Question

I am using WCF Async methods.

I face issue when I try to return the value of the callback function.

What are the possible methods of solution? (using .net 4.0, but not 4.5)

    public static Object LoadInfo()
    {
        var service = new SomeWcfService();
        service.BeginGetInfo(CallbackMethod, service);

        // HOW TO GET INFROMATION FROM CALLBACK??
        return INFORMATION;
    }

    private static void CallbackMethod(IAsyncResult ar)
    {
        // HOW TO PASS INFROMATION TO LoadInfo??
        var INFORMATION = (ar.AsyncState as SomeWcfService).EndGetInfo(ar);
    }

Note: All work should be asynchronously.

Thanks.

Answer 1

Better you design to use the async pattern than fight against it.

But, if you must access an async method synchronously, you can use a semaphore to block the calling thread until the async method returns.

public static Object LoadInfo()
{
    // this is our semaphore
    var blocker = new AutoResetEvent();
    object result = null;

    var service = new SomeWcfService();
    // use a lambda instead of a method as the callback.
    // this will result in a closure where we can access the result and blocker variables
    service.BeginGetInfo(x => 
    {
        // We are on a different thread within this lambda
        result = (x.AsyncState as SomeWcfService).EndGetInfo(ar);
        // release anybody calling blocker.WaitOne
        blocker.Set(); 
    }, service);

    // we are still on the original thread here, and
    // BeginGetInfo has possibly not yet executed, so we must wait until Set is called
    blocker.WaitOne(Timeout.Infinite); 
    return result;
}

This is rarely a good design choice. Aynchornous patterns are much better for responsive UI. Better to do something like this

public void LoadInfo()
{
    // Makes the UI show a loading indicator, blocking all actions except for CANCEL
    LoadingInfo = true;

    var service = new SomeWcfService();
    service.BeginGetInfo(CallbackMethod, service);
}

private void CallbackMethod(IAsyncResult ar)
{
    // the UI is now released from loading 
    LoadingInfo = false;
    // the UI is triggered to show our data
    ViewModel.Data = (ar.AsyncState as SomeWcfService).EndGetInfo(ar);
}