Threading 05 Signalling via CountdownEvent

signalling is way that that threads can communicate or signal each other, there are two types of signalling that can be implemented

• CountDownEvent which we'll cover in this post
• EventWaitHandle which we'll look at in the next post

the CountDownEvent signaling mechanism basically creates a barrier (I use this term loosely) that expects a certain number of signals before it will continue, first lets look at the implementation without the use of signalling

using System;

using System.Threading;

namespace pc.ThreadSignalling

{

    class Program

    {

        struct Data {

            public int Delay { get; set; }

            public string Value { get; set; }

            public Data(int Delay, string Value) {

                this.Delay = Delay;

                this.Value = Value;

            }

        }

        static void Main(string[] args)

        {

            var threads = new Thread[6];

            var data = "Hello world my name is Pawel".Split(' ');

            for (int i = 0, delay = 500; i < threads.Length; i++, delay += 500)

            {

                threads[i] = new Thread(Print);

                threads[i].Start(new Data(delay, data[i]));

            }

           

            //wait for each thread to finish before continuing with the main

            foreach (var t in threads)

                t.Join();

            Console.WriteLine("Woot Woot");

        }

        static void Print(object o) {

            var data = (Data)o;

            Thread.Sleep(data.Delay);

            Console.WriteLine(data.Value + " ");

        }

    }

}

Above we join all of our threads to the main before continuing to print out "Woot Woot" now we can accomplish the same thing using countdownevent

using System;

using System.Threading;

namespace pc.ThreadSignalling

{

    class Program

    {

        public struct Data

        {

            public int Delay { get; set; }

            public string Value { get; set; }

            public CountdownEvent cdEvent { get; set; }

            public Data(int Delay, string Value, CountdownEvent cdEvent)

            {

                this.Delay = Delay;

                this.Value = Value;

                this.cdEvent = cdEvent;

            }

        }

        static void Main(string[] args)

        {

            using (var cdEvent = new CountdownEvent(6))

            {

                var threads = new Thread[6];

                var data = "Hello world my name is Pawel".Split(' ');

                for (int i = 0, delay = 500; i < threads.Length; i++, delay += 500)

                {

                    threads[i] = new Thread(Print);

                    threads[i].Start(new Data(delay, data[i], cdEvent));

                }

                //Wait for 6 signals before continuing

                cdEvent.Wait();

            }

            Console.WriteLine("WOot woot");

        }

        static void Print(object o)

        {

            var data = (Data)o;

            Thread.Sleep(data.Delay);

            Console.Write(data.Value + " ");

            data.cdEvent.Signal();

        }

    }

}

In the above instead of joining all the threads in the main, we have each thread signal that it's in a state that can pass the cdEvent.Wait() barrier.