En un esfuerzo por perfeccionar algunos servicios de ejemplo que se utilizarán como referencia para nuestros escenarios internos, he creado este ejemplo de canal dúplex WCF, reuniendo varios ejemplos encontrados a lo largo de los años.

La parte dúplex no funciona y espero que todos podamos resolverlo juntos. Odio publicar tanto código, pero creo que lo he recortado lo más breve posible, al tiempo que incorporo todas las partes que espero haber examinado la comunidad. Puede haber algunas ideas realmente malas aquí, no estoy diciendo que sea correcto, es solo lo que tengo hasta ahora.

Hay tres partes El canal, el servidor y el cliente. Tres proyectos, y aquí, tres archivos de código. Sin configuración XML, todo está codificado. Seguido de la salida del código.

Channel.proj / Channel.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ServiceModel;

namespace Channel
{
    public interface IDuplexSyncCallback
    {
        [OperationContract]
        string CallbackSync(string message, DateTimeOffset timestamp);
    }

    [ServiceContract(CallbackContract = typeof(IDuplexSyncCallback))]
    public interface IDuplexSyncContract
    {
        [OperationContract]
        void Ping();

        [OperationContract]
        void Enroll();

        [OperationContract]
        void Unenroll();
    }
}

Server.proj / Server.cs, referencias Channel, System.Runtime.Serialization, System.ServiceModel

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ServiceModel;
using System.Timers;
using Channel;
using System.Diagnostics;
using System.Net.Security;

namespace Server
{
    class Program
    {
        // All of this just starts up the service with these hardcoded configurations
        static void Main(string[] args)
        {
            ServiceImplementation implementation = new ServiceImplementation();
            ServiceHost service = new ServiceHost(implementation);

            NetTcpBinding binding = new NetTcpBinding(SecurityMode.Transport);
            binding.Security.Message.ClientCredentialType = MessageCredentialType.Windows;
            binding.Security.Mode = SecurityMode.Transport;
            binding.Security.Transport.ClientCredentialType = TcpClientCredentialType.Windows;
            binding.Security.Transport.ProtectionLevel = ProtectionLevel.EncryptAndSign;
            binding.ListenBacklog = 1000;
            binding.MaxConnections = 30;
            binding.MaxReceivedMessageSize = 2147483647;
            binding.ReaderQuotas.MaxStringContentLength = 2147483647;
            binding.ReaderQuotas.MaxArrayLength = 2147483647;
            binding.SendTimeout = TimeSpan.FromSeconds(2);
            binding.ReceiveTimeout = TimeSpan.FromSeconds(10 * 60); // 10 minutes is the default if not specified
            binding.ReliableSession.Enabled = true;
            binding.ReliableSession.Ordered = true;

            service.AddServiceEndpoint(typeof(IDuplexSyncContract), binding, new Uri("net.tcp://localhost:3828"));

            service.Open();

            Console.WriteLine("Server Running ... Press any key to quit");
            Console.ReadKey(true);

            service.Abort();
            service.Close();
            implementation = null;
            service = null;
        }
    }

    /// <summary>
    /// ServiceImplementation of IDuplexSyncContract
    /// </summary>
    [ServiceBehavior(InstanceContextMode = InstanceContextMode.Single,
        MaxItemsInObjectGraph = 2147483647,
        IncludeExceptionDetailInFaults = true,
        ConcurrencyMode = ConcurrencyMode.Multiple,
        UseSynchronizationContext = false)]
    class ServiceImplementation : IDuplexSyncContract
    {
        Timer announcementTimer = new Timer(5000); // Every 5 seconds
        int messageNumber = 0; // message number incrementer - not threadsafe, just for debugging.

        public ServiceImplementation()
        {
            announcementTimer.Elapsed += new ElapsedEventHandler(announcementTimer_Elapsed);
            announcementTimer.AutoReset = true;
            announcementTimer.Enabled = true;
        }

        void announcementTimer_Elapsed(object sender, ElapsedEventArgs e)
        {
            AnnounceSync(string.Format("HELLO? (#{0})", messageNumber++));
        }

        #region IDuplexSyncContract Members
        List<IDuplexSyncCallback> syncCallbacks = new List<IDuplexSyncCallback>();

        /// <summary>
        /// Simple Ping liveness
        /// </summary>
        [OperationBehavior]
        public void Ping() { return; }

