WeakReference is dead

c#, weak-references

Solution

There are a few issues here (others have mentioned some of them already), but the primary one is that the compiler is creating a new delegate object that no one is holding a strong reference to. The compiler takes

ea.Subscribe<SomeEvent>(SomeHandlerMethod);

and inserts the appropriate delegate conversion, giving effectively:

ea.Subscribe<SomeEvent>(new Action<SomeEvent>(SomeHandlerMethod));

Then later this delegate is collected (there is only your `WeakReference` to it) and the subscription is hosed.

You also have thread-safety issues (I'm assuming you are using ConcurrentDictionary for this purpose). Specifically the access to both the `ConcurrentDictionary` and the `List`s are not thread-safe at all. The Lists need to be locked and you need to properly use `ConcurrentDictionary` to make updates. For example, in your current code, it is possible that two separate threads are in the `TryAdd` block and one of them will fail causing a subscription to be lost.

We can fix these problems, but let me outline the solution. The weak event pattern can be tricky to implement in .Net because of those automatically generated delegate instances. What will do instead is capture the delegate's `Target` in a `WeakReference`, if it has one (It may not if it is a static method). Then if the method is an instance method we will construct an equivalent `Delegate` that has no Target and thus there will be no strong reference.

using System.Collections.Concurrent;
using System.Diagnostics;

public class EventAggregator
{
    private readonly ConcurrentDictionary<Type, List<Subscriber>> subscribers =
        new ConcurrentDictionary<Type, List<Subscriber>>();

    public void Subscribe<TMessage>(Action<TMessage> handler)
    {
        if (handler == null)
            throw new ArgumentNullException("handler");

        var messageType = typeof(TMessage);
        var handlers = this.subscribers.GetOrAdd(messageType, key => new List<Subscriber>());
        lock(handlers)
        {
            handlers.Add(new Subscriber(handler));
        }
    }

    public void Publish(object message)
    {
        if (message == null)
            throw new ArgumentNullException("message");

        var messageType = message.GetType();

        List<Subscriber> handlers;
        if (this.subscribers.TryGetValue(messageType, out handlers))
        {
            Subscriber[] tmpHandlers;
            lock(handlers)
            {
                tmpHandlers = handlers.ToArray();
            }

            foreach (var handler in tmpHandlers)
            {
                if (!handler.Invoke(message))
                {
                    lock(handlers)
                    {
                        handlers.Remove(handler);
                    }
                }
            }
        }
    }

    private class Subscriber
    {
        private readonly WeakReference reference;
        private readonly Delegate method;

        public Subscriber(Delegate subscriber)
        {
            var target = subscriber.Target;

            if (target != null)
            {
                // An instance method. Capture the target in a WeakReference.
                // Construct a new delegate that does not have a target;
                this.reference = new WeakReference(target);
                var messageType = subscriber.Method.GetParameters()[0].ParameterType;
                var delegateType = typeof(Action<,>).MakeGenericType(target.GetType(), messageType);
                this.method = Delegate.CreateDelegate(delegateType, subscriber.Method);
            }
            else
            {
                // It is a static method, so there is no associated target. 
                // Hold a strong reference to the delegate.
                this.reference = null;
                this.method = subscriber;
            }

            Debug.Assert(this.method.Target == null, "The delegate has a strong reference to the target.");
        }

        public bool IsAlive
        {
            get
            {
                // If the reference is null it was a Static method
                // and therefore is always "Alive".
                if (this.reference == null)
                    return true;

                return this.reference.IsAlive;
            }
        }

        public bool Invoke(object message)
        {
            object target = null;
            if (reference != null)
                target = reference.Target;

            if (!IsAlive)
                return false;

            if (target != null)
            {
                this.method.DynamicInvoke(target, message);
            }
            else
            {   
                this.method.DynamicInvoke(message);
            }

            return true;                
        }
    }
}

And a test program:

public class Program
{
    public static void Main(string[] args)
    {
        var agg = new EventAggregator();
        var test = new Test();
        agg.Subscribe<Message>(test.Handler);
        agg.Subscribe<Message>(StaticHandler);
        agg.Publish(new Message() { Data = "Start test" });
        GC.KeepAlive(test);

        for(int i = 0; i < 10; i++)
        {
            byte[] b = new byte[1000000]; // allocate some memory
            agg.Publish(new Message() { Data = i.ToString() });
            Console.WriteLine(GC.CollectionCount(2));
            GC.KeepAlive(b); // force the allocator to allocate b (if not in Debug).
        }

        GC.Collect();
        agg.Publish(new Message() { Data = "End test" });
    }

    private static void StaticHandler(Message m)
    {
        Console.WriteLine("Static Handler: {0}", m.Data);
    }
}

public class Test
{
    public void Handler(Message m)
    {
        Console.WriteLine("Instance Handler: {0}", m.Data);
    }
}

public class Message
{
    public string Data { get; set; }
}

Problem

Original source

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