Calling async methods from a WCF service

Question

I want to call asynchronous methods from a WCF service, something like:

[ServiceContract]
interface IService
{
    [OperationContract]
    int SomeMethod(int data);
}

int SomeMethod(int data)
{
    var query = ... build LINQ query;
    var response = await query.ToListAsync();
    return response.Length;
}

I don't want to add async to the IService interface or SomeMethod method. Using asynchronous methods is an internal issue that shouldn't be reflected in the interface.

How can I do that?

CLARIFICATION:

My problem here is using await in a non-async method. I don't want the service contract to change (the client doesn't necessarily know what async is), and I don't want to split the method into BeginSomeMethod and EndSomeMethod. I want one method that uses await internally.

Answer 1

Whether the server is using sync or async code does not matter for the client. Client and server are separated by a well-specified wire-protocol (often SOAP). SOAP has no notion of asynchronous completion.

You can have a sync server and an async client, or vice versa. The client cannot even detect whether the server is sync or async. This is an implementation detail. The server could be a wrist watch running Linux and you still couldn't tell.

The style of IO you use is an implementation detail and does not influence the bytes that go over the network.

So pick what you like. The client can still use async IO to access the server.

I'm not sure why this is such a surprise to people. In other contexts this seems very intuitive: You can have a asynchronous TCP server and a synchronous client. I can say new WebClient().DownloadString(url) and download a string synchronously from a web-server that is implemented in an asynchronous way. I cannot even tell what server software is running.

Use Fiddler to look at what goes over the wire when you make a WCF call. There is no notion of synchronous or asynchronous calls.

Under the hood, when you invoke a service asynchronously, the WCF client library using TCP sockets in an asynchronous way. When you invoke synchronously, TCP sockets are being used with blocking calls. That's the entire difference.

WCF generated clients can be made to have asynchronous methods in addition to the synchronous methods. Select the "Generate asynchronous operations" option in the UI. Now you have both versions. Both fully functional.

Here's how you can convince yourself of this with an experiment: Write a sync server, and call it both sync and async from the same .NET client. Now write a 2nd server asynchronously (in any style you like) and use the exact same client code to call it.

Task and IAsyncResult are not serializable over SOAP anyway so it cannot possibly be the case that a Task is transmitted to the client.