Thread progress to GUI

Question

I implemented threading in my application for scraping websites. After all the sites are scrapped I want to process them.

form creates queueworker(which creates 2 workers and processes tasks). After all the tasks are done I want to process them baack in the formthread. At this point I accieved this with

public void WaitForCompletion()
    {
        // Enqueue one null task per worker to make each exit.
        StopWorkers();

        //wait for the workers to finish
        foreach (Thread worker in workers)
        {
            worker.Join();
        }
    }

After a task is preformed I fire (from queueworker):

public event EventHandler<ProgressEvent> UrlScanned;   

if (UrlScanned != null)
{ 
         UrlScanned(this, new ProgressEvent(task.Name, 1));
}

And catch that event with:

urlscanner.UrlScanned += new EventHandler<ProgressEvent>(UrlScanningProgress);     

private void UrlScanningProgress(object sender, ProgressEvent args)
{
   if (pbarurlScan.InvokeRequired)
   {
       //don't use invoke when Thread.Join() is used! Deadlock
       Invoke(new MethodInvoker(delegate() { UrlScanningProgress(sender, args); 
       //BeginInvoke(new MethodInvoker(delegate() { UrlScanningProgress(sender, args)};
   }
   else
   {
                pbarurlScan.Value++;
   }
}

The problem is that the formthread gets blocked and with calling Invoke the whole application is now in a deadlock situation.

How can I give update to the formthread without having a deadlock and have an immidiate update (begininvoke occurs if the workers threads are done)

Answer 1

Why are you doing the Join? I would raise a callback at the end of each thread - perhaps decrementing a counter. When the counter gets to 0, then call back to the UI thread and do the "all done" code; something like:

using System.Windows.Forms;
using System.Threading;
using System;
class MyForm : Form
{

    public MyForm()
    {
        Button btn = new Button();
        Controls.Add(btn);
        btn.Text = "Go";
        btn.Click += btn_Click;
    }
    int counter;
    void btn_Click(object sender, System.EventArgs e)
    {
        for (int i = 0; i < 5; i++)
        {
            Interlocked.Increment(ref counter);
            ThreadPool.QueueUserWorkItem(DoWork, i);
        }
    }
    void DoWork(object state)
    {
        for (int i = 0; i < 10; i++)
        { // send progress
            BeginInvoke((Action)delegate { Text += state.ToString(); });
            Thread.Sleep(500);
        }
        EndThread(); // this thread done
    }
    void EndThread()
    {
        if (Interlocked.Decrement(ref counter) == 0)
        {
            AllDone();
        }
    }
    void AllDone()
    {
        Invoke((Action)delegate { this.Text += " all done!"; });
    }
    [STAThread]
    static void Main()
    {
        Application.Run(new MyForm());
    }
}

Answer 2

I fire a seperate event now when all tasks are done. I moved the logic for processing the tasks to a seperate function that will be called when receiving the AllUrlsScanned event.