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");