Can an object containing an active Timer (System.Timers.Timer) be garbage collected, if no other object is referencing it?
There are two basic ways that a Timer stays referenced even if you don't store a reference somewhere:
As long as the timer is enabled. There's an underlying System.Threading.Timer that actually generates the event. The CLR keeps a list of active timers, the callback delegate object of them keeps the Timer referenced.
By the Elapsed event handler if it is not static. That extends the lifetime of the Timer instance to at least the lifetime of the class object that contains the Elapsed event handler.
There is no failure mode here. The only possible way to get a reference back to the timer is through the Elapsed event handler's sender argument. If you disable the timer then there is no longer a way to get a reference so it is appropriate for the object to be collected.
A fairly classic way to get in trouble with this Timer and lose Elapsed events is when you have the AutoReset property set to false. A very nasty issue is that any exceptions raised in the Elapsed event handler are swallowed without any diagnostic. Which will bypass the call you'd have to restart the timer at the end. Do favor System.Threading.Timer, it doesn't have this problem.
Yes. Timer will be GC'd if left unreferenced, even while running.
The documentation says:
The code contains declarations of the timer variable at the class level and inside Main. To see how aggressive garbage collection can affect a timer that is declared inside a long-running method, you can comment out the class-level declaration and uncomment the local variable. To keep the timer from being collected, uncomment the GC.KeepAlive method at the end of Main.
Related
which is the right approach to use a System.Timers.Timer?
I mean... I create the timer, set the interval and the method to be called on the Elapsed event.
double ms = 1000;
var t = new System.Timers.Timer(ms);
t.AutoReset = false;
t.Elapsed += (sender, e) => { runTask(); };
t.Start();
What next? Should a call dispose on the Timer? I suppose I can't, or the Elapsed event will never occur.
Should I register the Timer in some global variable to avoid to lose references to it and so the GC could dispose the timer before the Elapsed is called?
And if so, how can I dispose the Timer once the Elapsed event has been handled (thus my task has been executed)?
If you are using Timers during a long running process (e.g. a web application or a windows service), if you don't want to get a memory leak, you need to ensure that you un-subscribe from the Timer's elapsed event handler if you want the garbage collector to be able to reclaim the memory allocated for the Timer object.
System.Timers.Timer implements IDisposable and the guidelines here are that if you have a class that implements IDisposable, any class that has a dependency on an object implementing IDisposable should implement IDisposable itself and call the objects Dispose() method when it itself is called.
A perfect example of this is indeed with System.Timers.Timer. This uses System.Threading.Timer underneath the covers and if you look in reflector you can see the following for the Dispose method on System.Timers.Timer
public void Dispose()
{
this.timerBase.Dispose();
}
Here, timerBase is simply an instance of System.Threading.Timer and therefore the Dispose call cascades down to all dependencies of the class that implement IDisposable.
A short answer is you don't need to do anything. It will be collected by the Garbage Collector when function goes out of scope. If you want it available then declare it in class instead.
Usually when you declare a timer out in class level it is collected by GC when the Class is Disposed. However when you have your timer declare in a Function then the Timer still runs but if you are executing a very long process then GC can Aggressively Dispose it so you will need to use
GC.KeepAlive(youtimer_Instance);
Have a look at the end of the Timer's Documentation for reference to this scenario.
The Comments in the sample code says:
Normally, the timer is declared at the class level,
so that it stays in scope as long as it is needed.
If the timer is declared in a long-running method,
KeepAlive must be used to prevent the JIT compiler
from allowing aggressive garbage collection to occur
before the method ends.
You should call t.Stop() in the Close/Unload of your form/page or anywhere you seem fit. If you leave it running, it will utilize resources unnecessarily and also you might get exceptions while closing the application.
I'm trying to implement Undo function in my app. I have code like this Timer timer = new Timer(UndoDeleteTimerFinish, email, UndoBannerDisappearTime, Timeout.Infinite); and UndoDeleteTimerFinish() will make the banner disppear. However, sometimes the banner will stuck on the screen and I found that in this kind of scenario, UndoDeleteTimerFinish() won't be called. I think the timer must be GC collected. I'm wondering how to prevent timer get GC collected and at the same time be responsive and allow user to interact with the app.
