Timeout an async method implemented with TaskCompletionSource

Question

I have a blackbox object that exposes a method to kick of an async operation, and an event fires when the operation is complete. I have wrapped that into an Task<OpResult> BlackBoxOperationAysnc() method using TaskCompletionSource - that works well.

However, in that async wrapper I'd like to manage completing the async call with a timeout error if the event is not received after a given timeout. Currently I manage it with a timer as:

public Task<OpResult> BlackBoxOperationAysnc() {
    var tcs = new TaskCompletionSource<TestResult>();   
    const int timeoutMs = 20000;
    Timer timer = new Timer(_ => tcs.TrySetResult(OpResult.Timeout),
                            null, timeoutMs, Timeout.Infinite);

    EventHandler<EndOpEventArgs> eventHandler = (sender, args) => {
        ...
        tcs.TrySetResult(OpResult.BlarBlar);
    }
    blackBox.EndAsyncOpEvent += eventHandler;
    blackBox.StartAsyncOp();
    return tcs.Task;
}

Is that the only way to manage a timeout? Is there someway without setting up my own timer - I couldn't see anything timeout built into TaskCompletionSource?

Answer 1

You could use CancellationTokenSource with timeout. Use it together with your TaskCompletionSource like this.

E.g.:

public Task<OpResult> BlackBoxOperationAysnc() {
    var tcs = new TaskCompletionSource<TestResult>();

    const int timeoutMs = 20000;
    var ct = new CancellationTokenSource(timeoutMs);
    ct.Token.Register(() => tcs.TrySetCanceled(), useSynchronizationContext: false);

    EventHandler<EndOpEventArgs> eventHandler = (sender, args) => {
        ...
        tcs.TrySetResult(OpResult.BlarBlar);
    }
    blackBox.EndAsyncOpEvent += eventHandler;
    blackBox.StartAsyncOp();
    return tcs.Task;
}

Updated, here's a complete functional example:

using System;
using System.ComponentModel;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication
{
    public class Program
    {
        // .NET 4.5/C# 5.0: convert EAP pattern into TAP pattern with timeout
        public async Task<AsyncCompletedEventArgs> BlackBoxOperationAsync(
            object state,
            CancellationToken token,
            int timeout = Timeout.Infinite)
        {
            var tcs = new TaskCompletionSource<AsyncCompletedEventArgs>();
            using (var cts = CancellationTokenSource.CreateLinkedTokenSource(token))
            {
                // prepare the timeout
                if (timeout != Timeout.Infinite)
                {
                    cts.CancelAfter(timeout);
                }

                // handle completion
                AsyncCompletedEventHandler handler = (sender, args) =>
                {
                    if (args.Cancelled)
                        tcs.TrySetCanceled();
                    else if (args.Error != null)
                        tcs.SetException(args.Error);
                    else
                        tcs.SetResult(args);
                };

                this.BlackBoxOperationCompleted += handler;
                try
                {
                    using (cts.Token.Register(() => tcs.SetCanceled(), useSynchronizationContext: false))
                    {
                        this.StartBlackBoxOperation(null);
                        return await tcs.Task.ConfigureAwait(continueOnCapturedContext: false);
                    }
                }
                finally
                {
                    this.BlackBoxOperationCompleted -= handler;
                }
            }
        }

        // emulate async operation
        AsyncCompletedEventHandler BlackBoxOperationCompleted = delegate { };

        void StartBlackBoxOperation(object state)
        {
            ThreadPool.QueueUserWorkItem(s =>
            {
                Thread.Sleep(1000);
                this.BlackBoxOperationCompleted(this, new AsyncCompletedEventArgs(error: null, cancelled: false, userState: state));
            }, state);
        }

        // test
        static void Main()
        {
            try
            {
                new Program().BlackBoxOperationAsync(null, CancellationToken.None, 1200).Wait();
                Console.WriteLine("Completed.");
                new Program().BlackBoxOperationAsync(null, CancellationToken.None, 900).Wait();
            }
            catch (Exception ex)
            {
                while (ex is AggregateException)
                    ex = ex.InnerException;
                Console.WriteLine(ex.Message);
            }
            Console.ReadLine();
        }
    }
}

A .NET 4.0/C# 4.0 vesion can be found here, it takes advantage of the compiler-generated IEnumerator state machine.

Answer 2

You could use an extension for Task from here (https://stackoverflow.com/a/22078975/2680660) which also uses CancellationTokenSource.

With slight modification:

public static async Task<TResult> TimeoutAfter<TResult>(this Task<TResult> task, TimeSpan timeout)
{
    using (var timeoutCancellationTokenSource = new CancellationTokenSource())
    {
        var completedTask = await Task.WhenAny(task, Task.Delay(timeout, timeoutCancellationTokenSource.Token));
        if (completedTask == task)
        {
            timeoutCancellationTokenSource.Cancel();
            return await task;  // Very important in order to propagate exceptions
        }
        else
        {
            throw new TimeoutException($"{nameof(TimeoutAfter)}: The operation has timed out after {timeout:mm\\:ss}");
        }
    }
}

public Task<OpResult> BlackBoxOperationAysnc()
{
    var tcs = new TaskCompletionSource<TestResult>();   

    EventHandler<EndOpEventArgs> eventHandler = (sender, args) => {
        ...
        tcs.TrySetResult(OpResult.BlarBlar);
    }
    blackBox.EndAsyncOpEvent += eventHandler;
    blackBox.StartAsyncOp();
    return tcs.Task.TimeoutAfter(TimeSpan.FromSeconds(20));
}