Kako da ispravim ovu gresku C#

using System;
public class Duzina
{
    private double duz;
    private const double NULA = 0;


    public double DUZ
    {
        set
        {
            duz = (value > NULA) ? value : 0;
        }

        get
        {
            return duz;
        }
    }
    public double toKilometar(double metar)
    {
        return metar / 1000;
    }
}


class Program
{
    public static void Main()
    {
        Duzina duzina = new Duzina();
        char izbor;
        char duzine;



        do
        {
            do
            {
                Console.WriteLine("1 Duzina");
                Console.WriteLine("2 Povrsina");
                Console.WriteLine("3 Izlaz");

               do
               {
                    izbor = (char)Console.Read();
               } while (izbor == '\n' | izbor == '\r');
            } while (izbor < '1' | izbor > '3');
       



        switch (izbor)
        {
            case '1':
                {
                
                    Console.WriteLine("DUZINA");
                    Console.WriteLine();
                    Console.WriteLine("1 Metar");
                    Console.WriteLine("2 Kilometar");
                    Console.WriteLine("3 Centimetar");
                    do
                    {
                        duzine = (char)Console.Read();
                    } while (duzine == '\n' | duzine == '\r');
                    switch (duzine)
                    {
                        case '1':                                                                        //GRESKA!!!
                            {
                                 duzina.DUZ = duzina.toKilometar(Convert.ToDouble(Console.ReadLine()));//Input string was not in a //correct format ,kako da ispravim ovu gresku
                                 Console.WriteLine(duzina.DUZ);
                            }
                            break;

                    }
                } break;
        }
    } while (izbor != '3');
      


     
    }
}

Možeš svoj zadatak i na ovaj način možda riješiti(samo dio napisan ostalo možeš i sam po šablonu  :  

 public class Duzina
    {
        private double duz;
      
        public double DUZ
        {
            set
            {
                duz = (value > 0) ? value : 0;
            }

            get
            {
                return duz;
            }
        }
        public double toKilometar(double metar)
        {
            return (metar /  1000);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Duzina duzina = new Duzina();
            string unos = " ";
            string izbor = " ";
            do
            {

                Console.WriteLine("Unesite što želite ?");
                Console.WriteLine("1 Duzina");
                Console.WriteLine("2 Površina");
                Console.WriteLine("3 Izlaz");
             
                unos = Console.ReadLine();
                switch (unos)
                {
                    case "1":
                        Console.WriteLine("Duzina");
                        Console.WriteLine("Izaberite :  1 Metri");
                        Console.WriteLine("Unesite izbor: ");

                        izbor = Console.ReadLine();
                        switch (izbor)
                        {
                            case "1":
                                Console.WriteLine("Unesite metre : ");
                                duzina.DUZ = double.Parse(Console.ReadLine());
                                Console.WriteLine("Duzina u kilometrima {0} km .",
                                    duzina.toKilometar(duzina.DUZ));
                                break;

                        }
                        break;
                    case "2":
                        Console.WriteLine("Površina");
                        break;
                    case "3":
                        Console.WriteLine("Gotovo!");
                        break;
                }
            } while (unos != "3");

        }
    }

kakvo elegantno rijesenje Private

Trebat ce mi dosta dosta iskustva i vremena da na ovakav nacin razmisljam.

Hvala za kod citljiv jednostavan i radi ono sto zelim