I want to wait for a Task<T> to complete with some special rules: If it hasn't completed after X milliseconds, I want to display a message to the user. And if it hasn't completed after Y milliseconds, I want to automatically request cancellation.

I can use Task.ContinueWith to asynchronously wait for the task to complete (i.e. schedule an action to be executed when the task is complete), but that doesn't allow to specify a timeout. I can use Task.Wait to synchronously wait for the task to complete with a timeout, but that blocks my thread. How can I asynchronously wait for the task to complete with a timeout?

Solution 1

How about this:

int timeout = 1000;
var task = SomeOperationAsync();
if (await Task.WhenAny(task, Task.Delay(timeout)) == task) {
    // task completed within timeout
} else { 
    // timeout logic
}

And here's a great blog post "Crafting a Task.TimeoutAfter Method" (from MS Parallel Library team) with more info on this sort of thing.

Addition: at the request of a comment on my answer, here is an expanded solution that includes cancellation handling. Note that passing cancellation to the task and the timer means that there are multiple ways cancellation can be experienced in your code, and you should be sure to test for and be confident you properly handle all of them. Don't leave to chance various combinations and hope your computer does the right thing at runtime.

int timeout = 1000;
var task = SomeOperationAsync(cancellationToken);
if (await Task.WhenAny(task, Task.Delay(timeout, cancellationToken)) == task)
{
    // Task completed within timeout.
    // Consider that the task may have faulted or been canceled.
    // We re-await the task so that any exceptions/cancellation is rethrown.
    await task;

}
else
{
    // timeout/cancellation logic
}

Solution 2

Here's a extension method version that incorporates cancellation of the timeout when the original task completes as suggested by Andrew Arnott in a comment to his answer.

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("The operation has timed out.");
        }
    }
}

Solution 3

You can use Task.WaitAny to wait the first of multiple tasks.

You could create two additional tasks (that complete after the specified timeouts) and then use WaitAny to wait for whichever completes first. If the task that completed first is your "work" task, then you're done. If the task that completed first is a timeout task, then you can react to the timeout (e.g. request cancellation).

Solution 4

This is a slightly enhanced version of previous answers.

  • In addition to Lawrence's answer, it cancels the original task when timeout occurs.
  • In addtion to sjb's answer variants 2 and 3, you can provide CancellationToken for the original task, and when timeout occurs, you get TimeoutException instead of OperationCanceledException.
async Task<TResult> CancelAfterAsync<TResult>(
    Func<CancellationToken, Task<TResult>> startTask,
    TimeSpan timeout, CancellationToken cancellationToken)
{
    using (var timeoutCancellation = new CancellationTokenSource())
    using (var combinedCancellation = CancellationTokenSource
        .CreateLinkedTokenSource(cancellationToken, timeoutCancellation.Token))
    {
        var originalTask = startTask(combinedCancellation.Token);
        var delayTask = Task.Delay(timeout, timeoutCancellation.Token);
        var completedTask = await Task.WhenAny(originalTask, delayTask);
        // Cancel timeout to stop either task:
        // - Either the original task completed, so we need to cancel the delay task.
        // - Or the timeout expired, so we need to cancel the original task.
        // Canceling will not affect a task, that is already completed.
        timeoutCancellation.Cancel();
        if (completedTask == originalTask)
        {
            // original task completed
            return await originalTask;
        }
        else
        {
            // timeout
            throw new TimeoutException();
        }
    }
}

Usage

InnerCallAsync may take a long time to complete. CallAsync wraps it with a timeout.

async Task<int> CallAsync(CancellationToken cancellationToken)
{
    var timeout = TimeSpan.FromMinutes(1);
    int result = await CancelAfterAsync(ct => InnerCallAsync(ct), timeout,
        cancellationToken);
    return result;
}

async Task<int> InnerCallAsync(CancellationToken cancellationToken)
{
    return 42;
}

Solution 5

From .Net 6 (Preview 7) or later, there is a new build-in method Task.WaitAsync to achieve this.

