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.
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
I have code like this:
var myTimer = new Timer(500);
myTimer.Elapsed += (o, a) =>
{
Application.Current.Dispatcher.Invoke(() =>
{
// Handle elapsed
}, DispatcherPriority.Render);
};
If the timer elapses and adds the anonymous method to the Dispatcher, then immediately goes out of scope and gets garbage collected before the dispatcher invokes the anonymous method, what will happen?
It's lifetime won't end specifically because the message loop is referencing it.
Even though the object is no longer in scope in code, that object is accessible through a rooted object, so the GC cannot collect it.
The whole point of using managed memory is that you can rely on the GC to only ever reclaim any managed resourced when it is impossible for them to be accessed from executable code. If it's possible for executable code to access an object, then that object will not be collected.
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?
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.
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.
Ok,
So I have a method which fires when someone clicks on our Icon in a silverlight application, seen below:
private void Logo_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
{
e.Handled = true;
ShowInfo(true);
DispatcherTimer autoCloseTimer = new DispatcherTimer();
autoCloseTimer.Interval = new TimeSpan(0, 0, 10);
autoCloseTimer.Tick +=new EventHandler((timerSender,args) =>
{
autoCloseTimer.Stop();
ShowInfo(false);
});
autoCloseTimer.Start();
}
Whats meant to happen is that the method ShowInfo() opens up a box with the company info in and the dispatch timer auto closes it after said timespan. And this all works...
But what I'm not sure about is because the dispatch timer is a local var, after the Logo_MouseLeftButtonUp method finishes, what is there to keep the dispatch timer referenced and not availible for GC collection before the anonymous method is fired?
Is it the reference to the ShowInfo() method in the anonymous method?
Just feels like some thing I should understand deeper as I can imagine with using events etc it can be very easy to create a leak with something like this.
Hope this all makes sense!
Andy.
The DispatcherTimer registers itself with the Dispatcher by calling the internal Dispatcher.AddTimer method when you call Start.
Since it also unregisters itself by calling Dispatcher.RemoveTimer when you call Stop, you won't leak memory.
The Timer keeps the anonymous method alive in its Tick event, which also keeps the variables in the method alive through the closure.
One of the more obscure causes of memory leaks in .NET is event handlers. An event handler is a reference to an object, and keeps the object in scope. When you're done with an event handler, it needs to be dropped. If the event handler is used only once, it can deregister itself, but if it gets used more than once, there will need to be some other object that knows when it's no longer useful, and drops the event handler.