private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
{
    WebBrowser wb = sender as WebBrowser;
    Encoding encoding = Encoding.GetEncoding(wb.Document.Encoding);
    using (StreamReader stream = new StreamReader(wb.DocumentStream, encoding))
    {
        textBox1.Text = stream.ReadToEnd();
    }
}