How does the Garbage Collector KeepAlive method work with a timer?

Question

I've just stumbled across some code that uses the GC.KeepAlive() method and I am trying to understand how it works. For example, in this code:

Timer timer = new System.Timers.Timer(5000);
timer.Elapsed += new ElapsedEventHandler(OnTimedEvent);
timer.Enabled = true;
GC.KeepAlive(timer);

In this code, my understanding is that a Timer object is created, which spawns a thread that runs every 5 seconds. Next, the GC line is ran. Then the method exits, destroying the timer when garbage collection runs.

The KeepAlive only keeps it alive prior to the call to KeepAlive, which it looks to me is about 0.0000001 seconds and it won't be destroyed there anyway since there is a local reference to it (unless it's destroying it because nothing else happens to the timer object?)

Either way, by the time the 5000 interval is hit the method will have ended ages ago and it's very likely the timer is destroyed. So what's the purpose of that line?

Answer 1

So what's the purpose of that line?

Nothing. You shouldn't be using it as it's not helping anything. That code was probably written by someone not particularly familiar with how the Timer class works.

Internally the Timer class will use entirely different means of ensuring that it is not garbage collected before it should be. See this question for the details.

Answer 2

In this context, there's no reason for that line. The only time it can help is if there is a lot of code between the timer.Enabled and the call to KeepAlive, as in:

    timer.Enabled = true;
    // lots of code here that does
    // things that take some time
    GC.KeepAlive(timer);
}

That will prevent the garbage collector from collecting the timer before the method ends. Without the call to KeepAlive, the GC could decide that timer isn't used any more after Enabled is set, and it could do an early collection.

If you want a persistent timer, you have to declare it at class scope.