// Using TimeSpan
await myTask.WaitAsync(TimeSpan.FromSeconds(10));

// Using CancellationToken
await myTask.WaitAsync(cancellationToken);

// Using both TimeSpan and CancellationToken
await myTask.WaitAsync(TimeSpan.FromSeconds(10), cancellationToken);

Solution 6

Using Stephen Cleary's excellent AsyncEx library, you can do:

TimeSpan timeout = TimeSpan.FromSeconds(10);

using (var cts = new CancellationTokenSource(timeout))
{
    await myTask.WaitAsync(cts.Token);
}

TaskCanceledException will be thrown in the event of a timeout.

Solution 7

What about something like this?

    const int x = 3000;
    const int y = 1000;

    static void Main(string[] args)
    {
        // Your scheduler
        TaskScheduler scheduler = TaskScheduler.Default;

        Task nonblockingTask = new Task(() =>
            {
                CancellationTokenSource source = new CancellationTokenSource();

                Task t1 = new Task(() =>
                    {
                        while (true)
                        {
                            // Do something
                            if (source.IsCancellationRequested)
                                break;
                        }
                    }, source.Token);

                t1.Start(scheduler);

                // Wait for task 1
                bool firstTimeout = t1.Wait(x);

                if (!firstTimeout)
                {
                    // If it hasn't finished at first timeout display message
                    Console.WriteLine("Message to user: the operation hasn't completed yet.");

                    bool secondTimeout = t1.Wait(y);

                    if (!secondTimeout)
                    {
                        source.Cancel();
                        Console.WriteLine("Operation stopped!");
                    }
                }
            });

        nonblockingTask.Start();
        Console.WriteLine("Do whatever you want...");
        Console.ReadLine();
    }

You can use the Task.Wait option without blocking main thread using another Task.

Solution 8

Here is a fully worked example based on the top voted answer, which is:

int timeout = 1000;
var task = SomeOperationAsync();
if (await Task.WhenAny(task, Task.Delay(timeout)) == task) {
    // task completed within timeout
} else { 
    // timeout logic
}

The main advantage of the implementation in this answer is that generics have been added, so the function (or task) can return a value. This means that any existing function can be wrapped in a timeout function, e.g.:

Before:

int x = MyFunc();

After:

// Throws a TimeoutException if MyFunc takes more than 1 second
int x = TimeoutAfter(MyFunc, TimeSpan.FromSeconds(1));

This code requires .NET 4.5.

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

namespace TaskTimeout
{
    public static class Program
    {
        /// <summary>
        ///     Demo of how to wrap any function in a timeout.
        /// </summary>
        private static void Main(string[] args)
        {

            // Version without timeout.
            int a = MyFunc();
            Console.Write("Result: {0}\n", a);
            // Version with timeout.
            int b = TimeoutAfter(() => { return MyFunc(); },TimeSpan.FromSeconds(1));
            Console.Write("Result: {0}\n", b);
            // Version with timeout (short version that uses method groups). 
            int c = TimeoutAfter(MyFunc, TimeSpan.FromSeconds(1));
            Console.Write("Result: {0}\n", c);

            // Version that lets you see what happens when a timeout occurs.
            try
            {               
                int d = TimeoutAfter(
                    () =>
                    {
                        Thread.Sleep(TimeSpan.FromSeconds(123));
                        return 42;
                    },
                    TimeSpan.FromSeconds(1));
                Console.Write("Result: {0}\n", d);
            }
            catch (TimeoutException e)
            {
                Console.Write("Exception: {0}\n", e.Message);
            }

            // Version that works on tasks.
            var task = Task.Run(() =>
            {
                Thread.Sleep(TimeSpan.FromSeconds(1));
                return 42;
            });

            // To use async/await, add "await" and remove "GetAwaiter().GetResult()".
            var result = task.TimeoutAfterAsync(TimeSpan.FromSeconds(2)).
                           GetAwaiter().GetResult();

            Console.Write("Result: {0}\n", result);

