TestWeakEventSource.cs

void Main()
{
    var pub = new Publisher();
    var sub = new Subscriber();
    sub.Subscribe(pub);
    
    GC.Collect();
    GC.WaitForPendingFinalizers();
    
    pub.Raise();
    
    // Ensure subscriber isn't collected too early
    GC.KeepAlive(sub);
}

class Publisher
{
    private readonly WeakEventSource<EventHandler> _foo = new WeakEventSource<EventHandler>();
    public event EventHandler Foo
    {
        add { _foo.Subscribe(value); }
        remove { _foo.Unsubscribe(value); }
    }

    public void Raise()
    {
        _foo.Raise(this, EventArgs.Empty);
    }
}

class Subscriber
{
    public void Subscribe(Publisher pub)
    {
        pub.Foo += OnFoo;
    }

    private void OnFoo(object sender, EventArgs e)
    {
        Console.WriteLine("Subscriber.OnFoo");
    }
}