        /// <summary>
        /// Add channel to subscribers
        /// </summary>
        [OperationBehavior]
        void IDuplexSyncContract.Enroll()
        {
            IDuplexSyncCallback current = System.ServiceModel.OperationContext.Current.GetCallbackChannel<IDuplexSyncCallback>();

            lock (syncCallbacks)
            {
                syncCallbacks.Add(current);

                Trace.WriteLine("Enrollment Complete");
            }
        }

        /// <summary>
        /// Remove channel from subscribers
        /// </summary>
        [OperationBehavior]
        void IDuplexSyncContract.Unenroll()
        {
            IDuplexSyncCallback current = System.ServiceModel.OperationContext.Current.GetCallbackChannel<IDuplexSyncCallback>();

            lock (syncCallbacks)
            {
                syncCallbacks.Remove(current);

                Trace.WriteLine("Unenrollment Complete");
            }
        }

        /// <summary>
        /// Callback to clients over enrolled channels
        /// </summary>
        /// <param name="message"></param>
        void AnnounceSync(string message)
        {
            var now = DateTimeOffset.Now;

            if (message.Length > 2000) message = message.Substring(0, 2000 - "[TRUNCATED]".Length) + "[TRUNCATED]";
            Trace.WriteLine(string.Format("{0}: {1}", now.ToString("mm:ss.fff"), message));

            lock (syncCallbacks)
            {
                foreach (var callback in syncCallbacks.ToArray())
                {
                    Console.WriteLine("Sending \"{0}\" synchronously ...", message);

                    CommunicationState state = ((ICommunicationObject)callback).State;

                    switch (state)
                    {
                        case CommunicationState.Opened:
                            try
                            {
                                Console.WriteLine("Client said '{0}'", callback.CallbackSync(message, now));
                            }
                            catch (Exception ex)
                            {
                                // Timeout Error happens here
                                syncCallbacks.Remove(callback);
                                Console.WriteLine("Removed client");
                            }
                            break;
                        case CommunicationState.Created:
                        case CommunicationState.Opening:
                            break;
                        case CommunicationState.Faulted:
                        case CommunicationState.Closed:
                        case CommunicationState.Closing:
                        default:
                            syncCallbacks.Remove(callback);
                            Console.WriteLine("Removed client");
                            break;
                    }
                }
            }
        }
        #endregion
    }
}

Client.proj / Client.cs, referencias Channel, System.Runtime.Serialization, System.ServiceModel

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ServiceModel;
using System.Timers;
using System.Diagnostics;
using Channel;
using System.Net;

namespace Client
{
    class Program
    {
        static void Main(string[] args)
        {
            using (var callbackSyncProxy = new CallbackSyncProxy(new Uri("net.tcp://localhost:3828"), CredentialCache.DefaultNetworkCredentials))
            {
                callbackSyncProxy.Faulted += (s, e) => Console.WriteLine("CallbackSyncProxy Faulted.");
                callbackSyncProxy.ConnectionUnavailable += (s, e) => Console.WriteLine("CallbackSyncProxy ConnectionUnavailable.");
                callbackSyncProxy.ConnectionRecovered += (s, e) => Console.WriteLine("CallbackSyncProxy ConnectionRecovered.");

                callbackSyncProxy.Ping();
                callbackSyncProxy.Ping();
                callbackSyncProxy.Ping();

                Console.WriteLine("Pings completed.  Enrolling ...");

                callbackSyncProxy.AnnouncementSyncHandler = AnnouncementHandler;

                Console.WriteLine("Enrolled and waiting.  Press any key to quit ...");
                Console.ReadKey(true); // Wait for quit
            }
        }

        /// <summary>
        /// Called by the server through DuplexChannel
        /// </summary>
        /// <param name="message"></param>
        /// <param name="timeStamp"></param>
        /// <returns></returns>
        static string AnnouncementHandler(string message, DateTimeOffset timeStamp)
        {
            Console.WriteLine("{0}: {1}", timeStamp, message);

            return string.Format("Dear Server, thanks for that message at {0}.", timeStamp);
        }
    }

    /// <summary>
    /// Encapsulates the client-side WCF setup logic.
    /// 
    /// There are 3 events Faulted, ConnectionUnavailable, ConnectionRecovered that might be of interest to the consumer
    /// Enroll and Unenroll of the ServiceContract are called when setting an AnnouncementSyncHandler
    /// Ping, when set correctly against the server's send/receive timeouts, will keep the connection alive
    /// </summary>
    public class CallbackSyncProxy : IDisposable
    {
        Uri listen;
        NetworkCredential credentials;
        NetTcpBinding binding;
        EndpointAddress serverEndpoint;
        ChannelFactory<IDuplexSyncContract> channelFactory;
        DisposableChannel<IDuplexSyncContract> channel;

        readonly DuplexSyncCallback callback = new DuplexSyncCallback();

        object sync = new object();
        bool enrolled;
        Timer pingTimer = new Timer();
        bool quit = false; // set during dispose

