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C#: wait async operation to finish for fixed amount of time, and then do some action based on the result [duplicate]

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?

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
}

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.");
}
}
}

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);
If the task isn't finished before the TimeSpan or CancellationToken then it throws TimeoutException or TaskCanceledException respectively
try
{
await myTask.WaitAsync(TimeSpan.FromSeconds(10), cancellationToken);
}
catch (TaskCanceledException)
{
Console.WriteLine("Task didn't get finished before the `CancellationToken`");
}
catch (TimeoutException)
{
Console.WriteLine("Task didn't get finished before the `TimeSpan`");
}

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).

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;
}

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.

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.

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.

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;
}

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)

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;

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

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.

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;
}

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;

A few variants of Andrew Arnott's answer:
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
}
}
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
}
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

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.

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.

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);
}
}

Async/await in .Net --> Calling a service method that has no async implementation

Basically I need to make a remote request using a vendor's .Net SDK for some information. Their SDK has no async implementations on their methods so I am trying to come up with something on my own. I bascially want to fire off this request to a synchronous method, and wait on it for only a certain amount of time. If the request takes too long, I need to act and report that down to the client in our web app.
I'm wondering if this is the best way to do this, or is there a better way? The code below is a service method that is called from a Controller action.
public async Task<bool> SignersAdded(string packageId)
{
var timeout = 5000;
var task = Task.Run(() =>
{
var package = _eslClient.GetPackage(new PackageId(packageId));
return package != null && package.Documents.Values.Any(x => x.Signatures.Any());
});
var stopwatch = Stopwatch.StartNew();
while (!task.IsCompleted)
{
if (stopwatch.ElapsedMilliseconds >= timeout)
return false;
}
return false;
}

Task.Wait has an overload that takes an int which defines timeout.
public Task<bool> SignersAdded(string packageId)
{
var timeout = 5000;
var task = Task.Run(() =>
{
var package = _eslClient.GetPackage(new PackageId(packageId));
return package != null && package.Documents.Values.Any(x => x.Signatures.Any());
});
if(!task.Wait(1000 /*timeout*/))
{
// timeout
return false;
}
return task.Result;
}

Your method doesn't await on anything, so it runs synchronously. Also, your while loop will spin the CPU, blocking the calling code until the task is complete.
A better approach might be this:
var task = Task.Run(/* your lambda */)
var finishedTask = await Task.WhenAny(Task.Delay(timeout), task);
return finishedTask == task;
This way we create a separate delay task for that time and we await until the first task is complete. This will run in a truly asynchronous manner - there is no while loop that will burn CPU cycles.
(The above assumes timeout is in milliseconds. If not, then use an overload to Delay taking a TimeSpan argument instead.)

You are correct: start a task that calls GetPackage. After that you can continue doing other things.
After a while when you need the result you can wait for the task to complete. However you don't have to do Task.Wait. It is much easier to use async / await.
To do this, you have to do three things:
Declare your function async
Instead of void return Task and instead of type TResult return Task<TResult>. You already did that.
Instead of waiting for the task to finish use await
Your function would look much simpler:
public **async** Task<bool> SignersAdded(string packageId)
{
var timeout = TimeSpan.FromSeconds(5);
var task = Task.Run(() =>
{
var package = _eslClient.GetPackage(new PackageId(packageId));
return package != null
&& package.Documents.Values
.Any(x => x.Signatures.Any());
});
// if desired you can do other things here
// once you need the answer start waiting for it and return the result:
return await Task;
}
if you have a function that returns TResult the async version of it returns Task<TResult>.
the return value of await Task<TResult> is TResult
However, if you want to be able to wait with a timeout you can do the following:
var tokenSource = new CancellationTokenSource(TimeSpan.FromSeconds(1);
// cancel after 1 second
try
{
return await task.Run( () => ..., tokenSource.Token);
}
catch (OperationCanceledException exc)
{
// handle timeout
}
finally
{
// do necessary cleanup
}
The disadvantage of making your function async is that all callers also have to be async and all have to return Task or Task<TResult>. There is one exception:
An event handler can be async but may return void
Example:
private async void OnButton1_clicked(object sender, )