            Console.Write("[any key to exit]");
            Console.ReadKey();
        }

        public static int MyFunc()
        {
            return 42;
        }

        public static TResult TimeoutAfter<TResult>(
            this Func<TResult> func, TimeSpan timeout)
        {
            var task = Task.Run(func);
            return TimeoutAfterAsync(task, timeout).GetAwaiter().GetResult();
        }

        private static async Task<TResult> TimeoutAfterAsync<TResult>(
            this Task<TResult> task, TimeSpan timeout)
        {
            var result = await Task.WhenAny(task, Task.Delay(timeout));
            if (result == task)
            {
                // Task completed within timeout.
                return task.GetAwaiter().GetResult();
            }
            else
            {
                // Task timed out.
                throw new TimeoutException();
            }
        }
    }
}

Caveats

Having given this answer, its generally not a good practice to have exceptions thrown in your code during normal operation, unless you absolutely have to:

  • Each time an exception is thrown, its an extremely heavyweight operation,
  • Exceptions can slow your code down by a factor of 100 or more if the exceptions are in a tight loop.

Only use this code if you absolutely cannot alter the function you are calling so it times out after a specific TimeSpan.

This answer is really only applicable when dealing with 3rd party library libraries that you simply cannot refactor to include a timeout parameter.

How to write robust code

If you want to write robust code, the general rule is this:

Every single operation that could potentially block indefinitely, must have a timeout.

If you do not observe this rule, your code will eventually hit an operation that fails for some reason, then it will block indefinitely, and your app has just permanently hung.

If there was a reasonable timeout after some time, then your app would hang for some extreme amount of time (e.g. 30 seconds) then it would either display an error and continue on its merry way, or retry.

Solution 9

Use a Timer to handle the message and automatic cancellation. When the Task completes, call Dispose on the timers so that they will never fire. Here is an example; change taskDelay to 500, 1500, or 2500 to see the different cases:

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

namespace ConsoleApplication1
{
    class Program
    {
        private static Task CreateTaskWithTimeout(
            int xDelay, int yDelay, int taskDelay)
        {
            var cts = new CancellationTokenSource();
            var token = cts.Token;
            var task = Task.Factory.StartNew(() =>
            {
                // Do some work, but fail if cancellation was requested
                token.WaitHandle.WaitOne(taskDelay);
                token.ThrowIfCancellationRequested();
                Console.WriteLine("Task complete");
            });
            var messageTimer = new Timer(state =>
            {
                // Display message at first timeout
                Console.WriteLine("X milliseconds elapsed");
            }, null, xDelay, -1);
            var cancelTimer = new Timer(state =>
            {
                // Display message and cancel task at second timeout
                Console.WriteLine("Y milliseconds elapsed");
                cts.Cancel();
            }
                , null, yDelay, -1);
            task.ContinueWith(t =>
            {
                // Dispose the timers when the task completes
                // This will prevent the message from being displayed
                // if the task completes before the timeout
                messageTimer.Dispose();
                cancelTimer.Dispose();
            });
            return task;
        }

        static void Main(string[] args)
        {
            var task = CreateTaskWithTimeout(1000, 2000, 2500);
            // The task has been started and will display a message after
            // one timeout and then cancel itself after the second
            // You can add continuations to the task
            // or wait for the result as needed
            try
            {
                task.Wait();
                Console.WriteLine("Done waiting for task");
            }
            catch (AggregateException ex)
            {
                Console.WriteLine("Error waiting for task:");
                foreach (var e in ex.InnerExceptions)
                {
                    Console.WriteLine(e);
                }
            }
        }
    }
}

Also, the Async CTP provides a TaskEx.Delay method that will wrap the timers in tasks for you. This can give you more control to do things like set the TaskScheduler for the continuation when the Timer fires.