        // Events of interest to consumer
        public event EventHandler Faulted;
        public event EventHandler ConnectionUnavailable;
        public event EventHandler ConnectionRecovered;

        // AnnouncementSyncHandler property.  When set to non-null delegate, Enrolls client with server.
        // passes through to the DuplexSyncCallback callback.AnnouncementSyncHandler
        public Func<string, DateTimeOffset, string> AnnouncementSyncHandler
        {
            get
            {
                Func<string, DateTimeOffset, string> temp = null;

                lock (sync)
                {
                    temp = callback.AnnouncementSyncHandler;
                }
                return temp;
            }
            set
            {
                lock (sync)
                {
                    if (callback.AnnouncementSyncHandler == null && value != null)
                    {
                        callback.AnnouncementSyncHandler = value;

                        Enroll();
                    }
                    else if (callback.AnnouncementSyncHandler != null && value == null)
                    {
                        Unenroll();

                        callback.AnnouncementSyncHandler = null;
                    }
                    else // null to null or function to function, just update it
                    {
                        callback.AnnouncementSyncHandler = value;
                    }
                }
            }
        }

        /// <summary>
        /// using (var proxy = new CallbackSyncProxy(listen, CredentialCache.DefaultNetworkCredentials) { ... }
        /// </summary>
        public CallbackSyncProxy(Uri listen, NetworkCredential credentials)
        {
            this.listen = listen;
            this.credentials = credentials;

            binding = new NetTcpBinding(SecurityMode.Transport);
            binding.Security.Message.ClientCredentialType = MessageCredentialType.Windows;
            binding.Security.Mode = SecurityMode.Transport;
            binding.Security.Transport.ClientCredentialType = TcpClientCredentialType.Windows;
            binding.MaxReceivedMessageSize = 2147483647;
            binding.ReaderQuotas.MaxArrayLength = 2147483647;
            binding.ReaderQuotas.MaxBytesPerRead = 2147483647;
            binding.ReaderQuotas.MaxDepth = 2147483647;
            binding.ReaderQuotas.MaxStringContentLength = 2147483647;
            binding.ReliableSession.Enabled = true;
            binding.ReliableSession.Ordered = true;
            serverEndpoint = new EndpointAddress(listen);

            pingTimer.AutoReset = true;
            pingTimer.Elapsed += pingTimer_Elapsed;
            pingTimer.Interval = 20000;
        }

        /// <summary>
        /// Keep the connection alive by pinging at some set minimum interval
        /// </summary>
        void pingTimer_Elapsed(object sender, ElapsedEventArgs e)
        {
            bool locked = false;

            try
            {
                locked = System.Threading.Monitor.TryEnter(sync, 100);
                if (!locked)
                {
                    Console.WriteLine("Unable to ping because synchronization lock could not be aquired in a timely fashion");
                    return;
                }
                Debug.Assert(channel != null, "CallbackSyncProxy.channel is unexpectedly null");

                try
                {
                    channel.Service.Ping();
                }
                catch
                {
                    Console.WriteLine("Unable to ping");
                }
            }
            finally
            {
                if (locked) System.Threading.Monitor.Exit(sync);
            }
        }

        /// <summary>
        /// Ping is a keep-alive, but can also be called by the consuming code
        /// </summary>
        public void Ping()
        {
            lock (sync)
            {
                if (channel != null)
                {
                    channel.Service.Ping();
                }
                else
                {
                    using (var c = new DisposableChannel<IDuplexSyncContract>(GetChannelFactory().CreateChannel()))
                    {
                        c.Service.Ping();
                    }
                }
            }
        }

        /// <summary>
        /// Enrollment - called when AnnouncementSyncHandler is assigned
        /// </summary>
        void Enroll()
        {
            lock (sync)
            {
                if (!enrolled)
                {
                    Debug.Assert(channel == null, "CallbackSyncProxy.channel is unexpectedly not null");

                    var c = new DisposableChannel<IDuplexSyncContract>(GetChannelFactory().CreateChannel());