The timer has a reference to the object that defines UndoDeleteTimerFinish. If that object's only refrence is to the Timer and vice-versa they will both be eligible for garbage collection.
Quick and dirty fix is to have an object that is not garbage collected hold a reference to your Timer.
class ObjectNotGettingGarbageCollected{
Timer _timer; //won't be GC'd
SomeMethod(){
_timer = new Timer(UndoDeleteTimerFinish, email, UndoBannerDisappearTime, Timeout.Infinite)
}
}
Also as long as the state object (email in you case) is referenced somewhere, the CLR keeps the timer in its timer queue and the timer object won't get garbage collected. So I would check what is the status of the 'email' object which you pass as the state parameter to your Timer.
More about this here:
The System.Threading.Timer constructor has several overloads; all except one take a state parameter which is passed to the TimerCallback delegate when the timer fires.
It turns out that this state parameter (and the TimerCallback delegate) have an interesting effect on garbage collection: if neither of them reference the System.Threading.Timer object, it may be garbage collected, causing it to stop. This is because both the TimerCallback delegate and the state parameter are wrapped into a GCHandle. If neither of them reference the timer object, it may be eligible for GC, freeing the GCHandle from its finalizer.
The single-parameter constructor does not suffer from this problem, because it passes this for the state (not null). Most real-world usage of System.Threading.Timer either references the timer from the callback or uses the timer for the state, so this interesting garbage collection behavior will probably not be noticed.
http://blog.stephencleary.com/2011/07/systemthreadingtimer-constructor-and.html
Whenever you add a delegate to an event handler, you should remove it later, right? So if you attach an anonymous method to an event, does this create an event handler leak since you can't later remove it? This code sample from http://msdn.microsoft.com/en-us/library/0yw3tz5k%28VS.80%29.aspx seems to imply that this an okay practice though.
// Create a handler for a click event
button1.Click += delegate(System.Object o, System.EventArgs e)
{ System.Windows.Forms.MessageBox.Show("Click!"); };
Is this really an okay practice?
Whenever you add a delegate to an event handler, you should remove it later, right?
Not necessarily, no. Often you want the event handler to stay valid for as long as the event itself can be raised - that's certainly very common with UIs.
Is this really an okay practice?
Absolutely, so long as you don't need to unhook the handler. Think about the point at which you'd unhook the event handler. If it's "when the form (or button, or whatever) is elegible for garbage collection" then what benefit is there in removing the handler? Just let it be garbage collected with the form...
Whenever you add a delegate to an event handler, you should remove it later, right?
Well, not always. There are two reasons why you'd want to remove an event handler that you're adding:
You're constantly adding handlers to the same instance that are short lived. If you didn't remove them then there would be more and more handlers added, when most of them aren't needed.
Your handler internally holds onto a reference to an object who's lifetime is much shorter than the lifetime of whatever object the event belongs to, and the event handler won't be (or can't be) called once that other object goes out of scope. Leaving the event handler attached will either force it to stay in memory longer than desired, or possibly result in using an object that's "stale" and shouldn't be used anymore. (For example, if the resource has already been disposed you don't want that event to fire anymore.)
The reason #2 is an issue is because of how Garbage Collection works in C#. It marks all objects that it can be 100% sure are in scope as "alive", and then follows everything that any of those "alive" objects reference as also being "alive" until it's followed every reference in every live object. Anything that was never marked as "alive" is then deemded "dead" and eligible for garbage collection.
When you have an event handler attached to an event that delegate contains two things, an instance of an object and a method to run on that object. That referenced object won't be able to be garbage collected until either:
The object with the event is no longer "alive".
You remove the event handler (i.e. the reference to) your delegate, allowing your object to be freed earlier.
That said, a significant percentage of cases don't apply to either of those, so there's no need to bother removing the event handlers.
As an example, I often see people removing event handlers just before the event object goes out of scope. That's pointless. If the object is out of scope there's no problem with it holding onto references to...whatever.
Now, if you are in one of those few situations in which you do need to unsubscribe the event handler, and you're using an anonymous method you need to...not. Just create a class that can make it a named method and use that.
Subscibing to event with anonymous method (or lambda expression) potentially can lead to memory leaks, but not in this case.
