C# Threading and using AutoResetEvent

Question

Hello,

I have the following code in a class library. And I wait for a call back into my main application. I am making a DownloadStringAsync call so I have to wait a few seconds to get the callback after it has finished. I have a 3 of these calls to wait for, so in my main application I am using AutoResetEvent to wait all of them to finish. So I will block until they have been set in the callback function.

However, after testing the callback don't get called. I am thinking when the code gets blocked by the AutoResetEvent its blocking the DownloadStringAsync. As when I comment out this code everything works fine.

So I think as soon as I make a call to: objNoGateway.NoGatewayStatus(sipUsername, statusDisplay1.PhoneNumber); And when the code reaches here: handle.WaitOne(); It will block the code in the class library.

Many thanks for any advice.

In my class library code sample.

     // Event handler that makes a call back in my main application
     // Event handler and method that handles the event
    public EventHandler<NoGatewayEventArgs> NoGatewayCompletedEvent;
    // The method that raises the event.
    private void OnNoGatewayCompleted(object sender, NoGatewayEventArgs e)
    {
        if (NoGatewayCompletedEvent != null)
        {
            NoGatewayCompletedEvent(this, e);
        }
    }

    // Start the Async call to find if NoGateway is true or false
    public void NoGatewayStatus(string sipUsername, string phoneNumber)
    {     
        string strURL = string.Format("http://xxxxxxxxxxxxxxx={0}&amp;CalledNumber={1}", sipUsername, phoneNumber);

        if (!wc.IsBusy)
        {
            try
            {
                string errorMsg = string.Empty;
                wc.DownloadStringAsync(new Uri(strURL));
            }
            catch (WebException ex)
            {
                Console.WriteLine("IsNoGateway: " + ex.Message);
            }
            catch (Exception ex)
            {
                Console.WriteLine("IsNoGateway: " + ex.Message);
            }
        }
        else
        {
            Console.WriteLine("WebClient: IsNoGateWay(): Busy please try again");
        }

    }

    void wc_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        if (e.Error == null)
        {
            if (e.Result == "No gateway")
            {
                OnNoGatewayCompleted(this, new NoGatewayEventArgs(validateResponse_e.VALIDATION_FAILED));
                Console.WriteLine("NoGatway() DownloadedCompleted: " + e.Result);
            }
            else
            {
                OnNoGatewayCompleted(this, new NoGatewayEventArgs(validateResponse_e.OK));
                Console.WriteLine("NoGateway() DownloadCompleted: " + e.Result);
            }
        }
        else
        {
            this.OnNoGatewayCompleted(this, new NoGatewayEventArgs(validateResponse_e.SERVER_FAILED));
            Console.WriteLine("No Gateway: DownloadCompleted() Error: " + e.Error.Message);
        }
    }

In my main application I register this callback. And wait for the for the result. Then set the AutoResetEvent.

 ManualResetEvent[] waitValidateCallResponse = new ManualResetEvent[] 
          { new ManualResetEvent(false), new ManualResetEvent(false), new ManualResetEvent(false) };
    // Event handler for NoGateway event
    private void OnNoGatewayCompleted(object sender, NoGatewayEventArgs e)
    {
        Console.WriteLine("OnNoGatewayComleted: " + e.noGateway);
        waitValidateCallResponse[0].Set();
    }

The part when I am calling and blocking.

NoGateway objNoGateway = new NoGateway()           
objNoGateway.NoGatewayCompletedEvent += new EventHandler<NoGatewayEventArgs>(this.OnNoGatewayCompleted);
objNoGateway.NoGatewayStatus(sipUsername, statusDisplay1.PhoneNumber);


// Block here - Wait for all reponses to finish before moving on
waitEvent.WaitOne(5000, true);                      
Console.WriteLine("All thread finished");