private static Task CreateTaskWithTimeout(
    int xDelay, int yDelay, int taskDelay)
{
    var cts = new CancellationTokenSource();
    var token = cts.Token;
    var task = Task.Factory.StartNew(() =>
    {
        // Do some work, but fail if cancellation was requested
        token.WaitHandle.WaitOne(taskDelay);
        token.ThrowIfCancellationRequested();
        Console.WriteLine("Task complete");
    });

    var timerCts = new CancellationTokenSource();

    var messageTask = TaskEx.Delay(xDelay, timerCts.Token);
    messageTask.ContinueWith(t =>
    {
        // Display message at first timeout
        Console.WriteLine("X milliseconds elapsed");
    }, TaskContinuationOptions.OnlyOnRanToCompletion);

    var cancelTask = TaskEx.Delay(yDelay, timerCts.Token);
    cancelTask.ContinueWith(t =>
    {
        // Display message and cancel task at second timeout
        Console.WriteLine("Y milliseconds elapsed");
        cts.Cancel();
    }, TaskContinuationOptions.OnlyOnRanToCompletion);

    task.ContinueWith(t =>
    {
        timerCts.Cancel();
    });

    return task;
}

Solution 10

With .Net 6 (preview 7 as the date of this answer), it is possible to use the new WaitAsync(TimeSpan, CancellationToken) which answers to this particular need. If you can use .Net6, this version is moreover described to be optimized if we compare to the majority of good solutions proposed in this posts.

(Thanks for all participants because I used your solution for years)

Solution 11

Another way of solving this problem is using Reactive Extensions:

public static Task TimeoutAfter(this Task task, TimeSpan timeout, IScheduler scheduler)
{
        return task.ToObservable().Timeout(timeout, scheduler).ToTask();
}

Test up above using below code in your unit test, it works for me

TestScheduler scheduler = new TestScheduler();
Task task = Task.Run(() =>
                {
                    int i = 0;
                    while (i < 5)
                    {
                        Console.WriteLine(i);
                        i++;
                        Thread.Sleep(1000);
                    }
                })
                .TimeoutAfter(TimeSpan.FromSeconds(5), scheduler)
                .ContinueWith(t => { }, TaskContinuationOptions.OnlyOnFaulted);

scheduler.AdvanceBy(TimeSpan.FromSeconds(6).Ticks);

You may need the following namespace:

using System.Threading.Tasks;
using System.Reactive.Subjects;
using System.Reactive.Linq;
using System.Reactive.Threading.Tasks;
using Microsoft.Reactive.Testing;
using System.Threading;
using System.Reactive.Concurrency;

Solution 12

A generic version of @Kevan's answer above, using Reactive Extensions.

public static Task<T> TimeoutAfter<T>(this Task<T> task, TimeSpan timeout, IScheduler scheduler)
{
    return task.ToObservable().Timeout(timeout, scheduler).ToTask();
}

With optional Scheduler:

public static Task<T> TimeoutAfter<T>(this Task<T> task, TimeSpan timeout, Scheduler scheduler = null)
{
    return scheduler is null 
       ? task.ToObservable().Timeout(timeout).ToTask() 
       : task.ToObservable().Timeout(timeout, scheduler).ToTask();
}

BTW: When a Timeout happens, a timeout exception will be thrown

Solution 13

For the fun of it I made a 'OnTimeout' extension to Task. On timeout Task executes the desired inline lambda Action() and returns true, otherwise false.

public static async Task<bool> OnTimeout<T>(this T t, Action<T> action, int waitms) where T : Task
{
    if (!(await Task.WhenAny(t, Task.Delay(waitms)) == t))
    {
        action(t);
        return true;
    } else {
        return false;
    }
}

The OnTimeout extension returns a bool result that can be assigned to a variable like in this example calling an UDP socket Async:

var t = UdpSocket.ReceiveAsync();

var timeout = await t.OnTimeout(task => {
    Console.WriteLine("No Response");
}, 5000);

The 'task' variable is accessible in the timeout lambda for more processing.

The use of Action receiving an object may inspire to various other extension designs.

Solution 14

I felt the Task.Delay() task and CancellationTokenSource in the other answers a bit much for my use case in a tight-ish networking loop.

And although Joe Hoag's Crafting a Task.TimeoutAfter Method on MSDN blogs was inspiring, I was a little weary of using TimeoutException for flow control for the same reason as above, because timeouts are expected more frequently than not.