Look at the TaskCompletionSource and the CancellationToken class. Samples here: Timeout an async method implemented with TaskCompletionSource or How to cancel a TaskCompletionSource using a timout

C# Task Ignoring Cancellation timeout

I'm trying to write a wrapper for arbitrary code that will cancel (or at least stop waiting for) the code after a given timeout period.
I have the following test and implementation
[Test]
public void Policy_TimeoutExpires_DoStuff_TaskShouldNotContinue()
{
var cts = new CancellationTokenSource();
var fakeService = new Mock<IFakeService>();
IExecutionPolicy policy = new TimeoutPolicy(new ExecutionTimeout(20), new DefaultExecutionPolicy());
Assert.Throws<TimeoutException>(async () => await policy.ExecuteAsync(() => DoStuff(3000, fakeService.Object), cts.Token));
fakeService.Verify(f=>f.DoStuff(),Times.Never);
}
and the "DoStuff" method
private static async Task DoStuff(int sleepTime, IFakeService fakeService)
{
await Task.Delay(sleepTime).ConfigureAwait(false);
var result = await Task.FromResult("bob");
var test = result + "test";
fakeService.DoStuff();
}
And the implementation of IExecutionPolicy.ExecuteAsync
public async Task ExecuteAsync(Action action, CancellationToken token)
{
var cts = new CancellationTokenSource();//TODO: resolve ignoring the token we were given!
var task = _decoratedPolicy.ExecuteAsync(action, cts.Token);
cts.CancelAfter(_timeout);
try
{
await task.ConfigureAwait(false);
}
catch(OperationCanceledException err)
{
throw new TimeoutException("The task did not complete within the TimeoutExecutionPolicy window of" + _timeout + "ms", err);
}
}
What should happen is that that the test method attempts to take >3000ms and the timeout should occur at 20ms, but this is not happening. Why does my code not timeout as expected?
EDIT:
As requested - the decoratedPolicy is as follows
public async Task ExecuteAsync(Action action, CancellationToken token)
{
token.ThrowIfCancellationRequested();
await Task.Factory.StartNew(action.Invoke, token);
}

If I understand correctly, you're trying to support timeout for a method which doesn't supports timeout / cancellation.
Typically this is done with a starting a timer with required timeout value. If the timer fires first, then you can throw exception. With TPL, you can use Task.Delay(_timeout) instead of a timer.
public async Task ExecuteAsync(Action action, CancellationToken token)
{
var task = _decoratedPolicy.ExecuteAsync(action, token);
var completed = await Task.WhenAny(task, Task.Delay(_timeout));
if (completed != task)
{
throw new TimeoutException("The task did not complete within the TimeoutExecutionPolicy window of" + _timeout + "ms");
}
}
Note: This doesn't stops the execution of _decoratedPolicy.ExecuteAsync method rather it ignores it.
If your method do support cancellation(but not in a timely manner) then it is better to cancel the Task after the timeout. You can do it by creating a Linked Token.
public async Task ExecuteAsync(Action action, CancellationToken token)
{
using(var linkedTokenSource = CancellationTokenSource.CreateLinkedTokenSource(token))
{
var task = _decoratedPolicy.ExecuteAsync(action, linkedTokenSource.Token);
var completed = await Task.WhenAny(task, Task.Delay(_timeout));
if (completed != task)
{
linkedTokenSource.Cancel();//Try to cancel the method
throw new TimeoutException("The task did not complete within the TimeoutExecutionPolicy window of" + _timeout + "ms");
}
}
}

Using CancellationToken means that you're doing cooperative cancellation. Setting CancellationTokenSource.CancelAfter will transition the underlying token to a canceled state after the specified amount of time, but if that token isn't monitored by the calling async method, then nothing will happen.
In order for this to actually generate a OperationCanceledException, you need to call cts.Token.ThrowIfCancellationRequested inside _decoratedPolicy.ExecuteAsync.
For example:
// Assuming this is _decoratedPolicy.ExecuteAsync
public async Task ExecuteAsync(Action action, CancellationToken token)
{
// This is what creates and throws the OperationCanceledException
token.ThrowIfCancellationRequested();
// Simulate some work
await Task.Delay(20);
}
Edit:
In order to actually cancel the token, you need to monitor it at all points where work is executed and execution may timeout. If you cannot make that guarantee, then defer to #SriramSakthivel answer where the actual Task is being discarded, rather than being actually canceled.

