WeakEvent - you wish it was here

At the first glance .NET events are an easy and harmless way to decouple components. The former statement is true but the latter is not. The reason is that whenever an instance of a class subscribes to an event published by another class a strong link between these two is established. By strong link I mean that the subscriber(listener) won't get garbage-collected as long as the publisher is alive. The only way to break that link is to unsubscribe from the event which might be easily omitted as the link is not explicit. Additionally there are cases when explicit(deterministic) cancellation of subscription is impossible. If additionally the publisher is a long living object than we might face a memory leak. In ideal world there would be a way of specifying that a subscription is weak which would mean that if the subscription is the only link to an object then the object can be garbage-collected and the subscription can be deleted. .NET does not provide that facility out of the box but fortunately it provides building blocks that in most cases let us build a good enough solution. The idea is to intercept calls that add and remove subscribers from/to an event and create a weak link between subscribers and publisher instead of the default, strong one.
When you define an event you don't have to write add/remove methods on your own because C# compiler generates them automatically. Basically the following code snippet:

public event EventHandler <EventArgs> MyEvent;

is just "syntactic sugar" that C# compiler transforms to much more verbose form. You can find detailed description of this process in CLR via C#  by Jeffrey Richter. From our perspective the most important thing is that we can overwrite the default behavior of the compiler and inject our own implementation of Add and Remove methods in a way that is completely transparent to subscribers. SomeClass and Subscriber classes show how it can be done. Don't worry about WeakEvent<T> class as it will be explained later.

public class SomeClass
{
    private WeakEvent <EventArgs> myWeakEvent;
    public event EventHandler <EventArgs> MyWeakEvent
    {
        add
        {
            myWeakEvent.Add(value);
        }

        remove
        {
            myWeakEvent.Remove(value);
        }
    }
    private void SomeMethodThatNeedsToRiseMyWeakEvent()
    {
        OnMyWeakEvent(new EventArgs());
    }

    protected void OnMyWeakEvent(EventArgs args)
    {
        myWeakEvent.Invoke(args);
    }
}
public class Subscriber
{
    private SomeClass someClass;
    public Subscriber()
    {
        someClass = new SomeClass();
        someClass.MyWeakEvent += Method;
    }

    private void Method(object sender, EventArgs e)
    {
    }
}

Add and Remove methods take a delegate as the input parameter. Every .NET delegate is an object with 2 properties. One of them is a reference to the target of the delegate(the object the delegate will be called on) and the second one is a description of the method which is provided as an instance of System.Reflection.MethodInfo class. Static delegates have the target property set to null. The target field is the root of all evil as it keeps the subscriber alive(it is a strong reference to the object the delegate will be called on). Fortunately .NET framework provides a class that can act as man in the middle between the method and its target which lets us break the direct link between them.
The class that makes it possible is called (no surprise) System.WeakReference. An instance of System.WeakReference class keeps a weak reference(instead of strong) to the object that is passed to its constructor. The weak reference can be transformed into the strong reference by accessing its Target property and storing its value in an ordinary variable. In this way we resurrect the object. If the object is already garbage-collected then the property returns null. All aforementioned functionality is encapsulated in a custom class that I called WeakDelegate.

internal class WeakDelegate
{
    private WeakReference target;
    private MethodInfo method;

    public object Target
    {
        get
        {
            return target.Target;
        }
        set
        {
            target = new WeakReference(value);
        }
    }

    public MethodInfo Method
    {
        get { return method; }
        set { method = value; }
    }
}

WeakEvent<T> is a class that takes advantage of WeakDelegate class to solve the problem outlined in the first paragraph. Its below implementation is rather straightforward but 2 pieces of code might need some explanation. The first one is inside Invoke method. Internally we store instances of WeakDelegate class which means that we can not invoke them directly and every time one of them needs to be executed we have to assemble an instance of  System.Delegate class. I don't know if the way the code creates delegates is the fastest one but I measured the execution time of that statement and the average time was 0.005384 ms per delegate which is fast enough for me. The second one is related to the fact that the locking is done in a way that prevents threads from waiting forever. If a thread can't enter the critical section within 15 seconds then it throws an exception. The rationale behind that approach is explained here.

