How can I callback the client and expose a new Channel with instance context

I'm making a WCF service with netTcpBinding which has a main lobby with multiple chatrooms which the clients can enter. The Lobby class implements ILobby as the service contract.

When a client wishes to enter a room I want to callback the client exposing a new Channel containing the InstanceContext for the room he just entered but after much searching I am doubting that this is possible.

For example on the Service side I might have

class Lobby : ILobby
{
    Dictionary<string, Chatroom> rooms;

    public void JoinRoom(string roomname)
    {
        if (rooms[roomname].TryEnter()) {}
    }
}

class ChatRoom : IChatRoom
{
    public bool TryEnter(string username)
    {
        ILobbyCallback callback =
            OperationContext.Current.GetCallbackChannel<ILobbyCallback>();
        // How do I do this next bit?
        callback.JoinedRoom(pass some instance context here);
        return true;
    }
}

On the client side callback method I want

public void JoinedRoom(InstanceContext for the room on the service side)
{
    // Create a new WCF proxy using above InstanceContext
    // Create a WPF UI for the new room passing the proxy so it can communicate
    // with the room class directly without going via the root service
}

Is this possible? What's the best practice for spawning new classes with their own contracts on the service side? Or do I just have to bundle everything into one massive MyService class and handle everything myself?

You cannot pass instance context as parameter to any operation contract. It doesn't make sense because that context has local scope. It is called "instance context" = it is context of current service instance. In duplex scenario both client and server has its own service:

• Clients calls server's service through its proxy
• Server calls client' service through received callback channel

Server's service instance context has meaning only on the server. It is not clear what you are trying to achieve (except very complex architecture).

• If you want to share context on client you can try to pass around the instance context used for the very first proxy you created - I'm not sure if it will work but you can try it
• If you want to share service instance context between multiple proxies you must develop your own IInstanceContextProvider and perhaps also your own IInstanceProvider (depending on what you want to achieve), wrap them in behavior and add them to the service. That will put whole complexity of session handling and correct instance releasing under your control (it obviously has its pros and cons).

But is it really needed? When I look at your code I see that one service and one proxy is enough. Also your JoinRoom operation doesn't need to use callback at all, it can be just request response method.