title: How do I solve the error that I received when implementing the callback method?
body: <p>I am currently developing a WCF duplex service and I am trying to implement the callback method in my client app however there is a error of </p>
  
  <pre><code>'App.CallbackHandler' does not implement interface member IPostingServiceCallback.retrieveNotification(Service.Posting)' 
  </code></pre>
  
  <p>the service contract for my service are as follow</p>
  
  <pre><code>[ServiceContract(SessionMode=SessionMode.Required , CallbackContract = typeof(IPostingServiceCallBack))]
  public interface IPostingService
  {
      [OperationContract(IsOneWay = true)]
      void postNotification(Posting post);
  }
  
  public interface IPostingServiceCallBack
  {
      [OperationContract]
      String retrieveNotification(Posting post);
  }
  </code></pre>
  
  <p>I have generated the proxy and added into the project file of my client and adding the endpoint address into the app.config.</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>The code I have in my client app currently is </p>
  
  <pre><code>public class CallBackHandler : IPostingServiceCallback
  {
      public void retrieveNotification()
      {
          //planning to do something
      }
  }
  </code></pre>