Вы можете использовать WebBrowser
вместо:
WebBrowser browser;
private void button1_Click(object sender, RoutedEventArgs e)
{
string url = @"your_really_long_url_here";
browser = new WebBrowser();
browser.Navigated += new EventHandler<NavigationEventArgs>(browser_Navigated);
browser.Navigate(new Uri(url, UriKind.Absolute));
}
void browser_Navigated(object sender, System.Windows.Navigation.NavigationEventArgs e)
{
string htmlContent = browser.SaveToString();
System.Diagnostics.Debug.WriteLine(htmlContent);
}