C#

private void button1_Click(object sender, EventArgs e)
        {
            // Create a new backgroundworker that supports progressreporting and cancellation.
            BackgroundWorker worker = new BackgroundWorker();
            worker.WorkerReportsProgress = true;
            worker.WorkerSupportsCancellation = true;

            // Hook up the events that will do the actual work
            worker.DoWork += new DoWorkEventHandler(worker_DoWork);
            worker.ProgressChanged += new ProgressChangedEventHandler(worker_ProgressChanged);
            worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(worker_RunWorkerCompleted);

            // Start the background worker
            worker.RunWorkerAsync();
        }

        void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            // When an exception happens in the do_work eventhandler,
            // the RunWorkerCompleted is called with the exception passed on the argument.
            // Always check for this
            if (e.Error != null)
            {
                MessageBox.Show(e.Error.Message);
            }
            else if (e.Cancelled)
            {
                // show the user that the background worker was cancelled
            }
            else
            {
                // show the user that the worker completed succesfully
            }
        }

        void worker_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            progressBar1.Value = e.ProgressPercentage;
        }

        void worker_DoWork(object sender, DoWorkEventArgs e)
        {
            BackgroundWorker worker = (BackgroundWorker)sender;

            // perform the time consuming task
            for (int i = 1; i <= 100; i++)
            {
                if (!worker.CancellationPending) // this is only needed if your task can be cancelled in the middle of the run
                {
                    Thread.Sleep(50); // do the actual task
                    worker.ReportProgress(i);
                }
                else
                {
                    e.Cancel = true;
                    break;
                }
            }
        }