Weak References C# with Example

In .NET, the GC allocates objects when there are no references left to them. Therefore, while an object can still be 
reached from code (there is a strong reference to it), the GC will not allocate this object. This can become a problem 
if there are a lot of large objects. 
A weak reference is a reference, that allows the GC to collect the object while still allowing to access the object. A 
weak reference is valid only during the indeterminate amount of time until the object is collected when no strong 
references exist. When you use a weak reference, the application can still obtain a strong reference to the object, 
which prevents it from being collected. So weak references can be useful for holding on to large objects that are 
expensive to initialize, but should be available for garbage collection if they are not actively in use. 
Simple usage: 
WeakReference reference = new WeakReference(new object(), false); 
GC.Collect(); 
object target = reference.Target; 
if (target != null) 
DoSomething(target); 
So weak references could be used to maintain, for example, a cache of objects. However, it is important to 
remember that there is always the risk that the garbage collector will get to the object before a strong reference is 
reestablished. 
Weak references are also handy for avoiding memory leaks. A typical use case is with events. 
Suppose we have some handler to an event on a source: 
Source.Event += new EventHandler(Handler) 
This code registers an event handler and creates a strong reference from the event source to the listening object. If 
the source object has a longer lifetime than the listener, and the listener doesn't need the event 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. 
In this case, it may be a good idea is to use the Weak Event Pattern. 
Something like: 
public static class WeakEventManager 
{ 
public static void SetHandler( 
Action> add, 
Action> remove, 
S subscriber, 
Action action) 
where TArgs : EventArgs 
where S : class 
{ 
var subscrWeakRef = new WeakReference(subscriber); 
EventHandler handler = null; 
 

handler = (s, e) => 
{ 
var subscrStrongRef = subscrWeakRef.Target as S; 
if (subscrStrongRef != null) 
{ 
action(subscrStrongRef, e); 
} 
else 
{ 
remove(handler); 
handler = null; 
} 
}; 
add(handler); 
} 
} 
and used like this: 
EventSource s = new EventSource(); 
Subscriber subscriber = new Subscriber(); 
WeakEventManager.SetHandler(a => s.Event += a, r => s.Event -= r, 
subscriber, (s,e) => { s.HandleEvent(e); }); 
In this case of course we have some restrictions - the event must be a 
public event EventHandler Event; 
As MSDN suggests: 
Use long weak references only when necessary as the state of the object is unpredictable after finalization. 
Avoid using weak references to small objects because the pointer itself may be as large or larger. 
Avoid using weak references as an automatic solution to memory management problems. Instead, develop 
an effective caching policy for handling your application's objects.