C#

1)             2)             

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public partial class Window : Form
{
    bool running = false;
    BackgroundWorker worker;
    int num = 0;
    public Window()
    {
        InitializeComponent();
        this.button.Click += new EventHandler(ButtonClick);
    }

    void ButtonClick(object sender, EventArgs e)
    {
        if (running)
        {
            // synchronization error here, I think it's
            // because there's no thread to pulse
            Monitor.Pulse(this);
        }
        else
        {
            running = true;
            worker = new BackgroundWorker();
            worker.DoWork += new DoWorkEventHandler(Work);
            // Work never executes w or w/o this:
            worker.RunWorkerAsync(); 
        }
    }

    void Work(object sender, DoWorkEventArgs w)
    {
        ++num;
        label.Text = num.ToString();
        Monitor.Wait(this);
        Work(null, null);
    }
}

void ButtonClick(object sender, EventArgs e)
{
    if (running)
    {
        trigger.Set();
        trigger.WaitOne();
        label.Text = data.ToString();
    }
    else
    {
        running = true;
        worker = new BackgroundWorker();
        worker.DoWork += new DoWorkEventHandler(Work);
        worker.RunWorkerAsync();
        trigger.WaitOne();
        label.Text = data.ToString();
    }
}

void Work(object sender, DoWorkEventArgs w)
{
    recursive(0);
}

void recursive(int num)
{
    if (num % 4 == 2)
    {
        data = num;
        trigger.Set();
        trigger.WaitOne();
        Thread.Sleep(1);
    }
    recursive(num + 1);
}

private static readonly object _syncLock = new object(); 

lock (_syncLock)
{
  Monitor.Pulse(_syncLock);
}