Mã dịch vòng

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace MaDichVong
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private string strInput = "";
        private string strOutput = "";
        private int iKey;

        private void btEncrypt_Click(object sender, EventArgs e)
        {
            strInput = tbInput.Text;
            strOutput = "";
            try
            {
                iKey = int.Parse(tbKey.Text.ToString());
                iKey = iKey % 26;
            }
            catch (Exception eikey)
            {
                iKey = 0;
            }

            char[] ch = strInput.ToCharArray();
            for (int i = 0; i < strInput.Length; i++)
            {
                // CHU HOA
                if (ch[i] >= 'A' && ch[i] <= 'Z')
                {
                    if (ch[i] + iKey > 'Z')
                    {
                        strOutput += ((char)(ch[i] + iKey - 26)).ToString();
                    }
                    else
                    {
                        strOutput += ((char)(ch[i] + iKey)).ToString();
                    }
                }
                // chu thuong
                else if (ch[i] >= 'a' && ch[i] <= 'z')
                {
                    if (ch[i] + iKey > 'z')
                    {
                        strOutput += ((char)(ch[i] + iKey - 26)).ToString();
                    }
                    else
                    {
                        strOutput += ((char)(ch[i] + iKey)).ToString();
                    }
                }
                else
                {
                    strOutput += ((char)ch[i]).ToString();
                }
            }

            tbOutput.Text = strOutput;
        }

        private void btDencrypt_Click(object sender, EventArgs e)
        {
            strInput = tbInput.Text;
            strOutput = "";
            try
            {
                iKey = int.Parse(tbKey.Text.ToString());
                iKey = iKey % 26;
            }
            catch (Exception eikey)
            {
                iKey = 0;
            }

            char[] ch = strInput.ToCharArray();
            for (int i = 0; i < strInput.Length; i++)
            {
                // CHU HOA
                if (ch[i] >= 'A' && ch[i] <= 'Z')
                {
                    if (ch[i] - iKey < 'A')
                    {
                        strOutput += ((char)(ch[i] - iKey + 26)).ToString();
                    }
                    else
                    {
                        strOutput += ((char)(ch[i] - iKey)).ToString();
                    }
                }
                // chu thuong
                else if (ch[i] >= 'a' && ch[i] <= 'z')
                {
                    if (ch[i] - iKey < 'a')
                    {
                        strOutput += ((char)(ch[i] - iKey + 26)).ToString();
                    }
                    else
                    {
                        strOutput += ((char)(ch[i] - iKey)).ToString();
                    }
                }
                else
                {
                    strOutput += ((char)ch[i]).ToString();
                }
            }

            tbOutput.Text = strOutput;
        }

        private void btClear_Click(object sender, EventArgs e)
        {
            strInput = "";
            strOutput = "";
            tbInput.Text = "";
            tbOutput.Text = "";
            tbKey.Text = "";
        }
    }
}