Caesar_interview.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;

namespace Caesar
{
    static class Program
    {
        static void Main(string[] args)
        {
            // Skip argument checks, it is not essential to a clean and simple implementation of the algorithm
            
            int offset = int.Parse(args[0]);            
            
            string inputFilename = args[1];
            
            if (args.Any(s => s == "-d"))
                offset = -offset; // Toggle Caesarian en/decryption, which is just shifting the opposite way
                
            string inputText = File.ReadAllText(inputFilename);
            
            // Use a range expression to avoid getting the alphabet wrong.
            // string/List<char> don't matter, but is slightly more convenient with strings 
            // as you can type out the alphabet directly "abcdef..." instead of { 'a', 'b' ... }
            string alphabet = Range('a', 'z').Concat("æøå").CharsToString();
            
            // Caesarian encryption is just independent mappings of each character in the string:
            string shiftedText = (from c in inputText 
                                  select CyclicShift(c, offset, alphabet)).CharsToString();
            
            File.WriteAllText(inputFilename, shiftedText);
            Console.WriteLine(shiftedText);
        }
        
        static char CyclicShift(char c, int offset, string alphabet)
        {
            char lowerC = char.ToLowerInvariant(c);

            int index = alphabet.IndexOf(lowerC);

            if (index < 0)
            {
                return c; // If not covered by the alphabet (spaces, line shifts etc.) just pass it through
            }
            else
            {
                var shifted = alphabet[(index + offset + alphabet.Length) % alphabet.Length];

                if (char.IsUpper(c))
                {
                    shifted = char.ToUpperInvariant(shifted);
                }

                return shifted;
            }
        }
        
        static IEnumerable<char> Range(char first, char last)
        {
            for (char c = first; c <= last; c++)
                yield return c;
        }
        
        static string CharsToString(this IEnumerable<char> @string)
        {
            return new string(@string.ToArray());
        }
    }
}

Caesar_short.cs

using System;
using System.Linq;
using System.IO;

namespace Caesar
{
    class Program
    {
        static void Main(string[] args)
        {
            int offset = int.Parse(args[0]) * (args.Any(s => s == "-d") ? -1 : 1);
            string alphabet = new string(Enumerable.Range('a', 26).Select(i => (char)i).Concat("æøå").ToArray());
            string result = new string(File.ReadAllText(args[1])
                .Select(c => char.IsWhiteSpace(c) ? c : alphabet[(alphabet.IndexOf(c) + offset + alphabet.Length) % alphabet.Length])
                .ToArray());
            Console.WriteLine(result);
            File.WriteAllText(args[1], result);
        }
    }
}