Tasks and the Event-based Asynchronous Pattern

As has been discussed previously, one of the new features in the Task Parallel Library is TaskCompletionSource<TResult>, which enables the creation of a Task<TResult> that represents any other asynchronous operation. 

There are a wide variety of sources in the .NET Framework for asynchronous work.  One comes from components that implement the Asynchronous Programming Model (APM) pattern, which we discussed here.  Another includes types that implement the Event-based Asynchronous Pattern (EAP).  For some synchronous method Xyz, the EAP provides an asynchronous counterpart XyzAsync.  Calling this method launches the asynchronous work, and when the work completes, a corresponding XyzCompleted event is raised. (That’s an oversimplification of the pattern, but it provides a grounding.)

For many situations, the EAP is quite straightforward and simple to use.  However, there are cases where you’d like to be able to do things like join across multiple EAP asynchronous invocations, such as to download three different web pages asynchronously, and only when all three have completed do something else.  Tasks in .NET 4.0 make this kind of operation easy, through Task.Factory.ContinueWhenAll (or ContinueWhenAny if you want to do something when any one of the items completes rather than when all of them do).  However, in order to use these methods, you need Tasks, thus to use them with components that implement the EAP, you need to create Tasks from EAP.  TaskCompletionSource<TResult> can be used to do exactly that.

First, let’s create two small helper functions that we can reuse over and over for multiple EAP-to-Task implementations:

private static TaskCompletionSource<T> CreateSource<T>(object state)
{
    return new TaskCompletionSource<T>(
        state, TaskCreationOptions.DetachedFromParent);
}

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();
    }
}

The first helper, CreateSource, simply creates an instance of TaskCompletionSource<T> with the right type and settings.  I’ve included TaskCreationOptions.DetachedFromParent under the assumption that tasks created for the EAP pattern aren’t necessarily meant to participate in parent/child relationships, but you could certainly change this if your scenarios needed that functionality.

The second helper, TransferCompletion, will be used whenever an EAP event signals completion of an asynchronous operation.  It takes the AsyncCompletedEventArgs provided through the XyzCompleted event and uses it to determine whether the operation completed due to cancellation or due to an unhandled exception.  Both of those pieces of data are standard to the base AsyncCompletedEventArgs class. However, each EAP operation typically comes with its own type derived from AsyncCompletedEventArgs: to be able to use this one helper function with any of those to mine the result of the operation, TransferCompletion also accepts a Func<T> that will return the results from the derived instance.

With those two helpers, writing a Task wrapper for an EAP operation is a cinch.  Consider System.Net.WebClient, which provides support for downloading and uploading to and from URIs, with methods like DownloadData.  We can write a Task-based wrapper for DownloadData in just a few lines (in this case, as an extension method):

public static Task<byte[]> DownloadDataTask(
    this WebClient webClient, Uri address)
{
    var tcs = CreateSource<byte[]>(address);
    webClient.DownloadDataCompleted +=
        (sender, e) => TransferCompletion(tcs, e, () => e.Result, null);
    webClient.DownloadDataAsync(address, tcs);
    return tcs.Task;
}

We first create a TaskCompletionSource<byte[]> whose Task<byte[]> will be returned from the method.  We then register with the DownloadDataCompleted event handler, such that when the event is raised, the downloaded data (or exception or cancellation information) will be transferred to the returned Task.  And then we start the operation.  Piece of cake.

One interesting thing to note about the EAP is that some implementations support multiple asynchronous operations concurrently on the same instance.  In such cases, a user-supplied token is needed to correlate the asynchronous invocations to the asynchronous completions (something that’s provided in Task implicitly by having a Task reference returned from and serving as a reference for an asynchronous operation).  For that purpose, we pass the created TaskCompletionSource<TResult> as the user-supplied token, and in TransferCompletion, we only transfer the results if the token received matches the target completion source.  If it doesn’t match, this is an event completion for another operation and should be ignored.

One potential issue with the previously shown implementation (and with the EAP pattern in general) is that, if they’re used over and over, the XyzCompleted event handlers will start to build up.  Delegates are being registered with the event but not released.  To fix that, we can also remove the event handler in the delegate handling the event.  For example, DownloadDataTask can be rewritten as:

public static Task<byte[]> DownloadDataTask(
    this WebClient webClient, Uri address)
{
    var tcs = CreateSource<byte[]>(address);

    DownloadDataCompletedEventHandler handler = null;
    handler = (sender, e) => TransferCompletionToTask(tcs, e, () => e.Result, () => webClient.DownloadDataCompleted -= handler);

    webClient.DownloadDataCompleted += handler;
    try { webClient.DownloadDataAsync(address, tcs); }
    catch {
        webClient.DownloadDataCompleted -= handler;
        tcs.TrySetCanceled();
        throw;
    }

    return tcs.Task;
}

The Beta 1 samples available at http://code.msdn.microsoft.com/ParExtSamples already include Task-based extensions for WebClient, as well as extensions for other EAP implementations like SmtpClient and Ping.  Download and enjoy!