另一篇在 CSDN 也是有問題的程式，程式的目的是限制一定數量的 threads 同時執行某項工作。我的想法是用 System.Threading.Monitor 就可以很優雅地解決這問題：

class Program
{
  static void Main(string[] args)
  {
    Program p = new Program();

    Thread[] threads = new Thread[10];
    for (int i = 0; i < threads.Length; i++)
    {
      threads[i] = new Thread(p.Run);
      threads[i].Name = "Thread(" + i.ToString() + ")";
      threads[i].Start();

      Thread.Sleep(1000);
    }

    foreach (Thread t in threads) {
      t.Join();
    }

    Console.ReadLine();
  }

  private int allowed = 3;

  public void Run()
  {
    Thread t = Thread.CurrentThread;

    lock (this)
    {
      while (allowed <= 0)
      {
        Console.WriteLine(t.Name + " is waiting.");
        Monitor.Wait(this);
      }

      --allowed;
      Console.WriteLine(t.Name + " started.");
    }

    Thread.Sleep(2000);

    lock (this)
    {
      Console.WriteLine(t.Name + " stopped.");
      ++allowed;
      Monitor.Pulse(this);
    }
  }
}

說穿了其實這程式只是用 Monitor 來實作 Semaphore 的一個特別例子。