C# Timers and Garbage Collection

Question

According to MSDN, a reference to System.Threading.Timer should be kept otherwise it will get garbage-collected. So if I run this code, it doesn't write any message (which is the expected behavior):

static void Main(string[] args)
{
    RunTimer();
    GC.Collect();
    Console.ReadKey();
}

public static void RunTimer()
{
    new Timer(s => Console.WriteLine("Hello"), null, TimeSpan.FromSeconds(1), TimeSpan.Zero);
}

However, if I modify the code slightly by storing the timer in a temporary local variable, it survives and writes the message:

public static void RunTimer()
{
    var timer = new Timer(s => Console.WriteLine("Hello"));
    timer.Change(TimeSpan.FromSeconds(1), TimeSpan.Zero);
}

During garbage collection, there is apparently no way how to access the timer from root or static objects. So can you please explain why the timer survives? Where is the reference preserved?

Answer 1

Each Timer references a TimerHolder, which references a TimerQueueTimer. The implementation keeps an internal reference to the TimerQueueTimer via the call to UpdateTimer().

Under normal circumstances your timer is free to be collected, finalizing the TimerHolder and removing the TimerQueueTimer from the internal queue. But the simple constructor Timer(TimerCallback) calls TimerSetup() with the Timer itself as the state. So in this particular case, the TimerQueueTimer's state references back to the Timer, preventing it from being collected.

The effect is not related to keeping a temporary local variable. It just so happens to work due to the internals of the Timer mechanism. It is much clearer and safer to keep a reference to your timer as recommended by MSDN.