c# - Why does a Timer keep my object alive?

Question

Preface: I know how to solve the problem. I want to know why it arises. Please read the question from top to bottom.

As we all (should) know, adding event handlers can cause memory leaks in C#. See Why and How to avoid Event Handler memory leaks?

On the other hand, objects often have similar or connected life cycles and deregistering event handlers is not necessary. Consider this example:

using System;

public class A
{
    private readonly B b;

    public A(B b)
    {
        this.b = b;
        b.BEvent += b_BEvent;
    }

    private void b_BEvent(object sender, EventArgs e)
    {
        // NoOp
    }

    public event EventHandler AEvent;
}

public class B
{
    private readonly A a;

    public B()
    {
        a = new A(this);
        a.AEvent += a_AEvent;
    }

    private void a_AEvent(object sender, EventArgs e)
    {
        // NoOp
    }

    public event EventHandler BEvent;
}

internal class Program
{
    private static void Main(string[] args)
    {
        B b = new B();

        WeakReference weakReference = new WeakReference(b);
        b = null;
        GC.Collect();
        GC.WaitForPendingFinalizers();

        bool stillAlive = weakReference.IsAlive; // == false
    }
}

A and B reference each other implicitly via events, yet the GC can delete them (because it's not using reference counting, but mark-and-sweep).

But now consider this similar example:

using System;
using System.Timers;

public class C
{
    private readonly Timer timer;

    public C()
    {
        timer = new Timer(1000);
        timer.Elapsed += timer_Elapsed;
        timer.Start(); // (*)
    }

    private void timer_Elapsed(object sender, ElapsedEventArgs e)
    {
        // NoOp
    }
}

internal class Program
{
    private static void Main(string[] args)
    {
        C c = new C();

        WeakReference weakReference = new WeakReference(c);
        c = null;
        GC.Collect();
        GC.WaitForPendingFinalizers();
        bool stillAlive = weakReference.IsAlive; // == true !
    }
}

Why can the GC not delete the C object? Why does the Timer keep the object alive? Is the timer kept alive by some "hidden" reference of the timer mechanics (e.g. a static reference)?

(*) NB: If the timer is only created, not started, the issue does not occur. If it's started and later stopped, but the event handler is not deregistered, the issue persists.

Answer 1

The timer logic relies on an OS functionality. It is actually the OS that fires the event. OS in turn uses CPU interrupts to implement that.

The OS API, aka Win32, does not hold references to any objects of any kind. It holds memory addresses of functions which it has to call when a timer event happens. .NET GC has no way to track such "references". As a result a timer object could be collected without unsubscribing from the low-level event. It is a problem because OS would try to call it anyway and would crash with some weird memory access exception. That's why .NET Framework holds all such timer objects in the statically referenced object and removes them from that collection only when you unsubscribe.

If you look at the root of your object using SOS.dll you will get the next picture:

!GCRoot 022d23fc
HandleTable:
    001813fc (pinned handle)
    -> 032d1010 System.Object[]
    -> 022d2528 System.Threading.TimerQueue
    -> 022d249c System.Threading.TimerQueueTimer
    -> 022d2440 System.Threading.TimerCallback
    -> 022d2408 System.Timers.Timer
    -> 022d2460 System.Timers.ElapsedEventHandler
    -> 022d23fc TimerTest.C

Then if you look at the System.Threading.TimerQueue class in something like dotPeek, you will see that it is implemented as a singleton and it holds a collection of timers.

That's how it works. Unfortunately the MSDN documentation is not crystal clear about it. They just assumed that if it implements IDisposable then you should dispose it no question asked.