You are calling Assert.Throws(Action action) and your anonymous async method is casted to async void. The method will be called asynchronously with Fire&Forget semantics without throwing exception.
However the process would likely crash shortly after due to the uncatched exception in an async void method.
You should call ExecuteAsync synchronously:
[Test]
public void Policy_TimeoutExpires_DoStuff_TaskShouldNotContinue()
{
var cts = new CancellationTokenSource();
var fakeService = new Mock<IFakeService>();
IExecutionPolicy policy = new TimeoutPolicy(new ExecutionTimeout(20), new DefaultExecutionPolicy());
Assert.Throws<AggregateException>(() => policy.ExecuteAsync(() => DoStuff(3000, fakeService.Object), cts.Token).Wait());
fakeService.Verify(f=>f.DoStuff(),Times.Never);
}
or use an async test method:
[Test]
public async Task Policy_TimeoutExpires_DoStuff_TaskShouldNotContinue()
{
var cts = new CancellationTokenSource();
var fakeService = new Mock<IFakeService>();
IExecutionPolicy policy = new TimeoutPolicy(new ExecutionTimeout(20), new DefaultExecutionPolicy());
try
{
await policy.ExecuteAsync(() => DoStuff(3000, fakeService.Object), cts.Token);
Assert.Fail("Method did not timeout.");
}
catch (TimeoutException)
{ }
fakeService.Verify(f=>f.DoStuff(),Times.Never);
}

I've decided to answer my own question here, as while each of the listed answers solved something that I needed to do, they did not identify the root cause of this issue. Many, many thanks to: Scott Chamberlain, Yuval Itzchakov,Sriram Sakthivel, Jeff Cyr. All advice gratefully received.
Root cause/Solution:
await Task.Factory.StartNew(action.Invoke, token);
which you see above in my "decorated policy" returns a Task and await waits only for the outer task. Replacing it with
await Task.Run(async () => await action.Invoke());
gets the correct result.
My code was suffering from a combination of Gotcha #4 and Gotcha #5 from an excellent article on C# async gotchas
The entire article (as well as answers posted to this question) has really improved my overall understanding.

Using CancellationToken for timeout in Task.Run does not work [duplicate]

This question already has answers here:
How to cancel a Task in await?
(4 answers)
Closed 3 years ago.
OK, my questions is really simple. Why this code does not throw TaskCancelledException?
static void Main()
{
var v = Task.Run(() =>
{
Thread.Sleep(1000);
return 10;
}, new CancellationTokenSource(500).Token).Result;
Console.WriteLine(v); // this outputs 10 - instead of throwing error.
Console.Read();
}
But this one works
static void Main()
{
var v = Task.Run(() =>
{
Thread.Sleep(1000);
return 10;
}, new CancellationToken(true).Token).Result;
Console.WriteLine(v); // this one throws
Console.Read();
}

Cancellation in Managed Threads:
Cancellation is cooperative and is not forced on the listener. The listener determines how to gracefully terminate in response to a cancellation request.
You didn't write any code inside your Task.Run method to access your CancellationToken and to implement cancellation - so you effectively ignored the request for cancellation and ran to completion.

