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);
}

// http://www.codeproject.com/Messages/3589122/Multi-threading-and-function-recursion-with-a-form.aspx
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        _tw = new ThreadingWrapper(this);
    }

    private ThreadingWrapper _tw;
    private void button1_Click(object sender, EventArgs e)
    {
        button1.Enabled = false;
        _tw.ButtonClicked();
    }

    // We are using object parameter so we can call this method with BeginInvoke using WaitCallback delegate
    // If you create your own delegate you can use whatever method signature you want
    public void SetText(object o)
    {
        string text = (string)o;
        label1.Text = text;
    }

    public void WaitingOnClick(object o)
    {
        label1.Text += " - WAITING";
        button1.Enabled = true;
    }
}

public class ThreadingWrapper
{
    private AutoResetEvent _are = new AutoResetEvent(false);
    private bool _waiting;

    private Form1 _form;
    public ThreadingWrapper(Form1 toControl)
    {
        _form = toControl;
    }

    internal void ButtonClicked()
    {
        if (_waiting)
        {
            _waiting = false;
            _are.Set();
        }
        else
        {
            // initialize new background thread to process call
            ThreadPool.QueueUserWorkItem(new WaitCallback(functionCall), 0);
        }
    }

    private void functionCall(object p)
    {
        // perform calculations and set label
        int num = (int)p;
        Thread.Sleep(500);
        // we need to use BeginInvoke because we are setting GUI elements from background thread
        _form.BeginInvoke(new WaitCallback(_form.SetText), num.ToString());

        if (num % 10 == 0)
        {
            // again setting of GUI element
            _form.BeginInvoke(new WaitCallback(_form.WaitingOnClick), "");

            _waiting = true;
            _are.WaitOne();
        }

        functionCall(num + 1);
    }
}

private void benchmarkThread()
{
    Stopwatch sw = new Stopwatch();
    sw.Start();
    for (int i = 0; i < 1000; i++)
    {
        Thread t = new Thread(foo);
        t.Start();
    }

    sw.Stop();
    MessageBox.Show(sw.ElapsedMilliseconds.ToString());
}

private void benchmarkQueue()
{
    Stopwatch sw = new Stopwatch();
    sw.Start();
    for (int i = 0; i < 1000; i++)
    {
        ThreadPool.QueueUserWorkItem(new WaitCallback(foo));
    }

    sw.Stop();
    MessageBox.Show(sw.ElapsedMilliseconds.ToString());
}

private void foo(object o)
{

}

private void button2_Click(object sender, EventArgs e)
{
    benchmarkThread();
}

private void button3_Click(object sender, EventArgs e)
{
    benchmarkQueue();
}

if (flag) 
    return; // exiting out of method