PhantomReferenceQueue.cs

public class PhantomReferenceQueue<THandle>
{
    private BlockingCollection<THandle> _queue = new BlockingCollection<THandle>();
    private ConditionalWeakTable<object, PhatomReference> _refs = new ConditionalWeakTable<object, PhatomReference>();

    public void Register(object instance, THandle handle)
    {
        _refs.Add(instance, new PhantomReference(this, handle));
    }

    public void Unregister(object instance)
    {
        if (_refs.TryGetValue(instance, out var val))
            GC.SuppressFinalize(val);
        _refs.Remove(instance);
    }
    
    public THandle Poll()
    {
        return _queue.Take();
    }

    private class PhantomReference
    {
        THandle _handle;
        PhantomReferenceQueue<THandle> _parent;

        public PhantomReference(PhantomReferenceQueue<THandle> parent, THandle handle)
        {
            _parent = parent;
            _handle = handle;
        }

        ~PhantomReference()
        {
            _parent._queue.Add(_handle);
        }
    }
}