C# Cäsar-Verschlüsselung

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Reflection.Emit;
using System.Runtime.Versioning;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using static System.Windows.Forms.VisualStyles.VisualStyleElement;
 
namespace Caesar_Verschluesselung
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        public string verschluessle(string klartext, int schluessel)
        {
            string geheimtext = "";
            char zeichen;
            //Verschlüsselung
            for (int i=0; i<klartext.Length; i++)
            {
                Console.WriteLine(klartext[i]);
                if (klartext[i]!=' ')
                    zeichen=(char)(klartext[i] + schluessel);
                else
                    zeichen=klartext[i];
 
                Console.WriteLine("Verschluesselt: " + zeichen);
                if (zeichen > 'Z')
                    zeichen =(char)(zeichen - 26);
 
                geheimtext = geheimtext + zeichen;
            }
 
 
 
            return geheimtext;
        }
 
 
        private void label2_Click(object sender, EventArgs e)
        {
 
        }
 
        private void bt_groesser_Click(object sender, EventArgs e)
        {
            int schluessel= int.Parse(lb_schluessel.Text);
            schluessel = schluessel + 1;
            if(schluessel == 26) schluessel = 0;
            lb_schluessel.Text = schluessel.ToString();
 
            tb_geheimtext.Text= verschluessle(tb_klartext.Text, int.Parse(lb_schluessel.Text));
 
        }
 
        private void bt_kleiner_Click(object sender, EventArgs e)
        {
            int schluessel = int.Parse(lb_schluessel.Text);
            schluessel = schluessel - 1;
            if (schluessel == -1) schluessel = 25;
            lb_schluessel.Text = schluessel.ToString();
            tb_geheimtext.Text = verschluessle(tb_klartext.Text, int.Parse(lb_schluessel.Text));
        }
 
        private void tb_klartext_TextChanged(object sender, EventArgs e)
        {
            lb_anzahl_zeichen.Text= "Anzahl der Zeichen: "+ tb_klartext.Text.Length.ToString();
            tb_geheimtext.Text=verschluessle(tb_klartext.Text, int.Parse(lb_schluessel.Text));
        }
 
        private void tb_klartext_KeyPress(object sender, KeyPressEventArgs e)
        {
            Console.Write(e.KeyChar + " ");
            Console.WriteLine((int)e.KeyChar);
 
            if ((e.KeyChar >= 'A'  && e.KeyChar <= 'Z') || e.KeyChar == 8 || e.KeyChar==32
                || (e.KeyChar >= 'a' && e.KeyChar <= 'z'))
            {
                Console.WriteLine("Zeichen erlaubt");
                //8...Löschtaste, 65==A, 70 == F
 
                if (e.KeyChar >= 'a' && e.KeyChar <= 'z') e.KeyChar = (char)(e.KeyChar - 32);
            }
            else
            {
                e.Handled = true;  //Abbruch
 
            }
        }
    }
}