There is a difference in canceling a running task, and a task scheduled to run.
After the call to the Task.Run method, the task is only scheduled, and probably have not been executed yet.
When you use the Task.Run(..., CancellationToken) family of overloads with cancellation support, the cancellation token is checked when the task is about to run. If the cancellation token has IsCancellationRequested set to true at this time, an exception of the type TaskCanceledException is thrown.
If the task is already running, it is the task's responsibility to call the ThrowIfCancellationRequested method, or just throw the OperationCanceledException.
According to MSDN, it's just a convenience method for the following:
if (token.IsCancellationRequested)
throw new OperationCanceledException(token);
Note the different kind of exception used in this two cases:
catch (TaskCanceledException ex)
{
// Task was canceled before running.
}
catch (OperationCanceledException ex)
{
// Task was canceled while running.
}
Also note that TaskCanceledException derives from OperationCanceledException, so you can just have one catch clause for the OperationCanceledException type:
catch (OperationCanceledException ex)
{
if (ex is TaskCanceledException)
// Task was canceled before running.
// Task was canceled while running.
}

In the first variant of your code, you're not doing anything to manage the cancellation token.
For example, you're not checking whether token.IsCancellationRequested returns true (and then throwing an exception) or calling ThrowIfCancellationRequested() from your CancellationToken object.
Also, the Task.Run overload you used checks whether the token is already canceled or not when the task is about to start and your code states that token will report cancellation after 500 ms.
So, your code is just ignoring the cancellation request and that's why the task ran to completion.
You should do something like this:
void Main()
{
var ct = new CancellationTokenSource(500).Token;
var v =
Task.Run(() =>
{
Thread.Sleep(1000);
ct.ThrowIfCancellationRequested();
return 10;
}, ct).Result;
Console.WriteLine(v); //now a TaskCanceledException is thrown.
Console.Read();
}
or this, without passing the token, as others already noted:
void Main()
{
var ct = new CancellationTokenSource(500).Token;
ct.ThrowIfCancellationRequested();
var v =
Task.Run(() =>
{
Thread.Sleep(1000);
return 10;
}).Result;
Console.WriteLine(v); //now a TaskCanceledException is thrown.
Console.Read();
}
The second variant of your code works, because you're already initializing a token with a Canceled state set to true. Indeed, as stated here:
If canceled is true, both CanBeCanceled and IsCancellationRequested will be true
the cancellation has already been requested and then the exception TaskCanceledException will be immediately thrown, without actually starting the task.

Another implementation using Task.Delay with token instead it Thread.Sleep.
static void Main(string[] args)
{
var task = GetValueWithTimeout(1000);
Console.WriteLine(task.Result);
Console.ReadLine();
}
static async Task<int> GetValueWithTimeout(int milliseconds)
{
CancellationTokenSource cts = new CancellationTokenSource();
CancellationToken token = cts.Token;
cts.CancelAfter(milliseconds);
token.ThrowIfCancellationRequested();
var workerTask = Task.Run(async () =>
{
await Task.Delay(3500, token);
return 10;
}, token);
try
{
return await workerTask;
}
catch (OperationCanceledException )
{
return 0;
}
}

Asynchronous Task.WhenAll with timeout

Is there a way in the new async dotnet 4.5 library to set a timeout on the Task.WhenAll method? I want to fetch several sources, and stop after say 5 seconds, and skip the sources that weren't finished.

You could combine the resulting Task with a Task.Delay() using Task.WhenAny():
await Task.WhenAny(Task.WhenAll(tasks), Task.Delay(timeout));
If you want to harvest completed tasks in case of a timeout:
var completedResults =
tasks
.Where(t => t.Status == TaskStatus.RanToCompletion)
.Select(t => t.Result)
.ToList();

I think a clearer, more robust option that also does exception handling right would be to use Task.WhenAny on each task together with a timeout task, go through all the completed tasks and filter out the timeout ones, and use await Task.WhenAll() instead of Task.Result to gather all the results.
Here's a complete working solution:
static async Task<TResult[]> WhenAll<TResult>(IEnumerable<Task<TResult>> tasks, TimeSpan timeout)
{
var timeoutTask = Task.Delay(timeout).ContinueWith(_ => default(TResult));
var completedTasks =
(await Task.WhenAll(tasks.Select(task => Task.WhenAny(task, timeoutTask)))).
Where(task => task != timeoutTask);
return await Task.WhenAll(completedTasks);
}

Check out the "Early Bailout" and "Task.Delay" sections from Microsoft's Consuming the Task-based Asynchronous Pattern.
Early bailout. An operation represented by t1 can be grouped in a
WhenAny with another task t2, and we can wait on the WhenAny task. t2
could represent a timeout, or cancellation, or some other signal that
will cause the WhenAny task to complete prior to t1 completing.

