HTTP Duplex messaging in Silverlight 4

One of the new feature introduced in WCF for Silverlight 4.0 is an improvement of PollingDuplex binding. You must be aware of commonly used request-reply pattern, where client (Silverlight) initiate the request and response from WCF service is delivered on the same HTTP connection. The duplex binding allows either WCF service or Silverlight 4.0 client to initiate message exchange. The duplex messaging is useful when server needs to notify the client.

In older version of Silverlight when client communicates to server, the client directly send request to server and server send response to client in the same connection. When server needs to be enabled to communicate with client, the client has to poll the server and server uses same poll response to send message to clients. In this model the server will send single message on every poll response, and server will wait for new poll to return every message. The Silverlight 4 added a new mode MultipleMessagesPerPoll in existing PollingDuplex binding to address this issue.
The new polling mode is using HTTP response streaming to send multiple messages in a single poll response. The server will return chunked response with as many messages as are pending and then it close the poll response.

Silverlight 4.0 offers duplex messaging over HTTP or TCP. In this article I am going to cover HTTP based duplex messaging. This article first talks about how to created Duplex WCF service and next we will see how Silverlight client consume the Duplex service.

Following are the steps to create Duplex service and consume it in Silverlight client.

Create new Silverlight application. Go to, File > New > Project. From the new Project dialog select "Silverlight Application", specify the project name HTTPDuplexMessaging.


Click on "OK" button.

Create Duplex Service:

  1. Add WCF service in web project

    Right click on "HTTPDuplexMessaging.Web" project, select Add > New Item. From the Add New Item dialog box, select Silverlight Enabled WCF service, give the name "ChatService" and click on Add button.

  2. HTTP2.gif
  3. Add IChatClient interface

    Right click on web project and select Add > New Item, select Class template give file name "IChatClient" and click on Add button.

    This interface will be used as callback by service. Add below code for IChatClient interface.

        public interface IChatClient
            [OperationContract(IsOneWay = true)]
            void NotifyClient(string message);
  4. Specify Callback Contract in service contract attribute

    [ServiceContract(Namespace = "", CallbackContract = typeof(IChatClient))]
  5. Add Service contract to register client

    SynchronizedCollection<IChatClient> clients = new SynchronizedCollection<IChatClient>();

       public void AttachClient()

    The clients will use AttchClient contract to register for chat.

  6. Implement client notification

    Add PublishMessage contract, this contract will be used to notify registered clients.

    public void PublishMessage(string message)
    foreach (IChatClient channel in clients)

  7. Add binding extension to use PollingDuplex

    Add PollingDuplex binding extension in web.config. Here I have added binding extension with the name "duplexHttpBinding".

    add name="duplexHttpBinding"
    " />

    Now use duplexHttpBinding declared above in binding section to make use of this binding. Here the important thing to note is, the duplexMode property is set to MultipleMessagesPerPoll mode.

    binding name="myDuplexHttpBinding" duplexMode="MultipleMessagesPerPoll" maxOutputDelay="00:00:07"/>


At this moment the Service changes are completed. Next we will see how the Silverlight client consumes the above created service.

Create Silverlight client application:
  1. Add ChatService service reference

    Right click on Silverlight project and select Add Service Reference. In the Add Service Reference dialog box click on "Discover" button, by default ChatService will be selected. Specify the namespace name to "ChatService" and click on OK button.

  2. HTTP3.gif
  3. Design the Silverlight UI

    Add below line of code in MainPage.xaml.

                <TextBox x:Name="txtReceivedMsg" Height="200" Width="300"/>
                <StackPanel Orientation="Horizontal">
                    <TextBox x:Name="txtMyMsg" Height="25"  Width="250"/>
                    <Button x:Name="btnSend" Height="25" Width="50" Content="Send" Click="btnSend_Click"/>

    Here when you type message in txtMyMsg textbox and click on Send button, this message will be submitted to service and the same message will be published to all registered clients. The message notified from server to client will be displayed on txtReceivedMsg textbox.
  4. Create ChatService object and define endpoint

    Declare ChatService object and define binding and endpoint.

    ChatService.ChatServiceClient proxy;

    proxy = new ChatService.ChatServiceClient(
                    new PollingDuplexHttpBinding { DuplexMode = PollingDuplexMode.MultipleMessagesPerPoll},
                    new EndpointAddress("../ChatService.svc"));

  5. Register client

    To register client to receive service notification add below code in MainPage constructor.

  6. Attach Received event

    Attach Received event in MainPage constructor to get notification message published from server.

    proxy.NotifyClientReceived += new EventHandler<ChatService.NotifyClientReceivedEventArgs>(proxy_NotifyClientReceived);

    void proxy_NotifyClientReceived(object sender, ChatService.NotifyClientReceivedEventArgs e)
                txtReceivedMsg.Text += e.message + "\n";

  7. Send message from client to server

    On Send button click event, call PublishMessage async contract to send message from client to server. When server will receive message from any client, the server will send the same message to all registered clients.

    private void btnSend_Click(object sender, RoutedEventArgs e)
                txtMyMsg.Text = "";
Now Run application:

When you run application it will be launched in browser. Copy the URL from this browse instance and open two more instances of browser and access the same URL. The chat application will be opened in all three browser instances. At this moment three clients are registered to get service notification.

Now when you type message in any of the client and click on send button, the same message will be notified to all clients. You can see the same message in all clients.


In this article we learned how we can implement PollingDuplex with MultipleMessagesPerPoll duplex mode.

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