Early in the article, I introduced you to the ManualResetEvent class. This class allowed you to set and reset (signal and unsignal) the event by calling the Set and Reset instance methods. The System.Threading.AutoResetEvent class is very similar to the ManualResetEvent class, but when a thread waiting on the event is signaled, the one thread is released and the event is returned to the unsignaled state. This removes the necessity to reset the signal after a thread is signaled.

Another great class in the System.Threading namespace is the Timer class. This class allows you to signal an event at a particular interval in time in the future. The Timer class is implemented using a delegate callback instance method. When the Timer is signaled, the class calls the instance method that you specified in the constructor of the Timer object. The Timer callback can also receive a parameter object passed in the call to the Timer constructor. Presented below is a small sample using the AutoResetEvent and Timer classes.

using System;
using System.Threading;

namespace ConsoleApplication8
{
    class Class1
    {
        public void TimerCallback(Object obj)
        {
            Console.WriteLine("Timer triggered");
            ((AutoResetEvent) obj).Set();
            Thread.Sleep(1000);
            ((AutoResetEvent) obj).Set();
        }

        static void Main(string[] args)
        {
            Class1 obj = new Class1();
            AutoResetEvent ev =
                new AutoResetEvent(false);
            Timer timer = new Timer(
                new TimerCallback(obj.TimerCallback),
                ev, 1000, 0);
            ev.WaitOne();
            Console.WriteLine("Event Fired");
            ev.WaitOne();
            Console.WriteLine("Event Fired");
        }
    }
}

Note that the Timer callback instance method is wrapped in a TimerCallback delegate object. The main thread will create an AutoResetEvent object and a Timer object. The main thread then waits on the event object. The TimerCallback instance method is called after one second, triggering the event object. Because the event object is automatically reset, when the main thread attempts to wait on the event again, the thread yields until the event is signaled a second time. The TimerCallback instance method waits another second and then signals the event a second time, releasing the main thread.

More

In the second part of this article, I will complete my discussion of the synchronization objects and will discuss thread local storage, COM interoperability and thread states.