What you describe seems like a very common demand however I could not find anywhere an example of this. And I searched a lot... I finally created the following:
TimeSpan timeout = TimeSpan.FromSeconds(5.0);
Task<Task>[] tasksOfTasks =
{
Task.WhenAny(SomeTaskAsync("a"), Task.Delay(timeout)),
Task.WhenAny(SomeTaskAsync("b"), Task.Delay(timeout)),
Task.WhenAny(SomeTaskAsync("c"), Task.Delay(timeout))
};
Task[] completedTasks = await Task.WhenAll(tasksOfTasks);
List<MyResult> = completedTasks.OfType<Task<MyResult>>().Select(task => task.Result).ToList();
I assume here a method SomeTaskAsync that returns Task<MyResult>.
From the members of completedTasks, only tasks of type MyResult are our own tasks that managed to beat the clock. Task.Delay returns a different type.
This requires some compromise on typing, but still works beautifully and quite simple.
(The array can of course be built dynamically using a query + ToArray).
Note that this implementation does not require SomeTaskAsync to receive a cancellation token.

In addition to timeout, I also check the cancellation which is useful if you are building a web app.
public static async Task WhenAll(
IEnumerable<Task> tasks,
int millisecondsTimeOut,
CancellationToken cancellationToken)
{
using(Task timeoutTask = Task.Delay(millisecondsTimeOut))
using(Task cancellationMonitorTask = Task.Delay(-1, cancellationToken))
{
Task completedTask = await Task.WhenAny(
Task.WhenAll(tasks),
timeoutTask,
cancellationMonitorTask
);
if (completedTask == timeoutTask)
{
throw new TimeoutException();
}
if (completedTask == cancellationMonitorTask)
{
throw new OperationCanceledException();
}
await completedTask;
}
}

Check out a custom task combinator proposed in http://tutorials.csharp-online.net/Task_Combinators
async static Task<TResult> WithTimeout<TResult>
(this Task<TResult> task, TimeSpan timeout)
{
Task winner = await (Task.WhenAny
(task, Task.Delay (timeout)));
if (winner != task) throw new TimeoutException();
return await task; // Unwrap result/re-throw
}
I have not tried it yet.

void result version of #i3arnon 's answer, along with comments and changing first argument to use extension this.
I've also got a forwarding method specifying timeout as an int using TimeSpan.FromMilliseconds(millisecondsTimeout) to match other Task methods.
public static async Task WhenAll(this IEnumerable<Task> tasks, TimeSpan timeout)
{
// Create a timeout task.
var timeoutTask = Task.Delay(timeout);
// Get the completed tasks made up of...
var completedTasks =
(
// ...all tasks specified
await Task.WhenAll(tasks
// Now finish when its task has finished or the timeout task finishes
.Select(task => Task.WhenAny(task, timeoutTask)))
)
// ...but not the timeout task
.Where(task => task != timeoutTask);
// And wait for the internal WhenAll to complete.
await Task.WhenAll(completedTasks);
}

Seems like the Task.WaitAll overload with the timeout parameter is all you need - if it returns true, then you know they all completed - otherwise, you can filter on IsCompleted.
if (Task.WaitAll(tasks, myTimeout) == false)
{
tasks = tasks.Where(t => t.IsCompleted);
}
...

