The problem: Registered event handlers create a reference
from the event to
the event handler's instance. If that instance fails to unregister
the event handler (via Dispose, presumably), then
the instance memory will not be freed by
the garbage collector.
Example:
class Foo
{
public event Action AnEvent;
public void DoEvent()
{
if (AnEvent != null)
AnEvent();
}
}
class Bar
{
public Bar(Foo l)
{
l.AnEvent += l_AnEvent;
}
void l_AnEvent()
{
}
}
If I instantiate a Foo, and pass this to a new Bar constructor, then let go of
the Bar object, it will not be freed by
the garbage collector because of
the AnEvent registration.
I consider this a memory leak, and seems just like my old C++ days. I can, of course, make Bar IDisposable, unregister
the event in
the Dispose() method, and make sure to call Dispose() on instances of it, but why should I have to do this?
I first question why events are implemented with strong references? Why not use weak references? An event is used to abstractly notify an object of changes in another object. It seems to me that if
the event handler's instance is no longer in use (i.e., there are no non-event references to
the object), then any events that it is registered with should automatically be unregistered. What am I missing?
I have looked at WeakEventManager. Wow, what a pain. Not only is it very difficult to use, but its documentation is inadequate (see http://msdn.microsoft.com/en-us/library/system.windows.weakeventmanager.aspx -- noticing
the "
Notes to Inheritors" section that has 6 vaguely described bullets).
I have seen other discussions in various places, but nothing I felt I could use. I propose a simpler solution based on WeakReference, as described here. My question is: Does this not meet
the requirements with significantly less complexity?
To use
the solution,
the above code is modified as follows:
class Foo
{
public WeakReferenceEvent AnEvent = new WeakReferenceEvent();
internal void DoEvent()
{
AnEvent.Invoke();
}
}
class Bar
{
public Bar(Foo l)
{
l.AnEvent += l_AnEvent;
}
void l_AnEvent()
{
}
}
Notice two things:
1.
The Foo class is modified in two ways:
The event is replaced with an instance of WeakReferenceEvent, shown below; and
the invocation of
the event is changed.
2.
The Bar class is UNCHANGED.
No need to subclass WeakEventManager, implement IWeakEventListener, etc.
OK, so on to
the implementation of WeakReferenceEvent. This is shown here. Note that it uses
the generic WeakReference that I borrowed
from here: http://damieng.com/blog/2006/08/01/implementingweakreferencet
I had to add Equals() and GetHashCode() to his class, which I include below for reference.
class WeakReferenceEvent
{
public static WeakReferenceEvent operator +(WeakReferenceEvent wre, Action handler)
{
wre._delegates.Add(new WeakReference<Action>(handler));
return wre;
}
public static WeakReferenceEvent operator -(WeakReferenceEvent wre, Action handler)
{
foreach (var del in wre._delegates)
if (del.Target == handler)
{
wre._delegates.Remove(del);
return wre;
}
return wre;
}
HashSet<WeakReference<Action>> _delegates = new HashSet<WeakReference<Action>>();
internal void Invoke()
{
HashSet<WeakReference<Action>> toRemove = null;
foreach (var del in _delegates)
{
if (del.IsAlive)
del.Target();
else
{
if (toRemove == null)
toRemove = new HashSet<WeakReference<Action>>();
toRemove.Add(del);
}
}
if (toRemove != null)
foreach (var del in toRemove)
_delegates.Remove(del);
}
}
public class WeakReference<T> : IDisposable
{
private GCHandle handle;
private bool trackResurrection;
public WeakReference(T target)
: this(target, false)
{
}
public WeakReference(T target, bool trackResurrection)
{
this.trackResurrection = trackResurrection;
this.Target = target;
}
~WeakReference()
{
Dispose();
}
public void Dispose()
{
handle.Free();
GC.SuppressFinalize(this);
}
public virtual bool IsAlive
{
get { return (handle.Target != null); }
}
public virtual bool TrackResurrection
{
get { return this.trackResurrection; }
}
public virtual T Target
{
get
{
object o = handle.Target;
if ((o == null) || (!(o is T)))
return default(T);
else
return (T)o;
}
set
{
handle = GCHandle.Alloc(value,
this.trackResurrection ? GCHandleType.WeakTrackResurrection : GCHandleType.Weak);
}
}
public override bool Equals(object obj)
{
var other = obj as WeakReference<T>;
return other != null && Target.Equals(other.Target);
}
public override int GetHashCode()
{
return Target.GetHashCode();
}
}
It's functionality is trivial. I override operator + and - to get
the += and -= syntactic sugar matching events. These create WeakReferences to
the Action delegate. This allows
the garbage collector to free
the event target object (Bar in this example) when nobody else is holding on to it.
In
the Invoke() method, simply run through
the weak references and call their Target Action. If any dead (i.e., garbage collected) references are found, remove them
from the list.
Of course, this only works with delegates of type Action. I tried making this generic, but ran into
the missing where T : delegate in C#!
As an alternative, simply modify class WeakReferenceEvent to be a WeakReferenceEvent, and replace
the Action with Action. Fix
the compiler errors and you have a class that can be used like so:
class Foo
{
public WeakReferenceEvent<int> AnEvent = new WeakReferenceEvent<int>();
internal void DoEvent()
{
AnEvent.Invoke(5);
}
}
Hopefully this will help someone else when they run into
the mystery .NET event memory leak!
