using System;

using System.Linq;

​

namespace Example

{

    static public class ExtensionMethods

    {

        static public string CaesarShift(this string input, int offset)

        {

            const string alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

            return new string

            (

                input.Select 

                (

                    c =>

                    { 

                        var index = alphabet.IndexOf(c);

                        return index == -1 

                            ? c

                            : alphabet[(index + offset + alphabet.Length) % alphabet.Length];

                    }

                ).ToArray()

            );

        }

    }

    public class Program

    {

        public static void Main()

        {

            var word = "THE QUICK RED FOX JUMPS OVER THE LAZY BROWN DOG. the quick red fox jumps over the lazy brown dog. 123456789!(*&#*";

            var encoded = word.CaesarShift(3);

            Console.WriteLine(encoded);

            var decoded = encoded.CaesarShift(-3);

            Console.WriteLine(decoded);

        }

    }

}