So I went with this, which also handles the optimizations mentioned in the blog:

public static async Task<bool> BeforeTimeout(this Task task, int millisecondsTimeout)
{
    if (task.IsCompleted) return true;
    if (millisecondsTimeout == 0) return false;

    if (millisecondsTimeout == Timeout.Infinite)
    {
        await Task.WhenAll(task);
        return true;
    }

    var tcs = new TaskCompletionSource<object>();

    using (var timer = new Timer(state => ((TaskCompletionSource<object>)state).TrySetCanceled(), tcs,
        millisecondsTimeout, Timeout.Infinite))
    {
        return await Task.WhenAny(task, tcs.Task) == task;
    }
}

An example use case is as such:

var receivingTask = conn.ReceiveAsync(ct);

while (!await receivingTask.BeforeTimeout(keepAliveMilliseconds))
{
    // Send keep-alive
}

// Read and do something with data
var data = await receivingTask;

Solution 15

A few variants of Andrew Arnott's answer:

  1. If you want to wait for an existing task and find out whether it completed or timed out, but don't want to cancel it if the timeout occurs:

    public static async Task<bool> TimedOutAsync(this Task task, int timeoutMilliseconds)
    {
        if (timeoutMilliseconds < 0 || (timeoutMilliseconds > 0 && timeoutMilliseconds < 100)) { throw new ArgumentOutOfRangeException(); }
    
        if (timeoutMilliseconds == 0) {
            return !task.IsCompleted; // timed out if not completed
        }
        var cts = new CancellationTokenSource();
        if (await Task.WhenAny( task, Task.Delay(timeoutMilliseconds, cts.Token)) == task) {
            cts.Cancel(); // task completed, get rid of timer
            await task; // test for exceptions or task cancellation
            return false; // did not timeout
        } else {
            return true; // did timeout
        }
    }
    
  2. If you want to start a work task and cancel the work if the timeout occurs:

    public static async Task<T> CancelAfterAsync<T>( this Func<CancellationToken,Task<T>> actionAsync, int timeoutMilliseconds)
    {
        if (timeoutMilliseconds < 0 || (timeoutMilliseconds > 0 && timeoutMilliseconds < 100)) { throw new ArgumentOutOfRangeException(); }
    
        var taskCts = new CancellationTokenSource();
        var timerCts = new CancellationTokenSource();
        Task<T> task = actionAsync(taskCts.Token);
        if (await Task.WhenAny(task, Task.Delay(timeoutMilliseconds, timerCts.Token)) == task) {
            timerCts.Cancel(); // task completed, get rid of timer
        } else {
            taskCts.Cancel(); // timer completed, get rid of task
        }
        return await task; // test for exceptions or task cancellation
    }
    
  3. If you have a task already created that you want to cancel if a timeout occurs:

    public static async Task<T> CancelAfterAsync<T>(this Task<T> task, int timeoutMilliseconds, CancellationTokenSource taskCts)
    {
        if (timeoutMilliseconds < 0 || (timeoutMilliseconds > 0 && timeoutMilliseconds < 100)) { throw new ArgumentOutOfRangeException(); }
    
        var timerCts = new CancellationTokenSource();
        if (await Task.WhenAny(task, Task.Delay(timeoutMilliseconds, timerCts.Token)) == task) {
            timerCts.Cancel(); // task completed, get rid of timer
        } else {
            taskCts.Cancel(); // timer completed, get rid of task
        }
        return await task; // test for exceptions or task cancellation
    }
    

Another comment, these versions will cancel the timer if the timeout does not occur, so multiple calls will not cause timers to pile up.

sjb

Solution 16

So this is ancient, but there's a much better modern solution. Not sure what version of c#/.NET is required, but this is how I do it:


... Other method code not relevant to the question.

// a token source that will timeout at the specified interval, or if cancelled outside of this scope
using var timeoutTokenSource = new CancellationTokenSource(TimeSpan.FromSeconds(5));
using var linkedTokenSource = CancellationTokenSource.CreateLinkedTokenSource(token, timeoutTokenSource.Token);

async Task<MessageResource> FetchAsync()
{
    try
    {
        return await MessageResource.FetchAsync(m.Sid);
    } catch (TaskCanceledException e)
    {
        if (timeoutTokenSource.IsCancellationRequested)
            throw new TimeoutException("Timeout", e);
        throw;
    }
}

return await Task.Run(FetchAsync, linkedTokenSource.Token);

the CancellationTokenSource constructor takes a TimeSpan parameter which will cause that token to cancel after that interval has elapsed. You can then wrap your async (or syncronous, for that matter) code in another call to Task.Run, passing the timeout token.

