Question

I’m tinkering away on a multithreaded downloader, using a producer/consumer queue construct; the downloading parts works fine, but I’m running into a problem keeping the GUI updated.

For now I’m using a listbox control on the form to display status messages and updates on downloading progress, eventually I hope to replace that with progressbars.

First the Form1 code behind; the form contains nothing but a button and the listbox:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    public void SetProgressMessage(string message) 
    { 
        if (listboxProgressMessages.InvokeRequired) 
        {
            listboxProgressMessages.Invoke(new MethodInvoker(delegate()
            { SetProgressMessage(message); })); 
        } 
        else 
        {
            listboxProgressMessages.Items.Add(message);
            listboxProgressMessages.Update();
        } 
    }

    private void buttonDownload_Click(object sender, EventArgs e)
    {
        SetProgressMessage("Enqueueing tasks");
        using (TaskQueue q = new TaskQueue(4))
        {
            q.EnqueueTask("url");
            q.EnqueueTask("url");
            q.EnqueueTask("url");
            q.EnqueueTask("url");
            q.EnqueueTask("url");
            q.EnqueueTask("url");
            q.EnqueueTask("url");
            q.EnqueueTask("url");
            q.EnqueueTask("url");
            q.EnqueueTask("url");
        }
        SetProgressMessage("All done!");
    }
}

Now the producer/consumer logic. The consumer downloads the files bit by bit, and should tell the listbox living on the GUI thread how the progress is coming along; This works, but the listbox doesn’t actually update until all is finished, also the messages appear after the ‘All done!’ message, which isn’t desirable.

TaskQueue.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.IO;
using System.Net;
using System.Windows.Forms;
using System.Runtime.Remoting.Messaging;

namespace WinformProducerConsumer
{
    public class TaskQueue : IDisposable
    {
        object queuelocker = new object();
        Thread[] workers;
        Queue<string> taskQ = new Queue<string>();

    public TaskQueue(int workerCount)
    {
        workers = new Thread[workerCount];
        for (int i = 0; i < workerCount; i++)
            (workers[i] = new Thread(Consume)).Start();
    }

    public void Dispose()
    {
        foreach (Thread worker in workers) EnqueueTask(null);
        foreach (Thread worker in workers) worker.Join();
    }

    public void EnqueueTask(string task)
    {
        lock (queuelocker)
        {
            taskQ.Enqueue(task);
            Monitor.PulseAll(queuelocker);
        }
    }

    void Consume()
    {
        while (true)
        {
            string task;
            Random random = new Random(1);
            lock (queuelocker)
            {
                while (taskQ.Count == 0) Monitor.Wait(queuelocker);
                task = taskQ.Dequeue();
            }
            if (task == null) return;

            try
            {
                Uri url = new Uri(task);
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                response.Close();
                Int64 iSize = response.ContentLength;
                Int64 iRunningByteTotal = 0;

                using (WebClient client = new System.Net.WebClient())
                {
                    using (Stream streamRemote = client.OpenRead(new Uri(task)))
                    {
                        using (Stream streamLocal = new FileStream(@"images\" + Path.GetFileName(task), FileMode.Create, FileAccess.Write, FileShare.None))
                        {
                            int iByteSize = 0;
                            byte[] byteBuffer = new byte[iSize];
                            while ((iByteSize = streamRemote.Read(byteBuffer, 0, byteBuffer.Length)) > 0)
                            {
                                streamLocal.Write(byteBuffer, 0, iByteSize);
                                iRunningByteTotal += iByteSize;

                                double dIndex = (double)iRunningByteTotal;
                                double dTotal = (double)byteBuffer.Length;
                                double dProgressPercentage = (dIndex / dTotal);
                                int iProgressPercentage = (int)(dProgressPercentage * 100);

                                string message = String.Format("Thread: {0} Done: {1}% File: {2}",
                                    Thread.CurrentThread.ManagedThreadId,
                                    iProgressPercentage,
                                    task);

                                Form1 frm1 = (Form1)FindOpenForm(typeof(Form1));
                                frm1.BeginInvoke(new MethodInvoker(delegate()
                                {
                                    frm1.SetProgressMessage(message);
                                })); 
                            }
                            streamLocal.Close();
                        }
                        streamRemote.Close();
                    }
                }

            }
            catch (Exception ex)
            {
                // Generate message for user
            }
        }
    }

    private static Form FindOpenForm(Type typ) 
    { 
        for (int i1 = 0; i1 < Application.OpenForms.Count; i1++) 
        { 
            if (!Application.OpenForms[i1].IsDisposed && (Application.OpenForms[i1].GetType() == typ))
            { 
                return Application.OpenForms[i1]; 
            } 
        } 
        return null; 
    }
}

}

Any suggestions, examples? I’ve looked around for solutions, but couldn’t find anything that I could follow or worked.

Replacing the frm1.BeginInvoke(new MethodInvoker(delegate() with a frm1.Invoke(new MethodInvoker(delegate() results in a deadlock. I’m rather stumped here.

Sources:
Producer/Consumer example: http://www.albahari.com/threading/part4.aspx

Update: I’m going about this the wrong way; instead of invoking back to the GUI from the worker threads, I’ll use events that the GUI thread will have to keep an eye on. A lesson learned. 🙂

Answer 1

You should eliminate FindOpenForm and add a ProgressChanged event to TaskQueue. It is absolutely not TaskQueue responsibility to make direct calls to the presentation-layer (forms or controls). It is then the form’s responsibility to listen for “progress-changed” events generated by tasks, and then update itself propertly.

This will easily solve your problem, it will keep simple, follow best-practices, and eliminate timing issues, threading issues, and so on.

• MSDN Search for raising events at http://social.msdn.microsoft.com/Search/en-US?query=raising+events&ac=8

• Events Tutorial (C#) at http://msdn.microsoft.com/en-us/library/aa645739(VS.71).aspx

• Raising Events and Responding to Events (VB.NET) at http://msdn.microsoft.com/en-us/library/ms973905.aspx

• [must read] BackgroundWorker Component Overview at http://msdn.microsoft.com/en-us/library/8xs8549b.aspx illustrates what you need to accomplish with TaskQueue events and the form’s handling of those events.