some messy code of a synchronization gateway that allows only a single thread of execution for all passed callbacks. It allows new callbacks to be enqueued while they are being executed by another thread. This implementation is almost certainly broken because I didn't care if it compiles at all and if all closure stuff is done correctly, but hop…

gistfile1.cs

    public class ConcurrentGateway
    {
        private ConcurrentQueue<Action> _workQueue = new ConcurrentQueue<Action>();
        private  int _writeLock = 0;

        [ThreadStatic]
        private static AutoResetEvent _waitEvent = new AutoResetEvent(false);

        protected static AutoResetEvent GetThreadWaitEvent()
        {
            var ev = _waitEvent;
            if (ev == null)
            {
                ev = new AutoResetEvent(false);
                _waitEvent = ev;
            }
            return ev;
        }

        public void DoSynchronized(Action act)
        {
            bool done = false; 
            var ev = GetThreadWaitEvent();
            _workQueue.Enqueue(delegate() //enqueue the work TBD
            {
                act();
                done = true;
                ev.Set();
            });
            int lk = Interlocked.CompareExchange(ref _writeLock, 1, 0);
            if (lk == 0)
            { //I'm a writer thread, do all the work now
                Action a2;
                while(_workQueue.TryDequeue(out a2)) {
                    a2();
                }
                
                var ov = Interlocked.Exchange(ref _writeLock, 0);
                System.Diagnostics.Debug.Assert(ov == 1); //release
            }
            else
            { //i'm not a writer so I have to wait
                if (lk != 1) throw new Exception("LK: " +lk);
                while (!done)
                {
                    ev.WaitOne();
                }
            }
        }
    }
    
    //with a dedicated handler thread
    public class ThreadedConcurrentGateway
    {
        private ConcurrentQueue<Action> _workQueue = new ConcurrentQueue<Action>();
        private Thread _worker;

        public ThreadedConcurrentGateway()
        {
            _worker = new Thread(new ThreadStart(() => {
                Action act;
                Console.WriteLine("GW thread started");
                while (true)
                {
                    if (_workQueue.TryDequeue(out act))
                    {
                        act();
                    }
                    else
                    {
                        _wakeUp.WaitOne();
                    }
                }
            }));
            _worker.Start();
        }

        private AutoResetEvent _wakeUp = new AutoResetEvent(false);

        [ThreadStatic]
        private static AutoResetEvent _waitEvent = new AutoResetEvent(false);

        protected static AutoResetEvent GetThreadWaitEvent()
        {
            var ev = _waitEvent;
            if (ev == null)
            {
                ev = new AutoResetEvent(false);
                _waitEvent = ev;
            }
            return ev;
        }

        public void DoSynchronized(Action act)
        {
            bool done = false; 
            var ev = GetThreadWaitEvent();
            _workQueue.Enqueue(delegate() //enqueue the work TBD
            {
                act();
                done = true;
                ev.Set();
            });
            _wakeUp.Set();
            while (!done)
            {
                ev.WaitOne();
            }
        }
    }