How can I push a string from one client connected to a WCF service to another connected as well?

Question

Here's what I have so far:

IService:

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

namespace ServiceLibrary
{
    [ServiceContract(SessionMode = SessionMode.Allowed, CallbackContract = typeof(IServiceCallback))]
    public interface IService
    {
        [OperationContract(IsOneWay = false, IsInitiating = true, IsTerminating = false)]
        void Join(string userName);
    }

    interface IServiceCallback
    {        
        [OperationContract(IsOneWay = true)]
        void UserJoined(string senderName);
    }
}

Service:

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

namespace ServiceLibrary
{
    [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession, ConcurrencyMode = ConcurrencyMode.Multiple)]
    public class Service:IService
    {
        IServiceCallback callback = null;

        public void Join(string userName)
        {
            callback = OperationContext.Current.GetCallbackChannel<IServiceCallback>();

        }
    }
}

Just a simple string passed from one client to another.

Answer 1

You can't actively "push" a string from one WCF client to another - unless you use the peer-to-peer binding.

But what you're trying to do appears to be somewhat along the lines of the publish/subscribe model: one node publishes information (the server - maybe getting it from one of the clients), and any number of subscribers then receive that information.

There are a number of approaches in WCF on how to do this - duplex communication, MSMQ or Windows Azure ServiceBus messaging - the approaches are quite varied.

Here are a few articles with various approaches to get you started:

• Design Patterns: List-Based Publish-Subscribe
• Using Callback Contracts in WCF for Asynchronous Publish/Subscribe Event-Style Communication
• Building a Pub/Sub Message Bus with WCF and MSMQ
• Windows Azure – Service Bus Publish/Subscribe Example

but you'll find plenty more when you search for "WCF publish subscribe pattern" either on Google or Bing.

Answer 2

Looks like you already use duplex binding. Just keep a list of callbacks(subscribers) in some static Collection (static because you use per-session instances). When somebody calls "join" just iterate through that collection and call UserJoined for each subscriber.

private static List<IServiceCallback> subscribers = new List<IServiceCallback>();
public void Join(string userName)
{
    IServiceCallback callback = OperationContext.Current.GetCallbackChannel<IServiceCallback>();
    foreach (var serviceCallback in subscribers)
    {
        serviceCallback.UserJoined(userName);
    }

    subscribers.Add(callback);
}