Why Is Webbrowser_Documentcompleted() Firing Twice

Why is WebBrowser_DocumentCompleted() firing twice?

You can check the WebBrowser.ReadyState when the event is fired:

if (browser.ReadyState != WebBrowserReadyState.Complete)
    return;

ReadyState will be set to Complete once the whole document is ready.

DocumentCompleted firing multiple times - accepted StackOverflow answer not working

DocumentComplete may get fired multiple times for many reasons (frames, ajax, etc). At the same time, for a particular document, window.onload event will be fired only once. So, perhaps, you can do your processing upon window.onload. I just answered a related question on how that can be done.

WebBrowser Document Completed Event C#

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");
}

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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");
}

Detect WebBrowser complete page loading

I think the DocumentCompleted event will get fired for all child documents that are loaded as well (like JS and CSS, for example). You could look at the WebBrowserDocumentCompletedEventArgs in DocumentCompleted and check the Url property and compare that to the Url of the main page.

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