Question

I am trying to setup multiple BackgroundWorkers to do work and when not busy start doing the next bit of work. I can’t seem to get them working properly. I have the below code.

When I set FilesToProcess equal or less than MaxThreads it works perfectly although if I make it higher, the app freezes.

I am sure it is something simple, but I just can’t see it.

using System;
using System.ComponentModel;
using System.Threading;
using System.Windows.Forms;

namespace bgwtest
{
    public partial class Form1 : Form
    {
        private const int MaxThreads = 20;
        private const int FilesToProcess = 21;
        private BackgroundWorker[] threadArray = new BackgroundWorker[MaxThreads];

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1Load(object sender, EventArgs e)
        {
            InitializeBackgoundWorkers();
        }

        private void InitializeBackgoundWorkers()
        {
            for (var f = 0; f < MaxThreads; f++)
            {
                threadArray[f] = new BackgroundWorker();
                threadArray[f].DoWork += new DoWorkEventHandler(BackgroundWorkerFilesDoWork);
                threadArray[f].RunWorkerCompleted += new RunWorkerCompletedEventHandler(BackgroundWorkerFilesRunWorkerCompleted);
                threadArray[f].WorkerReportsProgress = true;
                threadArray[f].WorkerSupportsCancellation = true;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            for (var f = 0; f < FilesToProcess; f++)
            {
                var fileProcessed = false;
                while (!fileProcessed)
                {
                    for (var threadNum = 0; threadNum < MaxThreads; threadNum++)
                    {
                        if (!threadArray[threadNum].IsBusy)
                        {
                            Console.WriteLine("Starting Thread: {0}", threadNum);

                            threadArray[threadNum].RunWorkerAsync(f);
                            fileProcessed = true;
                            break;
                        }
                    }
                    if (!fileProcessed)
                    {
                        Thread.Sleep(50);
                    }
                }
            }
        }

        private void BackgroundWorkerFilesDoWork(object sender, DoWorkEventArgs e)
        {
            ProcessFile((int)e.Argument);

            e.Result = (int)e.Argument;
        }

        private static void ProcessFile(int file)
        {
            Console.WriteLine("Processing File: {0}", file);
        }

        private void BackgroundWorkerFilesRunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            if (e.Error != null)
            {
                MessageBox.Show(e.Error.Message);
            }

            Console.WriteLine("Processed File: {0}", (int)e.Result);
        }
    }
}

Answer 1

The problem appears to be that your workers are never completing. Why this is, I’m not sure; it has something to do with the fact that the method (and thread) you are running them from is not itself completing. I was able to solve the problem by creating another worker to assign files to the worker array:

    private BackgroundWorker assignmentWorker;

    private void InitializeBackgoundWorkers() {
        assignmentWorker = new BackgroundWorker();
        assignmentWorker.DoWork += AssignmentWorkerOnDoWork;
        // ...
    }

    private void AssignmentWorkerOnDoWork( object sender, DoWorkEventArgs doWorkEventArgs ) {
        for( var f = 0; f < FilesToProcess; f++ ) {
            var fileProcessed = false;
            while( !fileProcessed ) {
                for( var threadNum = 0; threadNum < MaxThreads; threadNum++ ) {
                    if( !threadArray[threadNum].IsBusy ) {
                        Console.WriteLine( "Starting Thread: {0}", threadNum );

                        threadArray[threadNum].RunWorkerAsync( f );
                        fileProcessed = true;
                        break;
                    }
                }
                if( !fileProcessed ) {
                    Thread.Sleep( 50 );
                    break;
                }
            }
        }
    }

    private void button1_Click( object sender, EventArgs e ) {
        assignmentWorker.RunWorkerAsync();
    }

I’m not happy with this answer because I don’t know why, exactly, it didn’t work as you originally designed it. Perhaps someone else can answer that…? At least this will get you a working version.

EDIT: Your original version didn’t work because the BackgroundWorkerFilesRunWorkerCompleted runs on the same thread as button1_Click (the UI thread). Since you are not freeing up the UI thread, the thread is never getting marked as complete.