Converting async/await to Task.ContinueWith

by

I’ve figured out how to do it without async/await or TaskCompletionSource, using nested tasks and Task.Unwrap instead.

First, to address @mikez’s comment, here’s GetResponseAsync implementation for .NET 4.0:

static public Task<WebResponse> GetResponseTapAsync(this WebRequest request)
{
    return Task.Factory.FromAsync(
         (asyncCallback, state) =>
             request.BeginGetResponse(asyncCallback, state),
         (asyncResult) =>
             request.EndGetResponse(asyncResult), null);
}

Now, here’s GetResponseWithRetryAsync implementation:

static Task<HttpWebResponse> GetResponseWithRetryAsync(string url, int retries)
{
    if (retries < 0)
        throw new ArgumentOutOfRangeException();

    var request = WebRequest.Create(url);

    Func<Task<WebResponse>, Task<HttpWebResponse>> proceedToNextStep = null;

    Func<Task<HttpWebResponse>> doStep = () =>
    {
        return request.GetResponseTapAsync().ContinueWith(proceedToNextStep).Unwrap();
    };

    proceedToNextStep = (prevTask) =>
    {
        if (prevTask.IsCanceled)
            throw new TaskCanceledException();

        if (prevTask.IsFaulted && --retries > 0)
            return doStep();

        // throw if failed or return the result
        return Task.FromResult((HttpWebResponse)prevTask.Result);
    };

    return doStep();
}

It’s been an interesting exercise. It works, but I think its the way more difficult to follow, than the async/await version.

More Related Contents:

  1. Running multiple async tasks and waiting for them all to complete
  2. Task sequencing and re-entracy
  3. Enforce an async method to be called once
  4. Executing tasks in parallel
  5. Use Task.Run() in synchronous method to avoid deadlock waiting on async method?
  6. A pattern for self-cancelling and restarting task
  7. What is the correct way to cancel an async operation that doesn’t accept a CancellationToken?
  8. How to limit the amount of concurrent async I/O operations?
  9. WaitAll vs WhenAll
  10. HttpClient.GetAsync(…) never returns when using await/async
  11. Calling async method synchronously
  12. StaTaskScheduler and STA thread message pumping
  13. Why should I prefer single ‘await Task.WhenAll’ over multiple awaits?
  14. Why does this async action hang when I try and access the Result property of my Task?
  15. Regarding usage of Task.Start() , Task.Run() and Task.Factory.StartNew()
  16. How to use non-thread-safe async/await APIs and patterns with ASP.NET Web API?
  17. Waiting for async/await inside a task
  18. Revisiting Task.ConfigureAwait(continueOnCapturedContext: false)
  19. Async and await – difference between console, Windows Forms and ASP.NET
  20. multiple awaits vs Task.WaitAll – equivalent?
  21. Why use async and return await, when you can return Task directly?
  22. Difference between returning and awaiting a Task in an async method [duplicate]
  23. Proper way to start and fire-and-forget asynchronous calls?
  24. Async always WaitingForActivation
  25. How to correctly write Parallel.For with async methods
  26. A pattern to pause/resume an async task?
  27. How to Async Files.ReadAllLines and await for results?
  28. Await and SynchronizationContext in a managed component hosted by an unmanaged app
  29. How to implement Task.WhenAny() with a predicate
  30. Create a completed Task

Leave a Comment