I came to the following piece of code that does what I needed:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using System.Net.Http;
using System.Json;
using System.Threading;
namespace MyAsync
{
class Program
{
static void Main(string[] args)
{
var cts = new CancellationTokenSource();
Console.WriteLine("Start Main");
List<Task<List<MyObject>>> listoftasks = new List<Task<List<MyObject>>>();
listoftasks.Add(GetGoogle(cts));
listoftasks.Add(GetTwitter(cts));
listoftasks.Add(GetSleep(cts));
listoftasks.Add(GetxSleep(cts));
List<MyObject>[] arrayofanswers = Task.WhenAll(listoftasks).Result;
List<MyObject> answer = new List<MyObject>();
foreach (List<MyObject> answers in arrayofanswers)
{
answer.AddRange(answers);
}
foreach (MyObject o in answer)
{
Console.WriteLine("{0} - {1}", o.name, o.origin);
}
Console.WriteLine("Press <Enter>");
Console.ReadLine();
}
static async Task<List<MyObject>> GetGoogle(CancellationTokenSource cts)
{
try
{
Console.WriteLine("Start GetGoogle");
List<MyObject> l = new List<MyObject>();
var client = new HttpClient();
Task<HttpResponseMessage> awaitable = client.GetAsync("http://ajax.googleapis.com/ajax/services/search/web?v=1.0&q=broersa", cts.Token);
HttpResponseMessage res = await awaitable;
Console.WriteLine("After GetGoogle GetAsync");
dynamic data = JsonValue.Parse(res.Content.ReadAsStringAsync().Result);
Console.WriteLine("After GetGoogle ReadAsStringAsync");
foreach (var r in data.responseData.results)
{
l.Add(new MyObject() { name = r.titleNoFormatting, origin = "google" });
}
return l;
}
catch (TaskCanceledException)
{
return new List<MyObject>();
}
}
static async Task<List<MyObject>> GetTwitter(CancellationTokenSource cts)
{
try
{
Console.WriteLine("Start GetTwitter");
List<MyObject> l = new List<MyObject>();
var client = new HttpClient();
Task<HttpResponseMessage> awaitable = client.GetAsync("http://search.twitter.com/search.json?q=broersa&rpp=5&include_entities=true&result_type=mixed",cts.Token);
HttpResponseMessage res = await awaitable;
Console.WriteLine("After GetTwitter GetAsync");
dynamic data = JsonValue.Parse(res.Content.ReadAsStringAsync().Result);
Console.WriteLine("After GetTwitter ReadAsStringAsync");
foreach (var r in data.results)
{
l.Add(new MyObject() { name = r.text, origin = "twitter" });
}
return l;
}
catch (TaskCanceledException)
{
return new List<MyObject>();
}
}
static async Task<List<MyObject>> GetSleep(CancellationTokenSource cts)
{
try
{
Console.WriteLine("Start GetSleep");
List<MyObject> l = new List<MyObject>();
await Task.Delay(5000,cts.Token);
l.Add(new MyObject() { name = "Slept well", origin = "sleep" });
return l;
}
catch (TaskCanceledException)
{
return new List<MyObject>();
}
}
static async Task<List<MyObject>> GetxSleep(CancellationTokenSource cts)
{
Console.WriteLine("Start GetxSleep");
List<MyObject> l = new List<MyObject>();
await Task.Delay(2000);
cts.Cancel();
l.Add(new MyObject() { name = "Slept short", origin = "xsleep" });
return l;
}
}
}
My explanation is in my blogpost:
http://blog.bekijkhet.com/2012/03/c-async-examples-whenall-whenany.html

In addition to svick's answer, the following works for me when I have to wait for a couple of tasks to complete but have to process something else while I'm waiting:
Task[] TasksToWaitFor = //Your tasks
TimeSpan Timeout = TimeSpan.FromSeconds( 30 );
while( true )
{
await Task.WhenAny( Task.WhenAll( TasksToWaitFor ), Task.Delay( Timeout ) );
if( TasksToWaitFor.All( a => a.IsCompleted ) )
break;
//Do something else here
}

You can use the following code:
var timeoutTime = 10;
var tasksResult = await Task.WhenAll(
listOfTasks.Select(x => Task.WhenAny(
x, Task.Delay(TimeSpan.FromMinutes(timeoutTime)))
)
);
var succeededtasksResponses = tasksResult
.OfType<Task<MyResult>>()
.Select(task => task.Result);
if (succeededtasksResponses.Count() != listOfTasks.Count())
{
// Not all tasks were completed
// Throw error or do whatever you want
}
//You can use the succeededtasksResponses that contains the list of successful responses
How it works:
You need to put in the timeoutTime variable the limit of time for all tasks to be completed. So basically all tasks will wait in maximum the time that you set in timeoutTime. When all the tasks return the result, the timeout will not occur and the tasksResult will be set.
After that we are only getting the completed tasks. The tasks that were not completed will have no results.

