Non-reentrant timers

Question

I have a function that I want to invoke every x seconds, but I want it to be thread-safe.

Can I set up this behavior when I am creating the timer? (I don't mind which .NET timer I use, I just want it to be thread-safe).

I know I can implement locks inside my callback function, but I think it would be more elegant if it were in the timer level.

My callback function, and environment are not related to a UI.

[Edit 1] I just don't want there to be more than one thread inside my callback function.

[Edit 2] I want to keep the locking inside the timer level, because the timer is responsible for when to call my callback, and here there is a particular situation when I don't want to call my callback function. So I think when to call is the responsibility of the timer.

Answer 1

I'm guessing, as your question is not entirely clear, that you want to ensure that your timer cannot re-enter your callback whilst you are processing a callback, and you want to do this without locking. You can achieve this using a System.Timers.Timer and ensuring that the AutoReset property is set to false. This will ensure that you have to trigger the timer on each interval manually, thus preventing any reentrancy:

public class NoLockTimer : IDisposable
{
    private readonly Timer _timer;

    public NoLockTimer()
    {
        _timer = new Timer { AutoReset = false, Interval = 1000 };

        _timer.Elapsed += delegate
        {
            //Do some stuff

            _timer.Start(); // <- Manual restart.
        };

        _timer.Start();
    }

    public void Dispose()
    {
        if (_timer != null)
        {
            _timer.Dispose();
        }
    }
} 

Answer 2

Complementing Tim Lloyd's solution for System.Timers.Timer, here's a solution to prevent reentrancy for cases where you want to use System.Threading.Timer instead.

TimeSpan DISABLED_TIME_SPAN = TimeSpan.FromMilliseconds(-1);

TimeSpan interval = TimeSpan.FromSeconds(1);
Timer timer = null; // assign null so we can access it inside the lambda

timer = new Timer(callback: state =>
{
  doSomeWork();
  try
  {
    timer.Change(interval, DISABLED_TIME_SPAN);
  }
  catch (ObjectDisposedException timerHasBeenDisposed)
  {
  }
}, state: null, dueTime: interval, period: DISABLED_TIME_SPAN);

I believe you don't want interval to be accessed inside of the callback, but that is be easy to fix, if you want to: Put the above into a NonReentrantTimer class that wraps the BCL's Timer class. You would then pass the doSomeWork callback in as a parameter. An example of such a class:

public class NonReentrantTimer : IDisposable
{
    private readonly TimerCallback _callback;
    private readonly TimeSpan _period;
    private readonly Timer _timer;

    public NonReentrantTimer(TimerCallback callback, object state, TimeSpan dueTime, TimeSpan period)
    {
        _callback = callback;
        _period = period;
        _timer = new Timer(Callback, state, dueTime, DISABLED_TIME_SPAN);
    }

    private void Callback(object state)
    {
        _callback(state);
        try
        {
            _timer.Change(_period, DISABLED_TIME_SPAN);
        }
        catch (ObjectDisposedException timerHasBeenDisposed)
        {
        }
    }


    public void Dispose()
    {
        _timer.Dispose();
    }
}

Answer 3

I know I can implement locks inside my callback function, but I think it will be more elegant if it will be in the timer level

If locking is necessary then how could a timer arrange that? You're looking for a magical freebie.

Re Edit1:

Your choices are System.Timers.Timer and System.Threading.Timer, both need precautions against re-entrance. See this page and look for the Dealing with Timer Event Reentrance section.

Answer 4

using System;
using System.Diagnostics;

/// <summary>
///     Updated the code.
/// </summary>
public class NicerFormTimer : IDisposable {

    public void Dispose() {
        using ( this.Timer ) { }

        GC.SuppressFinalize( this );
    }

    private System.Windows.Forms.Timer Timer { get; }

    /// <summary>
    ///     Perform an <paramref name="action" /> after the given interval (in <paramref name="milliseconds" />).
    /// </summary>
    /// <param name="action"></param>
    /// <param name="repeat">Perform the <paramref name="action" /> again. (Restarts the <see cref="Timer" />.)</param>
    /// <param name="milliseconds"></param>
    public NicerFormTimer( Action action, Boolean repeat, Int32? milliseconds = null ) {
        if ( action == null ) {
            return;
        }

        this.Timer = new System.Windows.Forms.Timer {
            Interval = milliseconds.GetValueOrDefault( 1000 )
        };

        this.Timer.Tick += ( sender, args ) => {
            try {
                this.Timer.Stop();
                action();
            }
            catch ( Exception exception ) {
                Debug.WriteLine( exception );
            }
            finally {
                if ( repeat ) {
                    this.Timer.Start();
                }
            }
        };

        this.Timer.Start();
    }

}

/// <summary>
///     Updated the code.
/// </summary>
public class NicerSystemTimer : IDisposable {

    public void Dispose() {
        using ( this.Timer ) { }

        GC.SuppressFinalize( this );
    }

    private System.Timers.Timer Timer { get; }

    /// <summary>
    ///     Perform an <paramref name="action" /> after the given interval (in <paramref name="milliseconds" />).
    /// </summary>
    /// <param name="action"></param>
    /// <param name="repeat">Perform the <paramref name="action" /> again. (Restarts the <see cref="Timer" />.)</param>
    /// <param name="milliseconds"></param>
    public NicerSystemTimer( Action action, Boolean repeat, Double? milliseconds = null ) {
        if ( action == null ) {
            return;
        }

        this.Timer = new System.Timers.Timer {
            AutoReset = false,
            Interval = milliseconds.GetValueOrDefault( 1000 )
        };

        this.Timer.Elapsed += ( sender, args ) => {
            try {
                this.Timer.Stop();
                action();
            }
            catch ( Exception exception ) {
                Debug.WriteLine( exception );
            }
            finally {
                if ( repeat ) {
                    this.Timer.Start();
                }
            }
        };

        this.Timer.Start();
    }

}