using System;

enum Duration { Day, Week, Month };

​

static class DurationExtensions 

{

  public static DateTime From(this Duration duration, DateTime dateTime) 

  {

    switch (duration) 

    {

      case Duration.Day:   return dateTime.AddDays(1);

      case Duration.Week:  return dateTime.AddDays(7);

      case Duration.Month: return dateTime.AddMonths(1);

      default:    throw new ArgumentOutOfRangeException("duration");

    }

  }

}

​

public class Program

{

    public static void Main()

    {

        DateTime datetime = new DateTime();

        Duration d = Duration.Week;

        d.From(datetime);

    }

}