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 Service Reference). So I am able to use webservice functions.

   client = new YChatWebService.WebServiceControllerPortTypeClient();

   client.loginCompleted += client_loginCompleted;   // this.token = e.Result;
   client.loginAsync(this.username, this.password); 

   client.getTestCompleted += client_getTestCompleted;
   client.getTestAsync(this.token); 

function getTestAsync and loginAsync return void and both are asynchronous. Is it possible for the functions to return Task<T>? I would like to use async/await keywords in my program.

Answer 1

Assuming that loginAsync returns void, and loginCmpleted event fires when login is done, this is called the Event-based Asynchronous Pattern, or EAP.

To convert EAP to await/async, consult Tasks and the Event-based Asynchronous Pattern. In particular, you'll want to make use of the TaskCompletionSource to convert the event-based model to a Task-based model. Once you've got a Task-based model, you can use C# 5's sexy await feature.

Here's an example:

// Use LoginCompletedEventArgs, or whatever type you need out of the .loginCompleted event
// This is an extension method, and needs to be placed in a static class.
public static Task<LoginCompletedEventArgs> LoginAsyncTask(this YChatWebService.WebServiceControllerPortTypeClient client, string userName, string password) 
{ 
    var tcs = CreateSource<LoginCompletedEventArgs>(null); 
    client.loginCompleted += (sender, e) => TransferCompletion(tcs, e, () => e, null); 
    client.loginAsync(userName, password);
    return tcs.Task; 
}

private static TaskCompletionSource<T> CreateSource<T>(object state) 
{ 
    return new TaskCompletionSource<T>( 
        state, TaskCreationOptions.None); 
}

private static void TransferCompletion<T>( 
    TaskCompletionSource<T> tcs, AsyncCompletedEventArgs e, 
    Func<T> getResult, Action unregisterHandler) 
{ 
    if (e.UserState == tcs) 
    { 
        if (e.Cancelled) tcs.TrySetCanceled(); 
        else if (e.Error != null) tcs.TrySetException(e.Error); 
        else tcs.TrySetResult(getResult()); 
        if (unregisterHandler != null) unregisterHandler();
    } 
}

Now that you've converted the Event-based async programming model to a Task-based one, you can now use await:

var client = new YChatWebService.WebServiceControllerPortTypeClient();
var login = await client.LoginAsyncTask("myUserName", "myPassword");

Answer 2

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<T> 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<LoginCompletedEventArgs> RaiseInvoiceAsync(this Client client, string userName, string password)
{
    var tcs = CreateSource<LoginCompletedEventArgs>();
    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.

Answer 3

While adding your service reference make sure you selected Generate Task based operations in Advanced section. this will create awaitable methods like LoginAsync returning Task<string>