EncryptTheMessage

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

public class EncryptTheMessage
{
    private static string _convertedString;

    private static void Main()
    {
        var fromAtoMupper = new char[13];
        var fromNtoZupper = new char[13];
        var fromAtoMlower = new char[13];
        var fromNtoZlower = new char[13];
        var convertedInputString = new List<char>();
        var theList = new List<string>();
        var inputString = Console.ReadLine();

        while (inputString.ToUpper() != "START")
        {
            inputString = Console.ReadLine();
        }

        inputString = Console.ReadLine();

        while (inputString != null && inputString.ToUpper() != "END")
        {
            if (string.IsNullOrWhiteSpace(inputString))
            {
                inputString = Console.ReadLine();
                continue;
            }
            var indexes = new int[inputString.Length];

            for (var i = 0; i < 13; i++)
            {
                fromAtoMupper[i] = (Convert.ToChar(i + 65));
                fromNtoZupper[i] = (Convert.ToChar((i + 13) + 65));
                fromAtoMlower[i] = (Convert.ToChar(i + 97));
                fromNtoZlower[i] = (Convert.ToChar((i + 13) + 97));
            }

            for (var i = 0; i < inputString.Length; i++)
            {
                var currentChar = inputString[i];
                var charToString = inputString[i].ToString();
                if (Regex.IsMatch(charToString, "^[a-zA-Z0-9]+$"))
                {
                    if (inputString[i] < 97)
                    {
                        if (Array.IndexOf(fromAtoMupper, currentChar) > -1)
                        {
                            indexes[i] = Array.IndexOf(fromAtoMupper, currentChar);
                            convertedInputString.Add(fromNtoZupper[indexes[i]]);
                        }

                        else if (inputString[i] > 47 && inputString[i] < 58)
                        {
                            convertedInputString.Add(inputString[i]);
                        }

                        else
                        {
                            indexes[i] = Array.IndexOf(fromNtoZupper, currentChar);
                            convertedInputString.Add(fromAtoMupper[indexes[i]]);
                        }
                    }
                    else
                    {
                        if (Array.IndexOf(fromAtoMlower, currentChar) > -1)
                        {
                            indexes[i] = Array.IndexOf(fromAtoMlower, currentChar);
                            convertedInputString.Add(fromNtoZlower[indexes[i]]);
                        }
                        else
                        {
                            indexes[i] = Array.IndexOf(fromNtoZlower, currentChar);
                            convertedInputString.Add(fromAtoMlower[indexes[i]]);
                        }
                    }
                }
                else
                {
                    switch (inputString[i])
                    {
                        case ' ':
                            convertedInputString.Add('+');
                            break;
                        case ',':
                            convertedInputString.Add('%');
                            break;
                        case '.':
                            convertedInputString.Add('&');
                            break;
                        case '?':
                            convertedInputString.Add('#');
                            break;
                        case '!':
                            convertedInputString.Add('$');
                            break;
                    }
                }
            }
            convertedInputString.Reverse();
            var currr = convertedInputString.ToArray();
            _convertedString = string.Join("", currr);
            theList.Add(_convertedString);
            convertedInputString.Clear();
            inputString = Console.ReadLine();
        }
        if (theList.Count == 0)
        {
            Console.WriteLine("No messages sent.");
        }
        else
        {
            Console.WriteLine("Total number of messages: {0}", theList.Count);
            for (int i =0 ; i < theList.Count ; i++)
            {
                Console.WriteLine(theList[i]);
            }
        }
    }
}