                    ((ICommunicationObject)c.Service).Open();

                    ((ICommunicationObject)c.Service).Faulted += new EventHandler(CallbackChannel_Faulted);

                    c.Service.Enroll();

                    channel = c;

                    Debug.Assert(!pingTimer.Enabled, "CallbackSyncProxy.pingTimer unexpectedly Enabled");

                    pingTimer.Start();

                    enrolled = true;
                }
            }
        }

        /// <summary>
        /// Unenrollment - called when AnnouncementSyncHandler is set to null
        /// </summary>
        void Unenroll()
        {
            lock (sync)
            {
                if (callback.AnnouncementSyncHandler != null)
                {
                    Debug.Assert(channel != null, "CallbackSyncProxy.channel is unexpectedly null");

                    channel.Service.Unenroll();

                    Debug.Assert(!pingTimer.Enabled, "CallbackSyncProxy.pingTimer unexpectedly Disabled");

                    pingTimer.Stop();

                    enrolled = false;
                }
            }
        }

        /// <summary>
        /// Used during enrollment to establish a channel.
        /// </summary>
        /// <returns></returns>
        ChannelFactory<IDuplexSyncContract> GetChannelFactory()
        {
            lock (sync)
            {
                if (channelFactory != null &&
                    channelFactory.State != CommunicationState.Opened)
                {
                    ResetChannel();
                }

                if (channelFactory == null)
                {
                    channelFactory = new DuplexChannelFactory<IDuplexSyncContract>(callback, binding, serverEndpoint);

                    channelFactory.Credentials.Windows.ClientCredential = credentials;

                    foreach (var op in channelFactory.Endpoint.Contract.Operations)
                    {
                        var b = op.Behaviors[typeof(System.ServiceModel.Description.DataContractSerializerOperationBehavior)] as System.ServiceModel.Description.DataContractSerializerOperationBehavior;

                        if (b != null)
                            b.MaxItemsInObjectGraph = 2147483647;
                    }
                }
            }

            return channelFactory;
        }

        /// <summary>
        /// Channel Fault handler, set during Enrollment
        /// </summary>
        void CallbackChannel_Faulted(object sender, EventArgs e)
        {
            lock (sync)
            {
                if (Faulted != null)
                {
                    Faulted(this, new EventArgs());
                }

                ResetChannel();

                pingTimer.Stop();
                enrolled = false;

                if (callback.AnnouncementSyncHandler != null)
                {
                    while (!quit) // set during Dispose
                    {
                        System.Threading.Thread.Sleep(500);

                        try
                        {
                            Enroll();

                            if (ConnectionRecovered != null)
                            {
                                ConnectionRecovered(this, new EventArgs());

                                break;
                            }
                        }
                        catch
                        {
                            if (ConnectionUnavailable != null)
                            {
                                ConnectionUnavailable(this, new EventArgs());
                            }
                        }
                    }
                }
            }
        }

        /// <summary>
        /// Reset the Channel & ChannelFactory if they are faulted and during dispose
        /// </summary>
        void ResetChannel()
        {
            lock (sync)
            {
                if (channel != null)
                {
                    channel.Dispose();
                    channel = null;
                }

                if (channelFactory != null)
                {
                    if (channelFactory.State == CommunicationState.Faulted)
                        channelFactory.Abort();
                    else
                        try
                        {
                            channelFactory.Close();
                        }
                        catch
                        {
                            channelFactory.Abort();
                        }

                    channelFactory = null;
                }
            }
        }

        // Disposing of me implies disposing of disposable members
        #region IDisposable Members
        bool disposed;
        void IDisposable.Dispose()
        {
            if (!disposed)
            {
                Dispose(true);
            }

            GC.SuppressFinalize(this);
        }

        void Dispose(bool disposing)
        {
            if (disposing)
            {
                quit = true;

                ResetChannel();

                pingTimer.Stop();

                enrolled = false;

                callback.AnnouncementSyncHandler = null;
            }

            disposed = true;
        }
        #endregion
    }

    /// <summary>
    /// IDuplexSyncCallback implementation, instantiated through the CallbackSyncProxy
    /// </summary>
    [CallbackBehavior(UseSynchronizationContext = false, 
    ConcurrencyMode = ConcurrencyMode.Multiple, 
    IncludeExceptionDetailInFaults = true)]
    class DuplexSyncCallback : IDuplexSyncCallback
    {
        // Passthrough handler delegates from the CallbackSyncProxy
        #region AnnouncementSyncHandler passthrough property
        Func<string, DateTimeOffset, string> announcementSyncHandler;
        public Func<string, DateTimeOffset, string> AnnouncementSyncHandler
        {
            get
            {
                return announcementSyncHandler;
            }
            set
            {
                announcementSyncHandler = value;
            }
        }
        #endregion

