WebBrowser control throws seemingly random NullReferenceException

Question

For a couple of days I am working on a WebBrowser based webscraper. After a couple of prototypes working with Threads and DocumentCompleted events, I decided to try and see

Answer 1

Busy waiting (while (_wb.IsBusy) ;) on UI thread isn't much advisable. If you use the new features async/await of .Net 4.5 you can get a similar effect (i.e. go to url, perform action, go to other url etc. etc.) you want

public static class SOExtensions
{
    public static Task NavigateAsync(this WebBrowser wb, string url)
    {
        TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();
        WebBrowserDocumentCompletedEventHandler completedEvent = null;
        completedEvent = (sender, e) =>
        {
            wb.DocumentCompleted -= completedEvent;
            tcs.SetResult(null);
        };
        wb.DocumentCompleted += completedEvent;

        wb.ScriptErrorsSuppressed = true;
        wb.Navigate(url);

        return tcs.Task;
    }
}



async void ProcessButtonClick()
{
    await webBrowser1.NavigateAsync("http://www.stackoverflow.com");
    MessageBox.Show(webBrowser1.DocumentTitle);

    await webBrowser1.NavigateAsync("http://www.google.com");
    MessageBox.Show(webBrowser1.DocumentTitle);
}