In this case compiler will generate an anonymous method inside current class and you don't have any memory related issues as long as your button will not live much longer than your object.
The use of inline anonymous delegates as event handlers does prevent those handlers being removed, as you stated. This can indeed cause problems, especially if the delegate comes from a different object. If you had a class A with a public event E, and E was subscribed to by a class B (that has no containing/contains relationship to A) attaching an anonymous handler H, H is technically a member of B, and so as long as it's referred to (by being attached to A's event), B won't be GCed. B's lifetime becomes tied to A's, and if you didn't want or expect that you now have a memory leak.
Now, that's not always a problem. If B contains A, or the other way around, then A and B's lifetimes are already tied together anyway; in your case, the button itself will very probably be disposed of before its containing control that attached the handler. Similarly, A and B could be long-lived objects (say, the program's main form and a data repository) and won't be GCed until the program terminates anyway. As long as you never have to care about a difference in lifetime between A and B, and you never need to detach H from E to stop exhibiting whatever behavior H has, then you can attach all the anonymous handlers you like.
Do I have to worry about removing event listeners assigned to local variables?
Consider the following sample:
var zipUtil = new ZipUtil();
zipUtil.ProgressChanged += ZipUtil_ProgressChanged;
I'm creating an instance of the ZipUtil class which is stored as a local variable within a method. Do I have to remove the listener (zipUtil.ProgressChanged -= ZipUtil_ProgressChanged;) before the method terminates or is it okay to skip the step?
No, you don't have to remove that event handling method.
When adding an event handler to an event, a reference from the (object containing the) event to the event handler is created, not the other way round. Once zipUtil goes out of scope, the event handler will not make any difference pertaining to references to zipUtil.
Two lines of code are never enough to make the call accurately. But it is very unlikely you'll need to unsubscribe explicitly. There are two possible problem scenarios:
the class may fire its event at an inopportune time, after your object is disposed for example. That's fairly unlikely in this scenario, surely the ZipUtil class stops raising ProgressChanged events when you stop calling its methods. Not completely out of the question, it could do work on a background thread for example. Not visible from your snippet.
you can have a garbage collection problem due to events. The event delegate keeps an implicit reference to your class object, necessary so it can call ZipUtil_ProgressChanged() method. If it is an instance method, not clear from your snippet. That reference will keep your outer class alive. But not the other way around. Given that the lifetime of the ZipUtil object is restricted to the method and you surely want your outer class to survive at least to the end of the method, this should never be a problem.
High odds that ZipUtil implements IDisposable btw. Do make sure you use the using statement if it does.
Is this a valid way to use a Lambda as an EventHandler? It seems to me that the handler has been correctly removed and that the garbage collector should clean this up. However, I haven't seen anyone else do it this way, so I thought I'd better double check.
Along the same lines, what's the best tool (preferably free) to use to test whether this is in fact being garbage collected?
DispatcherTimer timer = new DispatcherTimer();
timer.Interval = TimeSpan.FromSeconds(10);
EventHandler callback = null;
callback = (s, e) =>
{
timer.Stop();
timer.Tick -= callback;
};
timer.Tick += callback;
timer.Start();
A DispatcherTimer is kept alive by the Dispatcher class, it keeps a List<DispatcherTimer> internally that stores a reference to any timer that's enabled. As soon as you Stop() the timer, the object gets removed from that list, making it eligible for garbage collection if you don't store any additional reference to it. Which you don't in this case, the timer reference is a local variable.
You cannot get a finalizer on the auto-generated class that implements the lambda. Next best thing is to simply run this code a billion times. If you don't get runaway member consumption and OOM then it obviously doesn't leak. You'll want to make the Interval shorter so it doesn't take until Christmas, 15 msec is good. Use a Timer to call the method so you don't get too many active timers at the same time and allow the dispatcher to do its job.
Yes, the callback will be eligible for garbage collection after the callback has executed once. This is a reasonable way of subscribing to an event in order to execute the handler just once.
It's tough to prove that the delegate will be garbage collected - or indeed that the object used to hold the captured variables callback and timer will be garbage collected, admittedly... you'd really want to put a finalizer there (which affects garbage collection itself, of course), but you can't as it's generated code.