164

In a thread, I create some System.Threading.Task and start each task.

When I do a .Abort() to kill the thread, the tasks are not aborted.

How can I transmit the .Abort() to my tasks ?

Tim Lloyd
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Patrice Pezillier
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  • Does this answer your question? [Is it possible to abort a Task like aborting a Thread (Thread.Abort method)?](https://stackoverflow.com/questions/4359910/is-it-possible-to-abort-a-task-like-aborting-a-thread-thread-abort-method) – Edward Brey Feb 27 '20 at 11:33
  • @EdwardBrey superficially that question asks the same thing, but none of the answers (IMO) are actually relevant. They all seem to talk about Threads, not Tasks. – StayOnTarget Jul 15 '21 at 20:14

13 Answers13

245

You can't. Tasks use background threads from the thread pool. Also canceling threads using the Abort method is not recommended. You may take a look at the following blog post which explains a proper way of canceling tasks using cancellation tokens. Here's an example:

class Program
{
    static void Main()
    {
        var ts = new CancellationTokenSource();
        CancellationToken ct = ts.Token;
        Task.Factory.StartNew(() =>
        {
            while (true)
            {
                // do some heavy work here
                Thread.Sleep(100);
                if (ct.IsCancellationRequested)
                {
                    // another thread decided to cancel
                    Console.WriteLine("task canceled");
                    break;
                }
            }
        }, ct);

        // Simulate waiting 3s for the task to complete
        Thread.Sleep(3000);

        // Can't wait anymore => cancel this task 
        ts.Cancel();
        Console.ReadLine();
    }
}
Darin Dimitrov
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    Nice explanation. I have a question, how does it work when we don't have an anonymous method in the Task.Factory.StartNew ? like Task.Factory.StartNew(() => ProcessMyMethod(),cancellationToken) – Prerak K Sep 25 '13 at 05:04
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    what if there is a blocking call which does not return inside the executing task? – mehmet6parmak Jun 26 '14 at 06:47
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    @mehmet6parmak I think the only thing you can do then is use `Task.Wait(TimeSpan / int)` to give it a (time-based) deadline from the outside. – Mark Oct 20 '14 at 09:38
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    What if I have my custom class to manage the execution of methods inside a new `Task`? Something like: `public int StartNewTask(Action method)`. Inside the `StartNewTask` method I create a new `Task` by: `Task task = new Task(() => method()); task.Start();`. So how can I manage the `CancellationToken`? I would also like to know if like `Thread` I need to implement a logic to check if there're some Tasks that are still hanging and so kill them when `Form.Closing`. With `Threads` I use `Thread.Abort()`. – Cheshire Cat Dec 03 '14 at 15:31
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    Omg, what a bad example! It is a simple boolean condition, of course it is the first what one would try! But i.e. I have a function in a separate Task, which may take much time to be ended, and ideally it shouldn't know anything about a threading or whatever. So how do I cancel the function with your advice? – Hi-Angel Jan 29 '15 at 11:19
  • Does this method work for blocking functions. I am trying to reset a blocking function but this does not seem to help. The blocking function linger after calling- CancellationTokenSource.cancel(); – user2288650 Jan 20 '18 at 20:21
  • "You can't. Tasks use background threads from the thread pool." That's not necessarily true. You can [create a dedicated thread for your task](https://stackoverflow.com/a/60417715/145173) and then abort that thread. – Edward Brey Mar 31 '20 at 11:47
  • @Mark you are wrong, Task.Wait will just abandon the task – Toolkit Jan 13 '21 at 15:20
  • @darin - How would it work in such a scenario. https://stackoverflow.com/questions/tagged/cancellation-token – user204245 Nov 22 '22 at 05:46
  • Link is broken - http://blogs.msdn.com/b/csharpfaq/archive/2010/07/19/parallel-programming-task-cancellation.aspx – user204245 Nov 22 '22 at 05:47
36

Like this post suggests, this can be done in the following way:

int Foo(CancellationToken token)
{
    Thread t = Thread.CurrentThread;
    using (token.Register(t.Abort))
    {
        // compute-bound work here
    }
}

Although it works, it's not recommended to use such approach. If you can control the code that executes in task, you'd better go with proper handling of cancellation.

