Question

Below is the function I use as my browsers’ DocumentCompleted event, and also the navBtnClick() method which is responsible for creating the web browser and navigating to a specific url.

public void WebBrowser_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e) {

                MessageBox.Show( ((WebBrowser)sender).Url.ToString() );

}



private void navBtnClick(object sender, EventArgs e)
{
            WebBrowser wbrowser = new WebBrowser();
            wbrowser.DocumentCompleted +=new WebBrowserDocumentCompletedEventHandler(WebBrowser_DocumentCompleted);
            wbrowser.Navigate("http://www.google.com");

}

Now after this line wbrowser.Navigate("http://www.google.com"); is executed, there is a message box correctly showing the url, and then after a while another message box shows with the same url. So, what happens is, whatever is on the DocumentCompleted event handler, gets executed twice. Can someone help me make it execute once only?

Answer 1

As I recall, DocumentCompleted will fire multiple times if the document being navigated to has iframes that embed other web pages.

If you only want to receive the event exactly once, just unsubscribe from the DocumentCompleted handler:

public void WebBrowser_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e) 
{
   var webBrowser = sender as WebBrowser;
   webBrowser.DocumentCompleted -= WebBrowser_DocumentCompleted;
   MessageBox.Show(webBrowser.Url.ToString() );
}    

private void navBtnClick(object sender, EventArgs e)
{
     var wbrowser = new WebBrowser();
     wbrowser.DocumentCompleted +=new WebBrowserDocumentCompletedEventHandler(WebBrowser_DocumentCompleted);
     wbrowser.Navigate("http://www.google.com");
}

---

Alternately, you can use System.IObservable and ReactiveExtensions to subscribe to exactly one event firing:

private void navBtnClick(object sender, EventArgs e)
{
     var browser = new WebBrowser();
     var docCompleted = Observable.FromEventPattern<WebBrowserDocumentCompletedEventArgs>(browser, "DocumentCompleted")
     docCompleted
         .Take(1) // Take only one event firing
         .Subscribe(i => MessageBox.Show(browser.Url.ToString()));

     browser.Navigate("http://www.google.com");
}