Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace caesar_cipher
{
    class Program
    {
        public static char cipher(char ch, int key)
        {
            if (!char.IsLetter(ch))
            {

                return ch;
            }

            char d = char.IsUpper(ch) ? 'A' : 'a';
            return (char)((((ch + key) - d) % 26) + d);


        }

        public static string Encipher(string input, int key)
        {
            string output = string.Empty;

            foreach (char ch in input)
                output += cipher(ch, key);

            return output;
        }

        public static string Decipher(string input, int key)
        {
            return Encipher(input, 26 - key);
        }
        static void Main(string[] args)
        {
            Console.WriteLine("Type a string to encrypt:");
            string UserString = Console.ReadLine();

                int key=0;
            do
            {
                Console.WriteLine("\n");
                Console.Write("Enter your number for a key ");
                try { key = Convert.ToInt32(Console.ReadLine()); }
                catch (Exception e)
                {
                    Console.WriteLine("Enter a number for a key!");
                 
                }
            } while (key == 0);

            Console.WriteLine("\n");


            Console.WriteLine("Encrypted Data");

            string cipherText = Encipher(UserString, key);
            Console.WriteLine(cipherText);
            Console.Write("\n");

            Console.WriteLine("Decrypted Data:");

            string t = Decipher(cipherText, key);
            Console.WriteLine(t);
            Console.Write("\n");




            Console.ReadKey();

        }
    }
    
}