starteleport
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    +1 for giving a different approach while stating its fallbacks. I didn't know this could be done :) – Joel Jul 24 '14 at 14:39
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    Thanks for solution! We can just pass token to method and cancel tokensource, instead of somehow getting thread instance from method and aborting this instance directly. – Sam Sep 15 '16 at 13:28
  • The task's thread that gets aborted may be a thread-pool thread or the thread of the caller invoking the task. To avoid this, you can [use a TaskScheduler to specify a dedicated thread for the task](https://stackoverflow.com/a/60417715/145173). – Edward Brey Mar 31 '20 at 11:55
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    This is no longer supported in .NET Framework versions as of 5.0+ – Latency Jul 10 '21 at 01:40
  • https://stackoverflow.com/questions/tagged/cancellation-token – user204245 Nov 22 '22 at 05:44
34

Aborting a Task is easily possible if you capture the thread in which the task is running in. Here is an example code to demonstrate this:

void Main()
{
    Thread thread = null;

    Task t = Task.Run(() => 
    {
        //Capture the thread
        thread = Thread.CurrentThread;

        //Simulate work (usually from 3rd party code)
        Thread.Sleep(1000);

        //If you comment out thread.Abort(), then this will be displayed
        Console.WriteLine("Task finished!");
    });

    //This is needed in the example to avoid thread being still NULL
    Thread.Sleep(10);

    //Cancel the task by aborting the thread
    thread.Abort();
}

I used Task.Run() to show the most common use-case for this - using the comfort of Tasks with old single-threaded code, which does not use the CancellationTokenSource class to determine if it should be canceled or not.

Florian Rappl
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    Thanks for this idea. Used this approach to implement a timeout for some external code, which has no `CancellationToken` support... – Christoph Fink May 28 '14 at 16:47
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    AFAIK thread.abort will leave you unknown about your stack, it might be invalid. I have never tried it but I guess starting a thread in seperate app domain that thread.abort will be save! Besides, a whole thread is wasted in your solution just to abort one task. You would not have to use tasks but threads in first place. (downvote) – Martin Meeser Oct 13 '14 at 09:03
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    As I wrote - this solution is a last resort that might be considered under certain circumstances. Of course a `CancellationToken` or even simpler solutions that are race-condition free should be considered. The code above does only illustrate the method, not the area of usage. – Florian Rappl Oct 13 '14 at 20:14
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    I think this approach might have unknown consequences and I would not recommend it in production code. Tasks are not always guaranteed to be executed in a different thread, meaning they can be ran in the same thread as the one that created them if the scheduler decides so (which will mean the main thread will be passed to the `thread` local variable). In your code you might then end up aborting the main thread, which is not what you really want. Perhaps a check if the threads are the same before the abort would be a good think to have, if you insist on aborting – Ivaylo Slavov Nov 21 '15 at 20:36
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    @Martin - As I have researched this question on SO, I see you have down-voted several answers that use Thread.Abort to kill tasks, but you have not provided any alternative solutions. **How can you kill 3rd party code that does not support cancellation and is running in a Task**? – Gordon Bean Oct 07 '16 at 16:53
  • The only solution I can find is to use my own thread pool, and if it does become necessary to abort a stuck thread, replace it manually. It's just not practical to use Tasks where you might have to use Abort, which means that third party code and async also don't mix, as far as I'm concerned. – tekHedd Oct 13 '16 at 23:18
  • @GordonBean Here is a post that gives a nice general purpose way to abort threads safely. https://stackoverflow.com/a/55709530/1812732 – John Henckel Jun 27 '19 at 16:34
20

This sort of thing is one of the logistical reasons why Abort is deprecated. First and foremost, do not use Thread.Abort() to cancel or stop a thread if at all possible. Abort() should only be used to forcefully kill a thread that is not responding to more peaceful requests to stop in a timely fashion.

That being said, you need to provide a shared cancellation indicator that one thread sets and waits while the other thread periodically checks and gracefully exits. .NET 4 includes a structure designed specifically for this purpose, the CancellationToken.

Adam Robinson
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10

I use a mixed approach to cancel a task.

  • Firstly, I'm trying to Cancel it politely with using the Cancellation.
  • If it's still running (e.g. due to a developer's mistake), then misbehave and kill it using an old-school Abort method.

