How do events cause memory leaks in C# and how do Weak References help mitigate that?

Question

There are two ways (that I know of) to cause an unintentional memory leak in C#:

1. Not disposing of resources that implement IDisposable
2. Referencing and de-referencing events incorrectly.

I don't really understand the second point. If the source object has a longer lifetime than the listener, and the listener doesn't need the events anymore when there are no other references to it, using normal .NET events causes a memory leak: the source object holds listener objects in memory that should be garbage collected.

Can you explain how events can cause memory leaks with code in C#, and how I can code to get around it using Weak References and without Weak References?

Insufferable

You could also System.Runtime.InteropServices.Marshal.AllocHGlobal and fail to free it... old style leaks are harder but not impossible in .Net – Indelicacy

A bit late, but as a note, not disposing of IDisposable is not a memory leak. IDisposable's reason for existence is for "deterministic disposal of resources". In other words, you the developer control exactly when things have been freed. By and large, even IDisposable objects will eventually be dealt with by the garbage collector, but you don't know when, exactly, in that instance. – Heterochromosome

Update to @JesseC.Slicer's comment, for .net 5 and up: The GC's finalizer queue is not processed at program termination, even for a perfectly normal and clean exit. Anything you forgot to Dispose, anything a dependency left hanging, or anything that is otherwise in the finalizer queue will not be finalized. This can mean anything from no problem at all after the appdomain is unloaded all the way up to issues that persist across reboots, depending on what the leaked resources were. The first that comes to mind for me there is system-level locks on storage resources, for some OS/FS combos. – Farlay

Current link at MS Learn for my above comment: learn.microsoft.com/en-us/dotnet/csharp/programming-guide/… – Farlay

Answer 1

When a listener attaches an event listener to an event, the source object will get a reference to the listener object. This means that the listener cannot be collected by the garbage collector until either the event handler is detached, or the source object is collected.

Consider the following classes:

class Source
{
    public event EventHandler SomeEvent;
}

class Listener
{
    public Listener(Source source)
    {
        // attach an event listner; this adds a reference to the
        // source_SomeEvent method in this instance to the invocation list
        // of SomeEvent in source
        source.SomeEvent += new EventHandler(source_SomeEvent);
    }

    void source_SomeEvent(object sender, EventArgs e)
    {
        // whatever
    }
}

...and then the following code:

Source newSource = new Source();
Listener listener = new Listener(newSource);
listener = null;

Even though we assign null to listener, it will not be eligible for garbage collection, since newSource is still holding a reference to the event handler (Listener.source_SomeEvent). To fix this kind of leak, it is important to always detach event listeners when they are no longer needed.

The above sample is written to focus on the problem with the leak. In order to fix that code, the easiest will perhaps be to let Listener hold on to a reference to Source, so that it can later detach the event listener:

class Listener
{
    private Source _source;
    public Listener(Source source)
    {
        _source = source;
        // attach an event listner; this adds a reference to the
        // source_SomeEvent method in this instance to the invocation list
        // of SomeEvent in source
        _source.SomeEvent += source_SomeEvent;
    }

    void source_SomeEvent(object sender, EventArgs e)
    {
        // whatever
    }

    public void Close()
    {
        if (_source != null)
        {
            // detach event handler
            _source.SomeEvent -= source_SomeEvent;
            _source = null;
        }
    }
}

Then the calling code can signal that it is done using the object, which will remove the reference that Source has to ´Listener`;

Source newSource = new Source();
Listener listener = new Listener(newSource);
// use listener
listener.Close();
listener = null;

Vernettaverneuil

I don't think this is [entirely] true. Cyclic references within the CLR proper should take care of themselves (they can be detected). The real problem is when dealing across runtimes -- COM interop is a big case here. This is because the CLR can not adequately "see" into the COM (non-CLR) runtime/host and thus can not detect and clean-up cyclic references. I am not sure how this affects some standard .NET components such as WinForms (which run to native), though. – Hadik