public class WeakEvent <T> where T : EventArgs
{
    private readonly List <WeakDelegate> eventHandlers;
    private readonly object eventLock;

    public WeakEvent()
    {
        eventHandlers = new List <WeakDelegate>();
        eventLock = new object();
    }

    public void Invoke(T args)
    {
        ExecuteExclusively(delegate
        {
            for (int i = 0; i  < eventHandlers.Count; i++)
            {
                WeakDelegate weakDelegate = eventHandlers[i];
                // don't move this line to the ELSE block 
                //as the object needs to be resurrected
                Object target = weakDelegate.Target;

                if (IsWeakDelegateInvalid(target, weakDelegate.Method))
                {
                    eventHandlers.RemoveAt(i);
                    i--;
                }
                else
                {
                    Delegate realDelegate = Delegate.CreateDelegate(typeof(EventHandler <T>),
                    target, weakDelegate.Method);
                    EventHandler <T> eventHandler = (EventHandler <T>)realDelegate;
                    eventHandler(this, args);
                }
            }
        });
    }

    public void Remove(EventHandler <T> value)
    {
        ExecuteExclusively(delegate
        {
            for (int i = 0; i  < eventHandlers.Count; i++)
            {
                WeakDelegate weakDelegate = eventHandlers[i];
                Object target = weakDelegate.Target;

                if (IsWeakDelegateInvalid(target, weakDelegate.Method))
                {
                    eventHandlers.RemoveAt(i);
                    i--;
                }
                else
                {
                    if (value.Target == target && value.Method == weakDelegate.Method)
                    {
                        eventHandlers.RemoveAt(i);
                        i--;
                    }
                }
            }
        });
    }

    public void Add(EventHandler <T> value)
    {
        ExecuteExclusively(delegate
        {
            RemoveInvalidDelegates();

            WeakDelegate weakDelegate = new WeakDelegate();
            weakDelegate.Target = value.Target;
            weakDelegate.Method = value.Method;

            eventHandlers.Add(weakDelegate);
        });
    }

    private void RemoveInvalidDelegates()
    {
        for (int i = 0; i  < eventHandlers.Count; i++)
        {
            WeakDelegate weakDelegate = eventHandlers[i];

            if (IsWeakDelegateInvalid(weakDelegate))
            {
                eventHandlers.RemoveAt(i);
                i--;
            }
        }
    }

    private void ExecuteExclusively(Operation operation)
    {
        bool result = Monitor.TryEnter(eventLock, TimeSpan.FromSeconds(15));

        if (!result)
        {
            throw new TimeoutException("Couldn't acquire a lock");
        }

        try
        {
            operation();
        }
        finally
        {
            Monitor.Exit(eventLock);
        }
    }

    private bool IsWeakDelegateInvalid(WeakDelegate weakDelegate)
    {
        return IsWeakDelegateInvalid(weakDelegate.Target, weakDelegate.Method);
    }

    private bool IsWeakDelegateInvalid(object target, MethodInfo method)
    {
        return target == null && !method.IsStatic;
    }
}

You might have noticed that there is some housekeeping going on whenever one of Add, Remove or Invoke methods is called. The reason why we need to do this is that WeakEvent<T> keeps a collection of WeakDelegate objects that might contain methods bound to objects(targets) that have been garbage-collected. In other words we need to take care of getting rid of invalid delegates on our own. Solutions to this problem can vary from very simple to very sophisticated. The one that works in my case basically scans the collection of delegates and removes invalid ones every time a delegate is added, removed or the event is invoked. It might sound like overkill but it works fine for events that have around 1000-5000 subscribers and it's very simple. You might want to have a background thread that checks the collection every X seconds but then you need to figure out what is the value of X in your case. You can go even further and keep the value adaptive but then your solution gets even more complicated. In my case the simplest solutions works perfectly fine.
Hopefully this post will save someone an evening or two :).