Why do I get InvalidCastException from OperationContext.Current.GetCallbackChannel<>()

Question

So as to enable aysc callbacks to the clients, I need to add the Begin/End methods to the interface that is defined as the CallbackContract on my service. (I am using a she

Answer 1

As DxCK siad the type that is passed to OperationContext.Current.GetCallbackChannel>() must be exactly the same type as you specified in the [ServiceContract] attribute on the interface that the service implements.

However I need to use a different interface for the callbacks so I can add the Begin/End methods needed to support aysc callbacks.

So firstly my new callback interface.

[ServiceContract]
public interface IClientWithAsyncMethods : IClient
{
  [OperationContract(
    AsyncPattern = true, 
    Action = "ReceiveMessage", 
    ReplyAction = "ReceiveMessageResponse")]
  IAsyncResult BeginReceiveMessage(SimMessage message, 
    AsyncCallback callback, object asyncState);

  void EndReceiveMessage(IAsyncResult asyncResult);
}

Then need to define a new interface for my service to implment:

[ServiceContract(CallbackContract = typeof(IClientWithAsyncMethods))]
public interface IEngineManagerWithAsyncCallbacks : IEngineManager
{
}

The only change to to refare to the new callback interface as the CallbackContract, this is OK as IClientWithAsyncMethods is a subtype of IClient.

Then the last step is to change the serve implementation to use to the service interface:

• Change the interface type the service implements
• Pass the new interface into ServiceHost.AddServiceEndpoint() (and/or edit the WCF config files)
• Use IClientWithAsyncMethods in the call to OperationContext.Current.GetCallbackChannel>()

The rest is just calling the aysc method in the normal way.