FOR løkke

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

namespace WindowsFormsApp14
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnKnap_Click(object sender, EventArgs e)
        {
            int sum = 0;

            for (int i = 0; i < 5; i++)
            {
                sum = sum + i;
                txtUdTekst.Text = txtUdTekst.Text + i.ToString();
            }

            lblTalUd.Text = sum.ToString();
        }
    }
}