using System;
using System.Net;
using System.Windows.Forms;

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

        private delegate void FormDelegate();
        private void button1_Click(object sender, EventArgs e)
        {
            UseWaitCursor = true;
            button1.Enabled = false;

            BeginInvoke(new FormDelegate(delegate
            {
                using (WebClient web = new WebClient())
                {
                    web.Encoding = System.Text.Encoding.UTF8;
                    textBox1.Text = web.DownloadString("https://stackoverflow.com/");
                }

                UseWaitCursor = false;
                button1.Enabled = true;
            }));
        }
    }
}