Pow(to, j); }
                else if (value[i] == 'L') { dec += 21 * (int)Math.Pow(to, j); }
                else if (value[i] == 'M') { dec += 22 * (int)Math.Pow(to, j); }
                else if (value[i] == 'N') { dec += 23 * (int)Math.Pow(to, j); }
                else if (value[i] == 'O') { dec += 24 * (int)Math.Pow(to, j); }
                else if (value[i] == 'P') { dec += 25 * (int)Math.Pow(to, j); }
                else if (value[i] == 'Q') { dec += 26 * (int)Math.Pow(to, j); }
                else if (value[i] == 'R') { dec += 27 * (int)Math.Pow(to, j); }
                else if (value[i] == 'S') { dec += 28 * (int)Math.Pow(to, j); }
                else if (value[i] == 'T') { dec += 29 * (int)Math.Pow(to, j); }
                else if (value[i] == 'U') { dec += 30 * (int)Math.Pow(to, j); }
                else if (value[i] == 'V') { dec += 31 * (int)Math.Pow(to, j); }
                else if (value[i] == 'W') { dec += 32 * (int)Math.Pow(to, j); }
                else if (value[i] == 'X') { dec += 33 * (int)Math.Pow(to, j); }
                else if (value[i] == 'Y') { dec += 34 * (int)Math.Pow(to, j); }
                else if (value[i] == 'Z') { dec += 35 * (int)Math.Pow(to, j); }
                else { dec += (value[i] - '0') * (int)Math.Pow(to, j); }
            }
            return dec;
        }
        static void Main(string[] args)
        {
            while (true)
            {
                Console.WriteLine("From: ");
                int from = int.Parse(Console.ReadLine());
                Console.WriteLine("To: ");
                int to = int.Parse(Console.ReadLine());
                Console.WriteLine("Value: ");
                string value = Console.ReadLine();
                Console.WriteLine(from + "->" + to + " " + decTo(Convert.ToInt32(toDec(value, from)), to));
            }
        }