How does WCF support duplex communication?

Question

ICSM Computer · Answer

WCF supports duplex communication using a callback contract, enabling two-way communication between a service and its client. This is useful for scenarios like chat applications, real-time notifications, or event-based systems.
How Duplex Communication Works in WCF
Client calls the serviceService holds a reference to the client’s callbackService calls back the client asynchronously
Key Components
Component
Description
ServiceContract
The main service interface
CallbackContract
An interface that defines operations the client must implement
InstanceContextMode
Usually set to PerSession to maintain the callback reference
binding
Must support sessions (e.g., NetTcpBinding, WSDualHttpBinding)
Example
1. Define Callback Contract
public interface IMyCallback
{
    [OperationContract(IsOneWay = true)]
    void OnNotify(string message);
}
2. Define Service Contract with Callback
[ServiceContract(CallbackContract = typeof(IMyCallback))]
public interface IMyService
{
    [OperationContract]
    void Register();
    [OperationContract]
    void SendMessage(string msg);
}
3. Implement the Service
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)]
public class MyService : IMyService
{
    public void Register()
    {
        var callback = OperationContext.Current.GetCallbackChannelandlt;IMyCallbackandgt;();
        callback.OnNotify("Registered successfully.");
    }
    public void SendMessage(string msg)
    {
        var callback = OperationContext.Current.GetCallbackChannelandlt;IMyCallbackandgt;();
        callback.OnNotify("Server received: " + msg);
    }
}
4. Create Client with Callback Implementation
public class MyClientCallback : IMyCallback
{
    public void OnNotify(string message)
    {
        Console.WriteLine("Received from server: " + message);
    }
}
5. Connect Client
var instanceContext = new InstanceContext(new MyClientCallback());
var client = new MyServiceClient(instanceContext); // auto-generated proxy
client.Register();
client.SendMessage("Hello from client");
Required Bindings for Duplex
Binding
Supports Duplex?
Notes
NetTcpBinding
Yes
Best for intranet scenarios
WSDualHttpBinding
Yes
Allows duplex over HTTP (complex setup)
NetNamedPipeBinding
Yes
For on-machine communication
BasicHttpBinding
NO
No duplex support
and

Component	Description
`ServiceContract`	The main service interface
`CallbackContract`	An interface that defines operations the client must implement
`InstanceContextMode`	Usually set to `PerSession` to maintain the callback reference
`binding`	Must support sessions (e.g., `NetTcpBinding`, `WSDualHttpBinding`)

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How does WCF support duplex communication?

ICSM Computer

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1 Answers

How Duplex Communication Works in WCF

Key Components

Example

1. Define Callback Contract

2. Define Service Contract with Callback

3. Implement the Service

4. Create Client with Callback Implementation

5. Connect Client

Required Bindings for Duplex

Liked By

Binding	Supports Duplex?	Notes
`NetTcpBinding`	Yes	Best for intranet scenarios
`WSDualHttpBinding`	Yes	Allows duplex over HTTP (complex setup)
`NetNamedPipeBinding`	Yes	For on-machine communication
`BasicHttpBinding`	NO	No duplex support