public class GameService
{
    public void PlayAction(Connection peer, PlayPacket packet)
    {
        GameEngine g = GetGameEngineByPeer(peer)

        lock (g.SyncRoot)
        {
            g.Play(packet.Param);
        }
    }

    public void PlayHandler(object sender, PlayResultArgs e)
    {
        var g = (GameEngine)sender;
        lock (g.SyncRoot)
        {
            // boradcast to other peers
        }
    }
}

public class GameEngine
{
    private readonly object _syncRoot = new Object();

    public object SyncRoot { get { return _syncRoot; } }

    public void Play(int param)
    {
        // does something really cool
        OnPlayed();
    }

    public event EventHandler<PlayResultArgs> Played;

    private void OnPlayed()
    {
        if (Played != null) Played(this, new PlayedResultArgs());
    }
}