Question

I’m studying alternatives to implement a true asynchronous communication between a server and a client using WCF. I’ve, of course, bumped into AsyncPattern = true.

After some fussing around I discovered that AsyncPattern is used to make the service implementation rather than the interface asynchronous, that is, the service exposed to clients still is synchronous. For instance, assume a async pair called BeginMethod and EndMethod, like below:

[ServiceContract]
interface IMyService
{
    [OperationContract(AsyncPattern = true)]
    IAsyncResult BeginMyMethod(...);

    MyComplexResult EndMyMethod(...);
}

Now, the WSDL generated from that will only expose a single method called MyMethod:

<wsdl:portType name="IMyService">
    <wsdl:operation name="MyMethod">
         <wsdl:input wsaw:Action="http://tempuri.org/IMyService/MyMethod" message="tns:IMyService_MyMethod_InputMessage"/>
         <wsdl:output wsaw:Action="http://tempuri.org/IMyService/MyMethodResponse" message="tns:IMyService_MyMethod_OutputMessage"/>
    </wsdl:operation>
</wsdl:portType>

The question is, is there any configuration that will force WCF to actually expose the two methods of the APM to clients?

Answer 1

What you are trying to achieve is not what this settings is for. You could re-craft WSDL but this is not a good approach.

You can try other messaging options e.g. OneWay and Duplex to achieve async call pattern from the client. See MSDN: What You Need To Know About One-Way Calls, Callbacks, And Events by Juval Lowy.

More details about OperationContractAttribute.AsyncPattern Property:

“Clients remain unaffected because the asynchronous method pair on the server is an implementation detail that does not affect the underlying Web Services Description Language (WSDL) description of the operation. Such methods appear to clients as a single operation with and correlated messages. WCF automatically routes inbound messages to the Begin method and routes the results of the End call to the outbound message. Client channels, therefore, can represent the method pair as either a single synchronous operation or as an asynchronous operation pair. In no case does the client representation affect the asynchronous implementation on the server in any way.”

“Client contracts can use the AsyncPattern property to indicate an asynchronous method pair that the client can use to invoke the operation asynchronously. Typically, client applications use the ServiceModel Metadata Utility Tool (Svcutil.exe) tool and the /async option to generate a Begin and End method pair that the client can use to invoke the operation asynchronously“. [this method pair BeginXXXX/EndXXXX is generated on the client]