I tried to improve on the excellent i3arnon's solution, in order to fix some minor issues, but I ended up with a completely different implementation. The two issues that I tried to solve are:
In case more than one of the tasks fail, propagate the errors of all failed tasks, and not just the error of the first failed task in the list.
Prevent memory leaks in case all tasks complete much faster than the timeout.
Leaking an active Task.Delay might result in a non-negligible amount of leaked memory, in case the WhenAll is called in a loop, and the timeout is large.
On top of that I added a cancellationToken argument, XML documentation that explains what this method is doing, and argument validation. Here it is:
/// <summary>
/// Returns a task that will complete when all of the tasks have completed,
/// or when the timeout has elapsed, or when the token is canceled, whatever
/// comes first. In case the tasks complete first, the task contains the
/// results/exceptions of all the tasks. In case the timeout elapsed first,
/// the task contains the results/exceptions of the completed tasks only.
/// In case the token is canceled first, the task is canceled. To determine
/// whether a timeout has occured, compare the number of the results with
/// the number of the tasks.
/// </summary>
public static Task<TResult[]> WhenAll<TResult>(
Task<TResult>[] tasks,
TimeSpan timeout, CancellationToken cancellationToken = default)
{
if (tasks == null) throw new ArgumentNullException(nameof(tasks));
tasks = tasks.ToArray(); // Defensive copy
if (tasks.Any(t => t == null)) throw new ArgumentException(
$"The {nameof(tasks)} argument included a null value.", nameof(tasks));
if (timeout < TimeSpan.Zero && timeout != Timeout.InfiniteTimeSpan)
throw new ArgumentOutOfRangeException(nameof(timeout));
if (cancellationToken.IsCancellationRequested)
return Task.FromCanceled<TResult[]>(cancellationToken);
var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
cts.CancelAfter(timeout);
var continuationOptions = TaskContinuationOptions.DenyChildAttach |
TaskContinuationOptions.ExecuteSynchronously;
var continuations = tasks.Select(task => task.ContinueWith(_ => { },
cts.Token, continuationOptions, TaskScheduler.Default));
return Task.WhenAll(continuations).ContinueWith(allContinuations =>
{
cts.Dispose();
if (allContinuations.IsCompletedSuccessfully)
return Task.WhenAll(tasks); // No timeout or cancellation occurred
Debug.Assert(allContinuations.IsCanceled);
if (cancellationToken.IsCancellationRequested)
return Task.FromCanceled<TResult[]>(cancellationToken);
// Now we know that timeout has occurred
return Task.WhenAll(tasks.Where(task => task.IsCompleted));
}, default, continuationOptions, TaskScheduler.Default).Unwrap();
}
This WhenAll implementation elides async and await, which is not advisable in general. In this case it is necessary, in order to propagate all the errors in a not nested AggregateException. The intention is to simulate the behavior of the built-in Task.WhenAll method as accurately as possible.
Usage example:
string[] results;
Task<string[]> whenAllTask = WhenAll(tasks, TimeSpan.FromSeconds(15));
try
{
results = await whenAllTask;
}
catch when (whenAllTask.IsFaulted) // It might also be canceled
{
// Log all errors
foreach (var innerEx in whenAllTask.Exception.InnerExceptions)
{
_logger.LogError(innerEx, innerEx.Message);
}
throw; // Propagate the error of the first failed task
}
if (results.Length < tasks.Length) throw new TimeoutException();
return results;
Note: the above API has a design flaw. In case at least one of the tasks has failed or has been canceled, there is no way to determine whether a timeout has occurred. The Exception.InnerExceptions property of the task returned by the WhenAll may contain the exceptions of all tasks, or part of the tasks, and there is no way to say which is which. Unfortunately I can't think of a solution to this problem.