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C# Concurrency for Senior Engineering Interviews

... continued

Continues the discussion on the Monitor class.

We'll cover the following...

Ping Pong Example

We can rewrite our ping-pong example without busy waiting using Monitor. The two threads Wait() and Pulse() to not busy wait. The code is self-explanatory and appears below, the widget runs for ten seconds and then timees out.

C#
using System;

using System.Threading;



class Demonstration

{

    private readonly object padlock = new object();

    private bool flag = false;



    void ping()

    {

        while (true)

        {



            Monitor.Enter(padlock);



            while (flag)

            {

                Monitor.Wait(padlock);

            }



            Console.WriteLine("Ping");

            flag = true;

            Monitor.Pulse(padlock);



            Monitor.Exit(padlock);

        }

    }





    void pong()

    {

        while (true)

        {



            Monitor.Enter(padlock);



            while (!flag)

            {

                Monitor.Wait(padlock);

            }



            Console.WriteLine("Pong");

            flag = false;

            Monitor.Pulse(padlock);



            Monitor.Exit(padlock);

        }

    }



    public void runTest() {

        Thread pingThread = new Thread(() =>

        {

            ping();

        });





        Thread pongThread = new Thread(() =>

        {

            pong();

        });



        pongThread.Start();

        pingThread.Start();



        pongThread.Join();

        pingThread.Join();

    }



    static void Main()

    {

        new Demonstration().runTest();

    }

}

Using Value Types

One of the most common mistakes when using the Monitor class is to pass value types as the object to synchronize on. Consider the snippet below:

C#
int valueType = 5;
Monitor.Enter(valueType);
// Singal the monitor
Monitor.Pulse(valueType);
Monitor.Exit(valueType);

If we run the example above we get an object not synchronized exception. When we pass a value type to Monitor's static methods, it is automatically boxed to a reference type. The conversion is implicit and each of three methods in the above example receive different objects as arguments that box the value type! This is the reason the code widget below throws up an exception.

C#
using System;

using System.Threading;

class Demonstration

{

    static void Main()

    {

        int valueType = 5;

        

        Monitor.Enter(valueType);

        

        // Singal the monitor

        Monitor.Pulse(valueType);



        Monitor.Exit(valueType);

    }

}

Contrast the above with the ...