======================== Edit and added the other 2 callbacks as not to confuse the issue of me just having only one ======================

    private void OnCalledNumberBlockedCompleted(object sender, CalledNumberBlockedEventArgs e)
    {
        Console.WriteLine("OnCalledNumberBlockedCompleted: " + e.CalledNumberBlocked);
        waitValidateCallResponse[1].Set();
    }

    private void OnValidTelephoneNumberCompleted(object sender, ValidTelephoneNumberEventArgs e)
    {
        Console.WriteLine("OnValidTelephoneNumberCompleted: " + e.validTelephoneNumber);
        waitValidateCallResponse[2].Set();
    }

Answer 1

Is it as simple as: you always call Set on index 0?

private void OnNoGatewayCompleted(object sender, NoGatewayEventArgs e)
{
    Console.WriteLine("OnNoGatewayComleted: " + e.noGateway);
    waitValidateCallResponse[0].Set();
}

Answer 2

After lots of edits, I think I might understand the problem. Windows Forms applications have one main thread; this thread is used to process messages. So when your main thread is blocking, your application can't receive events. And you use WaitOne to keep the main thread blocked.

I'd move the WaitOne() checks to a separate timer thread.

Or you could wait for a limited time, and instruct the application to process messages in between:

foreach (WaitHandle handle in waitValidateCallResponse)
{
    while (!handle.WaitOne(300))
        Application.ProcessMessages();
    Console.WriteLine("events.WaitOne(): " + handle.ToString());
}

The later approach is not something you should do in a library though. It's something of an anti-pattern I think.

Answer 3

The snippet code is peculiar

// Event handler that makes a call back in my main application
     // Event handler and method that handles the event
    public EventHandler<NoGatewayEventArgs> NoGatewayCompletedEvent;
    // The method that raises the event.
    public void OnNoGatewayCompleted(object sender, NoGatewayEventArgs e)
    {
        if (NoGatewayCompletedEvent != null)
        {
            NoGatewayCompletedEvent(this, e);
        }
    }

However in the 2nd last snippet, you attach an event handler for this event as follows.. OnNoGatewayCompleted seems to be a helper method to raise the event.. (it should not be public) but here it seems you have the event handler raise the event again. Unless you have 2 methods named OnNoGatewayCompleted (I'm hoping not)

objNoGateway.NoGatewayCompletedEvent 
  += new EventHandler<NoGatewayEventArgs>(this.OnNoGatewayCompleted);

If you're looking for the waitHandles to be signalled in the event handler, shouldn't the methods OnCalledNumberBlockedCompleted be hooked up to the event instead.

PS: As Marc pointed out.. use WaitHandle.WaitAll ( the for loop demands that the async operations complete in order which may not be the case )

Answer 4

Use WaitHandle.WaitAny(handleArray); to wait on all the handles in the handle array instead of handle.WaitOne(); in a loop

Answer 5

Try something along these lines:

public void NoGatewayStatus (string sipUsername, string phoneNumber) {
    string strURL = string.Format( "http://xxxxxxxxxxxxxxx={0}&CalledNumber={1}", sipUsername, phoneNumber );

    ManualResetEvent wait1 = new ManualResetEvent( false );
    WebClient wc = new WebClient();
    Thread thr = new Thread( DownloadSomeStuff );
    thr.Start( new DlArguments( strURL, wait1 ) );

    // do the other three

    if ( !wait1.WaitOne( 10000 ) ) {
        Console.WriteLine( "DownloadSomeStuff timed out" );
        return;
    }
    if ( !wait2.WaitOne( 10000 ) ) {
        Console.WriteLine( "DownloadOtherStuff timed out" );
        return;
    }
    if ( !wait3.WaitOne( 10000 ) ) {
        Console.WriteLine( "DownloadMoreStuff timed out" );
        return;
    }
}

public void DownloadSomeStuff (object p_args) {
    DlArguments args = (DlArguments) p_args;
    try {
        WebClient wc = new WebClient();
        wc.DownloadString( args.Url );
        args.WaitHandle.Set();
    } catch ( Exception ) {
        // boring stuff
    }
}


private class DlArguments
{
    public DlArguments (string url, ManualResetEvent wait_handle) {
        this.Url = url;
        this.WaitHandle = wait_handle;
    }

    public string Url { get; set; }
    public ManualResetEvent WaitHandle { get; set; }
}

Does this do it?