Implementing async timeout using poor mans async/a

问题:

Motivation

C# 5.0 async/await constructs are awesome, unfortunately yet Microsoft only shown a release candidate of both .NET 4.5 and VS 2012, and it will take some time until these technologies will get widely adopted in our projects.

In Stephen Toub's Asynchronous methods, C# iterators, and Tasks I've found a replacement that can be nicely used in .NET 4.0. There are also a dozen of other implementations that make it possible using the approach even in .NET 2.0 though they seem little outdated and less feature-rich.

Example

So now my .NET 4.0 code looks like (the commented sections show how it is done in .NET 4.5):

//private async Task ProcessMessageAsync()
private IEnumerable<Task> ProcessMessageAsync()
{
    //var udpReceiveResult = await udpClient.ReceiveAsync();

    var task = Task<UdpAsyncReceiveResult>
               .Factory
               .FromAsync(udpClient.BeginReceive, udpClient.EndReceive, null);

    yield return task;

    var udpReceiveResult = task.Result;

    //... blah blah blah

    if (message is BootstrapRequest)
    {
        var typedMessage = ((BootstrapRequest)(message));

        // !!! .NET 4.0 has no overload for CancellationTokenSource that 
        // !!! takes timeout parameter :(
        var cts 
          = new CancellationTokenSource(BootstrapResponseTimeout); // Error here

        //... blah blah blah

        // Say(messageIPEndPoint, responseMessage, cts.Token);

        Task.Factory.Iterate(Say(messageIPEndPoint, responseMessage, cts.Token));
    }
}

Looks little ugly though it does the job

The question

When using CancellationTokenSource in .NET 4.5 there is a constructor that takes timespan as a timeout parameter, so that resulting CancellationTokenSource cancels within specified period of time.
.Net 4.0 is not able to timeout, so what is the correct way of doing that in .Net 4.0?

回答1:

Does this really have anything to do with async/await? Looks like you're just needing a way to cancel the token, independently of async/await, right? In that case, could you simply create a Timer that calls Cancel after the timeout?

new Timer(state => cts.Cancel(), null, BootstrapResponseTimeout, Timeout.Infinite);

EDIT

My initial response above is the basic idea, but a more robust solution is can be found in Is CancellationTokenSource.CancelAfter() leaky? (actually the .Net 4.5 implementation of the constructor you're seeking). Here's a function you can use to create timeout tokens based on that code.

public static CancellationTokenSource CreateTimeoutToken(int dueTime) {
    if (dueTime < -1) {
        throw new ArgumentOutOfRangeException("dueTime");
    }
    var source = new CancellationTokenSource();
    var timer = new Timer(self => {
        ((Timer)self).Dispose();
        try {
            source.Cancel();
        } catch (ObjectDisposedException) {}
    });
    timer.Change(dueTime, -1);
    return source;
}

回答2:

FWIW, you can use async/await in 4.0 projects, just use the async targeting pack. Works Great For Me!

回答3:

You can still use CancelAfter(), which is an extension method in Microsoft.Bcl.Async which is very similar to accepted answer above.

This is the refereance souce code when I press F12 to see the implementation of CancelAfter():

  /// <summary>Cancels the <see cref="T:System.Threading.CancellationTokenSource" /> after the specified duration.</summary>
  /// <param name="source">The CancellationTokenSource.</param>
  /// <param name="dueTime">The due time in milliseconds for the source to be canceled.</param>
  public static void CancelAfter(this CancellationTokenSource source, int dueTime)
  {
    if (source == null)
      throw new NullReferenceException();
    if (dueTime < -1)
      throw new ArgumentOutOfRangeException("dueTime");
    Contract.EndContractBlock();
    Timer timer = (Timer) null;
    timer = new Timer((TimerCallback) (state =>
    {
      timer.Dispose();
      TimerManager.Remove(timer);
      try
      {
        source.Cancel();
      }
      catch (ObjectDisposedException ex)
      {
      }
    }), (object) null, -1, -1);
    TimerManager.Add(timer);
    timer.Change(dueTime, -1);
  }