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by Jiří {x2} Činčura

Timer instance and race condition

20 Nov 2013 Best practice or not?, Lessons learned, Multithreading/Parallelism/Asynchronous/Concurrency

I’m spending now a lot of time working with Timer object. It’s a class with such a simple surface, but there’s still a lot to learn. How it works internally, the scheduling, … This time I learned, luckily not the hard way, just by studying, about interesting race condition, I never though about.

Let’s imagine you want to schedule operation every 10 seconds and the execution time does not count into interval. You will use the “kicking” trick (I blogged about it little here and here):

var timer = default(Timer);
timer = new Timer(_ =>
{
	Console.WriteLine("Tick");
	//...
	timer.Change(TimeSpan.FromSeconds(10), Timeout.InfiniteTimeSpan);
}, null, TimeSpan.FromSeconds(10), Timeout.InfiniteTimeSpan);

Looks good, right? Compiles. Runs fine. Or … You probably know there’s a catch. Else the blog post would not mention race condition. It runs fine in 99,99999% cases (100% for me so far 😉). The problem is, that the timer.Change call could happen before the assignment on L2. Who would have thought that. 😃

When I first saw it on Jeffrey Richter’s screen it was so clear to me. So easy. But honestly, I never thought about it.

Thus the correct way it to write:

var timer = default(Timer);
timer = new Timer(_ =>
{
	Console.WriteLine("Tick");
	//...
	timer.Change(TimeSpan.FromSeconds(10), Timeout.InfiniteTimeSpan);
}, null, Timeout.InfiniteTimeSpan, Timeout.InfiniteTimeSpan);
timer.Change(TimeSpan.FromSeconds(10), Timeout.InfiniteTimeSpan);

Never mind. Every day I learn something new, is a good day.

And what about you? Do you do the “safe” assignment or assign directly?