Using Timer class properly

Question

I would like to know if the code I wrote is a properly written one, it does function, but I've done bad designs before, so I need to know if I thought this in a proper way.

Code is about using a System.Timers.Timer to do a repeated action each X hours. I read some threads about this subject on stackoverflow and then I tried writing my own class. Here is what I wrote:

namespace MyTool
{
public  class UpdaterTimer : Timer
{
    private static UpdaterTimer _timer;
    protected UpdaterTimer()
    { }

    public static UpdaterTimer GetTimer()
    {
        if (_timer == null)
            _timer = new UpdaterTimer();

        SetTimer(_timer);
        return _timer;
    }

    private static void SetTimer(UpdaterTimer _timer)
    {
        _timer.AutoReset = true;
        _timer.Interval = Utils.TimeBetweenChecksInMiliseconds;
        _timer.Elapsed += new ElapsedEventHandler(_timer_Elapsed);
        _timer.Start();
        DoStuff();
    }

    static void _timer_Elapsed(object sender, ElapsedEventArgs e)
    {
        DoStuff();
    }

    private static void DoStuff()
    {
       //does stuff on each elapsed event occurrence
    }
}
}

Short description:

• tried using a singleton pattern since I only need one timer to work
• I'm not sure about calling DoStuff() inside the SetTimer() method, it seems redundant. But the logic is that, when the app starts, DoStuff() must run, and then it must run again on each Timer.Elapsed event.

My questions are:

1. Would you have written this behavior in a different way, given the specification?
2. Is it ok to use a singleton in this case, or it doesn't make sense?

Answer 1

I don't know what you are trying to achieve but here is a few point about your current design:

1. Your GetTimer is broken in multithreading mode:

   if (_timer == null) _timer = new UpdaterTimer();

   suppose you have two threads each one call GetTimer at the same time, the first one checks the _timer and found it null so it proceed. but at that time and before it reach _timer = new UpdateTimer() the thread context switching switch to the other thread and pause the current thread execution. so the other thread check the _timer and find it not null so it proceed and create a new timer, now the context switch rescheduled the first thread and its continue its execution and so create a new timer and update the old one. to correct use of singleton pattern use static constructor instead static UpdaterTimer() { _timer = new UpdaterTimer();}

2. The developer can call GetTimer() as much as he wanted so it will call _timer.Elapsed += new ElapsedEventHandler(_timer_Elapsed); again registering another Elapsed handler. Also note that whenever you call GetTimer, the timer will start "_timer.Start()" even if it were stopped.

3. Don't return the underlying timer, instead expose a public methods Start(), Stop(), UpdateInterval(int interval).

4. At SetTimer() you want to call DoStuff immediately, but that then will be blocked at the SetTimer method waiting DoStuff() to complete, a better way is to start that method in a new thread ThreadPool.QueueUserWorkItem(new WaitCallback((_) => DoStuff())); or use System.Threading.Timer instead of System.Timers.Timer and set it call the method start immediately.