        /// <summary>
        /// IDuplexSyncCallback.CallbackSync
        /// </summary>
        [OperationBehavior]
        public string CallbackSync(string message, DateTimeOffset timestamp)
        {
            if (announcementSyncHandler != null)
            {
                return announcementSyncHandler(message, timestamp);
            }
            else
            {
                return "Sorry, nobody was home";
            }
        }
    }

    // This class wraps an ICommunicationObject so that it can be either Closed or Aborted properly with a using statement
    // This was chosen over alternatives of elaborate try-catch-finally blocks in every calling method, or implementing a
    // new Channel type that overrides Disposable with similar new behavior
    sealed class DisposableChannel<T> : IDisposable
    {
        T proxy;
        bool disposed;

        public DisposableChannel(T proxy)
        {
            if (!(proxy is ICommunicationObject)) throw new ArgumentException("object of type ICommunicationObject expected", "proxy");

            this.proxy = proxy;
        }

        public T Service
        {
            get
            {
                if (disposed) throw new ObjectDisposedException("DisposableProxy");

                return proxy;
            }
        }

        public void Dispose()
        {
            if (!disposed)
            {
                Dispose(true);
            }

            GC.SuppressFinalize(this);
        }

        void Dispose(bool disposing)
        {
            if (disposing)
            {
                if (proxy != null)
                {
                    ICommunicationObject ico = null;

                    if (proxy is ICommunicationObject)
                        ico = (ICommunicationObject)proxy;

                    // This state may change after the test and there's no known way to synchronize
                    // so that's why we just give it our best shot
                    if (ico.State == CommunicationState.Faulted)
                        ico.Abort(); // Known to be faulted
                    else
                        try
                        {
                            ico.Close(); // Attempt to close, this is the nice way and we ought to be nice
                        }
                        catch
                        {
                            ico.Abort(); // Sometimes being nice isn't an option
                        }

                    proxy = default(T);
                }
            }

            disposed = true;
        }
    }
}

Salida clasificada:

>> Server Running ... Press any key to quit
                           Pings completed.  Enrolling ... <<
          Enrolled and waiting.  Press any key to quit ... <<
>> Sending "HELLO? (#0)" synchronously ...
                                CallbackSyncProxy Faulted. <<
                    CallbackSyncProxy ConnectionRecovered. <<
>> Removed client
>> Sending "HELLO? (#2)" synchronously ...
                   8/2/2010 2:47:32 PM -07:00: HELLO? (#2) <<
>> Removed client

Como Andrew ha señalado, el problema no es tan evidente. Esta "salida clasificada" no es la salida deseada. En cambio, me gustaría que el Servidor se esté ejecutando, los Pings y la inscripción tengan éxito, y luego cada 5 segundos, el servidor "ENVIARÁ" ¿HOLA? (#m) "sincrónicamente" e inmediatamente el Cliente se transformaría y regresaría y el Servidor recibiría e imprimiría.

En cambio, los pings funcionan, pero las fallas de devolución de llamada en el primer intento, llega al cliente en la reconexión pero no regresa al servidor, y todo se desconecta.

Las únicas excepciones que llego a ver se relacionan con que el canal falló previamente y, por lo tanto, no se puede usar, pero aún no hay nada sobre la falla real que hace que el canal alcance ese estado.

He usado un código similar con[OperationalBehavior(IsOneWay= true)] Muchas veces. Es extraño que este caso aparentemente más común me esté dando tanta pena.

La excepción atrapada en el lado del servidor, que no entiendo, es:
System.TimeoutException: "Esta operación de solicitud enviada a schemas.microsoft.com/2005/12/ServiceModel/Addressing/Anonymous no recibió una respuesta dentro del tiempo de espera configurado (00:00:00). Es posible que el tiempo asignado a esta operación ha sido una parte de un tiempo de espera más largo. Esto puede deberse a que el servicio aún está procesando la operación o porque el servicio no pudo enviar un mensaje de respuesta. Considere aumentar el tiempo de espera de la operación (enviando el canal / proxy a IContextChannel y configurando OperationTimeout propiedad) y asegúrese de que el servicio pueda conectarse al cliente ".