What is the mistake with this code???

shivamkalra

Hi, I've just started to learn multi-threading. I'm trying to make a console program that makes a web browser and then navigate to some page and post some data and invoke submit button and in method it waits for the response of the web browser. I'm using code below, could someone tell me how to do it correctly.

private AutoResetEvent ar;
private WebBrowser brs;

    \[STAThread\]
    private void Contribute\_Load(object sender, EventArgs e)
    {
        ar = new AutoResetEvent(false);
        brs = new WebBrowser();
        brs.Url = new Uri("http://www.gmail.com");
        brs.Navigated +=new WebBrowserNavigatedEventHandler(brs\_Navigated);
        ar.WaitOne();
        Console.WriteLine("Navigated to gmail.com");
    }

    void brs\_Navigated(object sender, WebBrowserNavigatedEventArgs e)
    {
        ar.Set();
    }

This code never prints navigated to gmail. Thank you Shivam

Luc Pattyn

this can't work, your Load handler is running on the main (aka GUI) thread and so is the Navigated handler; this means the Navigated handler will not execute as long as the Load handler hasn't finished. You are deadlocked. Most often it is a bad idea (or plain wrong) to have blocking calls (such as Thread.Sleep or AutoResetEvent.WaitOne) inside an event handler. probable solution: end your Load handler with the navigation command (you should first set the Navigated handler, then the URL); put everything that needs to be done after navigation in the Navigated handler. That is what it is for. :)

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shivamkalra

Hi. Thanks for the quick response. I've just started learning thread on c sharp. So could you please explain more? Basically I want a method "ValidateStudent" that will create a webbrowser object, navigate to school's website then post ID and Password and check if login succeed. But during this whole process the method should wait.

private bool nav = false;
private WebBrowser brs;

    public ValidateStudent()
    {
        ar = new AutoResetEvent(false);
        ar1 = new AutoResetEvent(false);

        brs = new WebBrowser();
        brs.Navigated += new WebBrowserNavigatedEventHandler(brs\_Navigated); 
        brs.Url = new Uri("https://rooms.library.dc-uoit.ca/studyrooms/myreservations.aspx");

Console.Write(nav);

    }

    private void brs\_Navigated(object sender, WebBrowserNavigatedEventArgs e)
    {
        nav = true;
       
    }

This code is printing "False" because it does't wait for event to complete and prints "False" instantly. I want it to wait until event completes. THANK YOU Shivam

David1987

Even if it could work (but see Luc's reply), there would be a race condition. What if the navigation would be done before you add the event handler?

Luc Pattyn

that is just a minor issue that gets fixed by swapping the two relevant statements. :)

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David1987

that's true

Luc Pattyn

a handler is not supposed to wait at all, it should be swift: its purpose is to handle one event as fast as it can, so your app is ready to handle the next event, which could be anything from a keyboard event, a mouse moving, a window uncovering yours, etc. Do not wait for anything inside a handler!!! :)

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Dave Kreskowiak

You may also run into a problem where the WebBrowser control will not work on a non-GUI thread. I've never tried it myself, but it's a possibility.

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shivamkalra

OK FINE. What is the solution for this problem? Suggest some alternative way of doing it.

Dev RoOo7

I agree with this answer ... Thank you.

BobJanova

Don't try to wait for the event, instead do the processing in the event handler. In your cut down example here, move the Console.WriteLine into the Navigated event handler.