Dec2Bin.cs

using System;

namespace Dec2Bin
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Dec2Bin - 10進数から2進数に変換");
            Console.WriteLine("プログラム能力を試すのにお薦め");
            while (true)
            {
                Console.Write("> ");
                string input = Console.ReadLine();

                try
                {
                    int dec = int.Parse(input);
                    if (dec < 0) throw new ApplicationException();
                    Console.WriteLine("Result: " + input + " -> " + dec2bin(dec));
                }
                catch (FormatException)
                {
                    break;
                }
                catch (ApplicationException)
                {
                    Console.WriteLine("負の数値は変換に対応していません");
                }
            }
        }

        static string dec2bin(int input)
        {
            if (input == 0) return "0";
            return dec2bin(input / 2) + input % 2;
        }
    }
}