using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        //Convert 10101001 from binary to decimal

        Console.WriteLine(NumberConversion.Convert(2, 10, "10101001"));

​

        //Convert 256 from decimal to binary

        Console.WriteLine(NumberConversion.Convert(10, 2, "256"));

​

        //Convert 19 from decimal to hex

        Console.WriteLine(NumberConversion.Convert(10, 16, "28"));

​

        //Convert 1034 from decimal to quaternary numeral system

        Console.WriteLine(NumberConversion.Convert(10, 4, "1034"));

    }

}

​

public class NumberConversion {

    private static readonly char[] alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray();

​

    public static string Convert(uint baseSystem, uint destinationSystem, string number) =>

        Convert(System.Convert.ToInt32(baseSystem), System.Convert.ToInt32(destinationSystem), number);

    private static string Convert(int baseSystem, int destinationSystem, string number) {

        if(baseSystem == 0 || destinationSystem == 0)

            throw new ArgumentException("Number System connot be zero");

        if(number == "0") 

            return "0";

        if(baseSystem == destinationSystem)

            return number.ToString();

​

        long decimalNumber;

        if(baseSystem != 10)

            decimalNumber = ConvertToDecimal(baseSystem, number);

        else

            decimalNumber = System.Convert.ToInt32(number);

​

        if(destinationSystem != 10)

            return ConvertFromDecimalToAny(decimalNumber, destinationSystem);

        else 

            return decimalNumber.ToString();

    }

​

    private static long ConvertToDecimal(int baseSystem, string number) {

        string numbers = number.ToString();

        long[] heavyNumbers = new long[numbers.Length];

​

        int i = 0;

        int p = numbers.Length - 1;

​

        while(i < numbers.Length) {

            heavyNumbers[i] = System.Convert.ToInt64(System.Math.Pow(baseSystem, p)) * System.Convert.ToInt64(numbers[i].ToString());

            i++;

            p--;

        }

​

        long decimalNumber = 0;

        for(int s = 0; s < heavyNumbers.Length; s++) {

            decimalNumber += heavyNumbers[s];

        }

​

        return decimalNumber;

    }

​

    private static string ConvertFromDecimalToAny(long decimalNumber, int destinationSystem) {

        List<long> rest = new List<long>();

        while(decimalNumber != 0) {

            rest.Add(decimalNumber % destinationSystem);

            decimalNumber = decimalNumber / destinationSystem;

        }

​

        string[] results = new string[rest.Count];

        for(int i = 0; i < rest.Count; i++) {

            if(rest[i] >= 10) {

                results[i] = alpha[rest[i] - 10].ToString();

            } else {

                results[i] = rest[i].ToString();

            }

        }

​

        string result = "";

        for(int i = rest.Count - 1; i >= 0; i--) {

            result += results[i];

        }

​

        return result;

    }

}