Canceling Task Delay in .Net 4.0

Question

I am currently trying to implement a substitute for .Net 4.5's Task.Delay() method in a program that must target .Net 4.0. I found the following code at this blog.

    /* You can write Task-based asynchronous methods by utilizing a TaskCompletionSource.
A TaskCompletionSource gives you a 'slave' Task that you can manually signal.
Calling SetResult() signals the task as complete, and any continuations kick off. */

void Main()
{    
    for (int i = 0; i < 10000; i++)
    {
        Task task = Delay (2000);
        task.ContinueWith (_ => "Done".Dump());
    }
}

Task Delay (int milliseconds)        // Asynchronous NON-BLOCKING method
{
    var tcs = new TaskCompletionSource<object>();
    new Timer (_ => tcs.SetResult (null)).Change (milliseconds, -1);
    return tcs.Task;
}

Tasks are fairly new to me. System.Threading.Timer and TaskCompletionSource are brand new to me (as of today), and I'm struggling a bit with them. All that aside, I'm wondering how I might add CancellationToken functionality to this code. I'm assuming I could add a parameter to the Delay() method like this:

Task Delay (int milliseconds, CancellationToken token)        // Asynchronous NON-BLOCKING method
{
    var tcs = new TaskCompletionSource<object>();
    new Timer (_ => tcs.SetResult (null)).Change (milliseconds, -1);
    return tcs.Task;
}

... but then, where do I put the logic for checking the token and getting out of the method? Somewhere in the callback? Is this even possible?

Answer 1

I've tried to change your code as little as possible but here is a working example that behaves in the same way as Task.Delay.

It's important to note that I use TrySetCanceled and TrySetResult because the Timer could finish after the task is canceled. Ideally you want to stop the timer.

Also note a canceled task will throw a TaskCanceledException

static void Main(string[] args)
{
    // A cancellation source that will cancel itself after 1 second
    var cancellationTokenSource = new CancellationTokenSource(TimeSpan.FromSeconds(1));

    try
    {
        // This will only wait 1 second because as it will be cancelled.
        Task t = Delay(5000, cancellationTokenSource.Token);                
        t.Wait();
        Console.WriteLine("The task completed");
    }
    catch (AggregateException exception)
    {
        // Expecting a TaskCanceledException
        foreach (Exception ex in exception.InnerExceptions)
            Console.WriteLine("Exception: {0}", ex.Message);
    }
    Console.WriteLine("Done");
    Console.ReadLine();
}

private static Task Delay(int milliseconds, CancellationToken token)
{
    var tcs = new TaskCompletionSource<object>();
    token.Register(() => tcs.TrySetCanceled());
    Timer timer = new Timer(_ => tcs.TrySetResult(null));
    timer.Change(milliseconds, -1);            
    return tcs.Task;
}

Reading a bit more into your question. If you need Task.Delay and you're targeting .NET 4.0 then you should use the Microsoft Async nuget package from http://www.nuget.org/packages/Microsoft.Bcl.Async/ it contains the method TaskEx.Delay

Answer 2

Like this:

token.Register(() => tcs.TrySetCancelled());