ActionBlock.cs

public static class ActionBlock
{
	private static readonly Dictionary<string, List<Action<Level, Player>>> Hooks = new Dictionary<string, List<Action<Level, Player>>>();
	private static readonly ReaderWriterLockSlim RwLock = new ReaderWriterLockSlim();

	// returns true if an interaction took place
	public static bool InteractHook(this Block block, Level level, Player player, BlockFace face)
	{
		string hash = GetHash(level, block);
		List<Action<Level, Player>> hooks;

		RwLock.EnterReadLock();
		try
		{
			if (!Hooks.TryGetValue(hash, out hooks) || !hooks.Any())
				return false;
		}
		finally
		{
			RwLock.ExitReadLock();
		}

		foreach (var hook in hooks)
			hook(level, player);

		return true;
	}

	public static void RegisterHook(this Block block, Level level, Action<Level, Player> callback)
	{
		string hash = GetHash(level, block);

		RwLock.EnterWriteLock();

		List<Action<Level, Player>> hooks;
		if (!Hooks.TryGetValue(hash, out hooks))
		{
			hooks = new List<Action<Level, Player>>();
			Hooks[hash] = hooks;
		}

		hooks.Add(callback);

		RwLock.ExitWriteLock();
	}

	public static void UnregisterAllHooks(this Block block, Level level)
	{
		string hash = GetHash(level, block);

		RwLock.EnterWriteLock();

		Hooks.Remove(hash);

		RwLock.ExitWriteLock();
	}

	// returns what uniquely identifies a block registration, in this case its location
	private static string GetHash(Level level, Block block)
	{
		var coords = block.Coordinates;
		return level.LevelId + $"({coords.X},{coords.Y},{coords.Z})";
	}
}

Inside Player override

if (message.face <= 5)
{
	var coords = message.blockcoordinates;
  var face = (BlockFace) message.face;
	Block target = Level.GetBlock(coords);
	if (target.InteractHook(Level, this, face))
		return;
	Level.Interact(Level, this, message.item.Id, coords, message.item.Metadata, face, message.facecoordinates);
} else {
  ...
}