Countdown

public class Countdown
{
  object _locker = new object ();
  int _value;

  public Countdown() { }
  public Countdown (int initialCount) { _value = initialCount; }

  public void Signal() { AddCount (-1); }

  public void AddCount (int amount)
  {
    lock (_locker)
    {
      _value += amount;
      if (_value <= 0) Monitor.PulseAll (_locker);
    }
  }

  public void Wait()
  {
    lock (_locker)
      while (_value > 0)
        Monitor.Wait (_locker);
  }
}