WFA og opdatering af textbox

Kode 

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;
using System.Threading;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public delegate void UpdateTextCallback(string message);
        Thread thread;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (btnStart.Text == "Start")
            {
                // Creates a new thread
                thread = new Thread(new ThreadStart(TestThread));
                thread.IsBackground = true;
                thread.Start();
                btnStart.Text = "Vent venligst";
                btnStart.Enabled = false;
            }
            else
            {
                thread.Abort();
                btnStart.Text = "Start";
            }
        }

        private void UpdateText(string message)
        {
            textBox1.Text = message;
        }

        private void TestThread()
        {
            for (int i = 0; i <= 1000; i++)
            {
                Thread.Sleep(1000);
                textBox1.BeginInvoke(new UpdateTextCallback(UpdateText), new object[] { i.ToString() });
            }
        }
    }
}