This assumes you're passing in a cancellation token (the token variable). If you don't have a need to cancel the task separately from the timeout, you can just use timeoutTokenSource directly. Otherwise, you create linkedTokenSource, which will cancel if the timeout ocurrs, or if it's otherwise cancelled.

We then just catch OperationCancelledException and check which token threw the exception, and throw a TimeoutException if a timeout caused this to raise. Otherwise, we rethrow.

Also, I'm using local functions here, which were introduced in C# 7, but you could easily use lambda or actual functions to the same affect. Similarly, c# 8 introduced a simpler syntax for using statements, but those are easy enough to rewrite.

Solution 17

Create a extension to wait for the task or a delay to complete, whichever comes first. Throw an exception if the delay wins.

public static async Task<TResult> WithTimeout<TResult>(this Task<TResult> task, TimeSpan timeout)
{
    if (await Task.WhenAny(task, Task.Delay(timeout)) != task)
        throw new TimeoutException();
    return await task;
}

Solution 18

If you use a BlockingCollection to schedule the task, the producer can run the potentially long running task and the consumer can use the TryTake method which has timeout and cancellation token built in.

Solution 19

I'm recombinging the ideas of some other answers here and this answer on another thread into a Try-style extension method. This has a benefit if you want an extension method, yet avoiding an exception upon timeout.

public static async Task<bool> TryWithTimeoutAfter<TResult>(this Task<TResult> task,
    TimeSpan timeout, Action<TResult> successor)
{

    using var timeoutCancellationTokenSource = new CancellationTokenSource();
    var completedTask = await Task.WhenAny(task, Task.Delay(timeout, timeoutCancellationTokenSource.Token))
                                  .ConfigureAwait(continueOnCapturedContext: false);

    if (completedTask == task)
    {
        timeoutCancellationTokenSource.Cancel();

        // propagate exception rather than AggregateException, if calling task.Result.
        var result = await task.ConfigureAwait(continueOnCapturedContext: false);
        successor(result);
        return true;
    }
    else return false;        
}     

async Task Example(Task<string> task)
{
    string result = null;
    if (await task.TryWithTimeoutAfter(TimeSpan.FromSeconds(1), r => result = r))
    {
        Console.WriteLine(result);
    }
}    

Solution 20

Here is a low level implementation of a WaitAsync method that accepts both a timeout and a CancellationToken, and in case of an exception propagates all the errors of the Task<T>, instead of just the first error:

public static Task<TResult> WaitAsync<TResult>(this Task<TResult> task,
    TimeSpan timeout, CancellationToken cancellationToken = default)
{
    if (task == null) throw new ArgumentNullException(nameof(task));
    if (timeout < TimeSpan.Zero && timeout != Timeout.InfiniteTimeSpan)
        throw new ArgumentOutOfRangeException(nameof(timeout));

    var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
    cts.CancelAfter(timeout);

    return task
        .ContinueWith(_ => { }, cts.Token,
            TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default)
        .ContinueWith(continuation =>
        {
            cts.Dispose();
            if (task.IsCompleted) return task;
            cancellationToken.ThrowIfCancellationRequested();
            if (continuation.IsCanceled) throw new TimeoutException();
            return task;
        }, TaskScheduler.Default).Unwrap();
}

A TimeoutException is thrown if the timeout elapses before the completion of the task.

Honestly propagating all the errors is not really a value-adding feature in this case. The reason is that if you use the WaitAsync like this: await someTask.WaitAsync(timeout), any extra errors will be swallowed by the await operator anyway, which by design propagates only the first exception of the awaited task. And there is not much point in storing the WaitAsync task in a variable and examining it inside the catch block, because you already have the someTask available, and you could examine this instead.