Hello dear readers and coders

Welcome to the third ‘Can You Spot the Deadlock?’ trivia. Today’s exercise is still dedicated to the Monitor Pattern. Next one will explore other horizons.

As usual the code is in C#, it does compile but it will not run as is.

So:

• What is the issue here?
• Where does it occur?
• Can you fix it ? if yes how, if not, why

And please share your ideas through the comment!

using System;
using System.Threading;

namespace SpotTheDeadLock
{
  internal class SpotTheDeadlock3
  {
    readonly object _synchro = new object(); // lock object

    public int _x = 1; // resulting data

    bool _mustStop;
    int _iteration = 0;
    int []_factor={2,3,5};

    // first running thread

    public void Runner1(int stepId) {
      while (true) {
        lock (_synchro) {
          if (_iteration % 3 != stepId && !_mustStop)
            Monitor.Wait(_synchro);

          if (_mustStop)
            return;

          _x = _x * _factor[stepId];
          NextStep();

        } // _synchro
      } // while
    }

  private void NextStep() {
    if (_iteration == 8)
      _mustStop = true;
    else
      _iteration++;
    Monitor.PulseAll(_synchro);
  }

  public static void Runner() {

    SpotTheDeadlock3 agent = new SpotTheDeadlock3();
    Thread thread1 = new Thread(() => agent.Runner1(0));
    Thread thread2 = new Thread(() => agent.Runner1(1));
    Thread thread3 = new Thread(() => agent.Runner1(2));

    thread1.Start();
    thread2.Start();
    thread3.Start();

    thread1.Join();
    thread2.Join();
    thread3.Join();

    if (agent._x != 2 * 2 * 2 * 3 * 3 * 3 * 5 * 5 * 5)
      throw new ApplicationException("Invalid result");
    }

  } // class
} // namespace

Hello dear readers and coders

Welcome to the second ‘Can You Spot the Deadlock?’ trivia. I surely hope you had fun with the first one. Yeah, I know, it was kinda easy. So today, I raise the bar a bit and bring you one of my favorites: the transactional subscription/un subscription pattern.

The code compiles but it will not run, as I did not show the scaffolding code. Suffice to say that there is a notification mechanism that lives in its own thread(s) and some client code logic that rely on the main(starting) thread.

So:

• What is the issue here?
• Where does it occur?
• Can you fix it ? if yes how, if now, why

Problem is now closed, please find the solution

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace SpotTheDeadLock2
{
///
/// Implements aynchronous notification
///
public class EventSource
{
    private EventHandler _eventHandler;
    ///
    /// notification event
    ///

    public event EventHandler EventOccured
    {
        add {
            // lock to secure subscription
            lock (this){
                _eventHandler += value;
            }
        }
        remove {
            // lock to secure unsubscription
            lock (this) {
                _eventHandler -= value;
            }
        }
    }
    // implementation of notification
    private void TriggerEvent() {
        lock (this) {
            if (_eventHandler != null) {
                _eventHandler(this, new EventArgs());
            }
        }
    }
}

///
/// client implementation
///
public class Client: IDisposable
{
    private EventSource _source;
    ///
    /// Simple constructor
    ///
    ///data source to subsribe to
    public Client(EventSource source) {
        _source = source;
        _source.EventOccured += OnEvent;
    }

    ///
    /// Method in charge of processing events
    ///
    private void OnEvent(object sender, EventArgs arguments) {
        // use lock as we do not have control of calling thread
       lock (this) {
           // do my job
           ...
       }
    }
    // unsubscribe on clean up
    public void Dispose() {
    // use lock to ensure no processing is in progress
        lock (this) {
            _source.EventOccured -= OnEvent;
        }
    }
}
}