using System;

enum Duration { Day, Week, Month };

​

static class DurationExtensions 

{

  public static string From(this Duration duration) 

  {

    switch (duration) 

    {

      case Duration.Day:   return "1";

      case Duration.Week:  return "2";

      case Duration.Month: return "3";

      default:    throw new ArgumentOutOfRangeException("duration");

    }

  }

}

​

public class Program

{

    public static void Main()

    {

        Duration d = Duration.Week;

        Console.WriteLine(d.From());

    }

}