Pils wird in einem neuen Thread ausgeführt und nach Fertigstellung wird bw_RunWorkerCompleted aufgerufen und im alten Thread ausgeführt.

private BackgroundWorker bw = new BackgroundWorker();

public Form1()
{
    InitializeComponent();
    bw.WorkerReportsProgress = true;
    bw.WorkerSupportsCancellation = true;
    bw.ProgressChanged += new ProgressChangedEventHandler(bw_ProgressChanged);
    bw.DoWork += new DoWorkEventHandler(bw_DoWork);
    bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);
}

public void buttonStart_Click(object sender, EventArgs e)
{
    if (bw.IsBusy != true)
        bw.RunWorkerAsync(12); //Start
}

public int Pils(int i)
{
    Thread.Sleep(2000);
    bw.ReportProgress(70, "In the middle of the work..");
    Thread.Sleep(2000);
    bw.ReportProgress(90, "Returning the result..");
    Thread.Sleep(2000);
    return (2 * i);
}

private void bw_DoWork(object sender, DoWorkEventArgs e)
{
    bw.ReportProgress(20, "Waiting for cancel..");
    Thread.Sleep(2000);
    if ((bw.CancellationPending == true))
        e.Cancel = true;
    else
    {
        bw.ReportProgress(50, "Starting process..");
        e.Result = Pils((int)e.Argument);
    }
}

private void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    bw.ReportProgress(100, "Work done..");
    if ((e.Cancelled == true))
         textBox1.Text = "Canceled!";
    else if (e.Error != null)
         textBox1.Text = ("Error: " + e.Error.Message);
    else textBox1.Text = e.Result.ToString();
}

private void buttonCancel_Click(object sender, EventArgs e)
{
    if (bw.WorkerSupportsCancellation == true)
        bw.CancelAsync();
}

private void bw_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    listBox1.Items.Add((e.ProgressPercentage.ToString() + "%") + " - " + e.UserState as String);
}