How can I use async/await to call a webservice?

Question

I have a webservice written in Yii (php framework).

I use C# and Visual Studio 2012 to develop a WP8 application. I added a service reference to my project (Add Serv

Answer 1

I've had to do this a couple of times over the last year and I've used both @Judah's code above and the original example he has referenced but each time I've hit on the following problem with both: the async call works but doesn't complete. If I step through it I can see that it will enter the TransferCompletion method but the e.UserState == tcs will always be false.

It turns out that web service async methods like the OP's loginAsync have two signatures. The second accepts a userState parameter. The solution is to pass the TaskCompletionSource object you created as this parameter. This way the e.UserState == tcs will return true.

In the OP, the e.UserState == tcs was removed to make the code work which is understandable - I was tempted too. But I believe this is there to ensure the correct event is completed.

The full code is:

public static Task RaiseInvoiceAsync(this Client client, string userName, string password)
{
    var tcs = CreateSource();
    LoginCompletedEventHandler handler = null;
    handler = (sender, e) => TransferCompletion(tcs, e, () => e, () => client.LoginCompleted -= handler);
    client.LoginCompleted += handler;

    try
    {
        client.LoginAsync(userName, password, tcs);
    }
    catch
    {
        client.LoginCompleted -= handler;
        tcs.TrySetCanceled();
        throw;
    }

    return tcs.Task;
}

Alternatively, I believe there is a tcs.Task.AsyncState property too that will provide the userState. So you could do something like:

if (e.UserState == taskCompletionSource || e.UserState == taskCompletionSource?.Task.AsyncState)
{
    if (e.Cancelled) taskCompletionSource.TrySetCanceled();
    else if (e.Error != null) taskCompletionSource.TrySetException(e.Error);
    else taskCompletionSource.TrySetResult(getResult());
    unregisterHandler();
}

This was what I tried initially as it seemed a lighter approach and I could pass a Guid rather than the full TaskCompletionSource object. Stephen Cleary has a good write-up of the AsyncState if you're interested.