@pst: there is no cyclic reference; note that Listener holds no reference to Source. Listener is being kept alive because Source is holding a reference to Listener. As long as something holds a reference to Source, listener cannot be collected, but Listener is not preventing Source from being collected. – Varix

You should edit your post to tell people how to dereference event handlers and where to do it. – Woodley

+1 Great description of what's going on Fredrik. It obviously gets a real problem when the Listener classes are created and subscribe to Source either on a certain event or in some sort of loop, making it happen a lot of times. – Napoli

@pst, This is not an issue of cyclic references but rather an issue of object lifetime. The example is not great, but it does seem to try show that the event source out lives the listener, in which case the listener will not be collected until the event source no longer has references or the listener explicitly removes it's event handler from the event source. A typical Windows Form would listen to events of it's child controls, in which case the event source has a shorter lifetime and the Form so no memory leak in this typical WinForms scenario, other scenarios exist and often bite. – Milestone

ty this really helped me :) Just one mroe question if we have just used ordinary "-=" before assigning null to listener would memory leak occur – Insufferable

@Fredrik Mörk Great explanation. I was thinking of asking the same question like listner = null scenario you explained. Once agina great! – Lusitania

@Insufferable @Avatar No, it would not. Assigning null to the listener doesn't really do anything here, other than tell the Garbage collector the item is ready to be collected; which it would do once it left the block anyway. It's seldom necessary to set an object to null to get it to be collected once it leaves the block, but some programmers put it there to explicitly state that they null'd the object. – Woodley

since i've spent quite some time researching this i'd just like to drop a link here, which in combination with this question makes it all so easy to understand and fix: thomaslevesque.com/2010/05/17/… – Uglify

In the second code there is Cyclic references . ref is kept in private Source _source;...no ? @GeorgeStocker ? Or was he talking about the first code ? – Aguste

Answer 2

Read Jon Skeet's excellent article on events. It's not a true "memory leak" in the classic sense, but more of a held reference that hasn't been disconnected. So always remember to -= an event handler that you += at a previous point and you should be golden.

Heterochromosome

Answer 3

There are, strictly speaking, no "memory leaks" within the "sandbox" of a managed .NET project; there are only references held longer than the developer would think necessary. Fredrik has the right of it; when you attach a handler to an event, because the handler is usually an instance method (requiring the instance), the instance of the class containing the listener stays in memory as long as this reference is maintained. If the listener instance contains references to other classes in turn (e.g. backreferences to containing objects), the heap can stay quite large long after the listener has gone out of all other scopes.

Maybe someone with a bit more esoteric knowledge of Delegate and MulticastDelegate can shed some light on this. The way I see it, a true leak COULD be possible if all of the following were true:

• The event listener requires external/unmanaged resources to be released by implementing IDisposable, but it either does not, or
• The event multicast delegate does NOT call the Dispose() methods from its overridden Finalize() method, and
• The class containing the event does not call Dispose() on each Target of the delegate through its own IDisposable implementation, or in Finalize().

I've never heard of any best practice involving calling Dispose() on delegate Targets, much less event listeners, so I can only assume the .NET developers knew what they were doing in this case. If this is true, and the MulticastDelegate behind an event tries to properly dispose of listeners, then all that is necessary is proper implementation of IDisposable on a listening class that requires disposal.

Gnu

My definition of memory leak would suggest that a program generally has a memory leak iff there exists a repeating sequence of inputs which will cause the program to require an unbounded amount of memory while passing through a bounded number of observable states. By such a definition, events can certainly cause memory leaks even within an entirely-managed .net program. Event listeners should implement IDisposable, and should unsubscribe from their events in Dispose, but Microsoft does not make it easy to do that consistently, and unfortunately sloppiness doesn't usually cause trouble. – Bahena

The MulticastDelegate does not itself do anything with IDisposable; calling Delegate.Remove (as event-unsubscribe methods usually do) simply creates a new MulticastDelegate which doesn't contain the removed item. Finalizers are generally useless with events, since an event will keep all its subscribers in scope as long as the publisher is in scope. By the time a subscriber could be finalized, the publisher will be out of scope and the event will be irrelevant. – Bahena