Тип ENUM

public enum DaysOfWeek
{
   Monday = 0,
   Tuesday = 1,
   Wednesday = 2,
   Thursday = 3,
   Friday = 4,
   Saturday = 5,
   Sunday = 6
}

using System;
using System.Collections.Generic;
using System.Text;

namespace EnumerationExample
{
   class Program
    {
       public enum DaysOfWeek
       {
           Monday = 0,
           Tuesday = 1,
           Wednesday = 2,
           Thursday = 3,
           Friday = 4,
           Saturday = 5,
           Sunday = 6
        }

        static void Main(string[] args)
        {
           WriteText(DaysOfWeek.Sunday);
           Console.ReadLine();
        }

       static void WriteText(DaysOfWeek days)
       {
           switch (days)
           {
               case DaysOfWeek.Monday:
                   Console.WriteLine("Понедельник");
                   break;
               case DaysOfWeek.Tuesday:
                   Console.WriteLine("Вторник");
                   break;
               case DaysOfWeek.Wednesday:
                   Console.WriteLine("Среда");
                   break;
               case DaysOfWeek.Thursday:
                   Console.WriteLine("Четверг");
                   break;
               case DaysOfWeek.Friday:
                   Console.WriteLine("Пятница");
                   break;
               case DaysOfWeek.Saturday:
                   Console.WriteLine("Суббота");
                   break;
               case DaysOfWeek.Sunday:
                   Console.WriteLine("Понедельник");
                   break;
           }
       }
   }
}

public enum E1 {One, Two, Three}

const int One = 0;
const int Two = 1;
const int Three = 2;