How to prevent my Timer from GC collecting before its callback excuted?

Question

I need to create a bunch of timer as local variable to do something like:

void Foo()
{
    Timer t = new Timer(myTimerCallback,null,1000,Timeout.Infinite);
}
         


        

Answer 1

A. System.Threading vs System.Timers

It's worth pointing out that the keeping reference issue is specific to System.Threading.Timer and not System.Timers.Timer.

---

B. Threading.Timer's Timer(TimerCallback callback) constructor

This is not a solution to your problem but I think it's still relevant to the topic.

System.Threading.Timer's Timer(TimerCallback callback) constructor (the one that doesn't take the dueTime and others) uses this for state which means that the timer will keep a reference to itself and this means that it will survive garbage collection.

public Timer(TimerCallback callback)
{
    (...)
    TimerSetup(callback, this, (UInt32)dueTime, (UInt32)period, ref stackMark);
}

Example

Timer 'C' is created using Timer(TimerCallback callback) and it just keeps going after GC.Collect().

Output

A
B
C
A
B
C
GC Collected
B
C
C
B
B
C

Code

class TimerExperiment
{
    System.Threading.Timer timerB;

    public TimerExperiment()
    {
        StartTimer("A"); // Not keeping this timer
        timerB = StartTimer("B"); // Keeping this timer
        StartTimer2("C"); // Not keeping this timer
    }

    static System.Threading.Timer StartTimer(string name) {
        return new System.Threading.Timer(_ =>
        {
            Console.WriteLine($"{name}");
        }, null, dueTime: Delay(name), period: TimeSpan.FromSeconds(1));
    }

    static System.Threading.Timer StartTimer2(string name)
    {
        //Create the timer using the constructor which only takes the callback
        var t = new System.Threading.Timer( _ => Console.WriteLine($"{name}"));
        t.Change(dueTime: Delay(name), period: TimeSpan.FromSeconds(1));
        return t;
    }

    static TimeSpan Delay(string name) 
            => TimeSpan.FromMilliseconds(Convert.ToInt64(name[0])*10);

}

class Program
{
    static async Task Main(string[] args)
    {
        var withTimers = new TimerExperiment();

        await Task.Delay(TimeSpan.FromSeconds(2));
        GC.Collect();
        Console.WriteLine("GC Collected");
        Console.ReadLine();
    }
}