Timers in .NET

A current C# project of mine required a timer where every couple of seconds a method would fire and a potentially fairly long-running process would run.

With .NET we have a few built-in options for timers:

System.Web.UI.Timer

Available in the .NET Framework 4.8 which performs asynchronous or synchronous Web page postbacks at a defined interval and was used back in the older WebForms days.

System.Windows.Forms.Timer

This timer is optimized for use in Windows Forms applications and must be used in a window.

System.Timers.Timer

Generates an event after a set interval, with an option to generate recurring events. This timer is almost what I need however this has quite a few stackoverflow posts where exceptions get swallowed.

System.Threading.Timer

Provides a mechanism for executing a method on a thread pool thread at specified intervals and is the one I decided to go with.

Issues

I came across a couple of minor issues the first being that even though I held a reference to my Timer object in my class and disposed of it in a Dispose method the timer would stop ticking after a while suggesting that the garbage collector was sweeping up and removing it.

My Dispose method looks like the first method below and I suspect it is because I am using the conditional access shortcut feature from C# 6 rather than explicitly checking for null first.

public void Dispose()
{ 
    // conditional access shortcut
    _timer?.Dispose(); 
} 

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

A workaround is to tell the garbage collector to not collect this reference by using this line of code in timer’s elapsed method.

GC.KeepAlive(_timer);

The next issue was that my TimerTick event would fire and before the method that was being called could finish another tick event would fire.

This required a stackoverflow search where the following code fixed my issue.

// private field
private readonly object _locker = new object();

// this in TimerTick event
if (Monitor.TryEnter(_locker))
{
    try
    {
        // do long running work here
        DoWork();
    }
    finally
    {
        Monitor.Exit(_locker);
    }
}

And so with these two fixes in place, my timer work was behaving as expected.

Solution

Here is a sample class with the above code all in context for future reference

Success 🎉

Comments

[Katie Nelson]

How would you handle a cancellation token? I have a need for a timer that should stop firing on cancellation and also the DoWork method should immediately quit.

[John Smith]

Hi Katie,

Take a look at this. hitting ctrl-c while its running or in the 5-second DoWork method will cancel gracefully and hopefully do what you want.

I plan to blog about it in more detail later but this should help.

Gist here

gist.github.com/solrevdev/f28ab813...

[Katie Nelson]

Thanks John. I see now the running timer callback is buried several levels down. Makes things a bit more complex, but I understand what you did.

[John Smith]

Ahh yes, the timer's TimerCallback method signature needed to be void DoWork(object state) hence needing another method call that uses async Task accepting a CancellationToken.

But looking at it again I think that DoWorkAsync and RunJobAsync could just be the one method.

I'll take another look later and will try and explain more in another blog post.

[Katie Nelson]

Great! Looking forward to it.