Checkout an example below:

private CancellationTokenSource taskToken;
private AutoResetEvent awaitReplyOnRequestEvent = new AutoResetEvent(false);

void Main()
{
    // Start a task which is doing nothing but sleeps 1s
    LaunchTaskAsync();
    Thread.Sleep(100);
    // Stop the task
    StopTask();
}

/// <summary>
///     Launch task in a new thread
/// </summary>
void LaunchTaskAsync()
{
    taskToken = new CancellationTokenSource();
    Task.Factory.StartNew(() =>
        {
            try
            {   //Capture the thread
                runningTaskThread = Thread.CurrentThread;
                // Run the task
                if (taskToken.IsCancellationRequested || !awaitReplyOnRequestEvent.WaitOne(10000))
                    return;
                Console.WriteLine("Task finished!");
            }
            catch (Exception exc)
            {
                // Handle exception
            }
        }, taskToken.Token);
}

/// <summary>
///     Stop running task
/// </summary>
void StopTask()
{
    // Attempt to cancel the task politely
    if (taskToken != null)
    {
        if (taskToken.IsCancellationRequested)
            return;
        else
            taskToken.Cancel();
    }

    // Notify a waiting thread that an event has occurred
    if (awaitReplyOnRequestEvent != null)
        awaitReplyOnRequestEvent.Set();

    // If 1 sec later the task is still running, kill it cruelly
    if (runningTaskThread != null)
    {
        try
        {
            runningTaskThread.Join(TimeSpan.FromSeconds(1));
        }
        catch (Exception ex)
        {
            runningTaskThread.Abort();
        }
    }
}
Alex Klaus
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9

To answer Prerak K's question about how to use CancellationTokens when not using an anonymous method in Task.Factory.StartNew(), you pass the CancellationToken as a parameter into the method you're starting with StartNew(), as shown in the MSDN example here.

e.g.

var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;

Task.Factory.StartNew( () => DoSomeWork(1, token), token);

static void DoSomeWork(int taskNum, CancellationToken ct)
{
    // Do work here, checking and acting on ct.IsCancellationRequested where applicable, 

}
Jools
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8

You should not try to do this directly. Design your tasks to work with a CancellationToken, and cancel them this way.

In addition, I would recommend changing your main thread to function via a CancellationToken as well. Calling Thread.Abort() is a bad idea - it can lead to various problems that are very difficult to diagnose. Instead, that thread can use the same Cancellation that your tasks use - and the same CancellationTokenSource can be used to trigger the cancellation of all of your tasks and your main thread.

This will lead to a far simpler, and safer, design.

Reed Copsey
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4

Tasks have first class support for cancellation via cancellation tokens. Create your tasks with cancellation tokens, and cancel the tasks via these explicitly.

Tim Lloyd
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4

You can use a CancellationToken to control whether the task gets cancelled. Are you talking about aborting it before it's started ("nevermind, I already did this"), or actually interrupting it in middle? If the former, the CancellationToken can be helpful; if the latter, you will probably need to implement your own "bail out" mechanism and check at appropriate points in the task execution whether you should fail fast (you can still use the CancellationToken to help you, but it's a little more manual).

MSDN has an article about cancelling Tasks: http://msdn.microsoft.com/en-us/library/dd997396.aspx

Hank
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3

Task are being executed on the ThreadPool (at least, if you are using the default factory), so aborting the thread cannot affect the tasks. For aborting tasks, see Task Cancellation on msdn.

Tim Lloyd
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Oliver Hanappi
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1

I tried CancellationTokenSource but i can't do this. And i did do this with my own way. And it works.

namespace Blokick.Provider
{
    public class SignalRConnectProvider
    {
        public SignalRConnectProvider()
        {
        }

        public bool IsStopRequested { get; set; } = false; //1-)This is important and default `false`.

        public async Task<string> ConnectTab()
        {
            string messageText = "";
            for (int count = 1; count < 20; count++)
            {
                if (count == 1)
                {
                //Do stuff.
                }

                try
                {
                //Do stuff.
                }
                catch (Exception ex)
                {
                //Do stuff.
                }
                if (IsStopRequested) //3-)This is important. The control of the task stopping request. Must be true and in inside.
                {
                    return messageText = "Task stopped."; //4-) And so return and exit the code and task.
                }
                if (Connected)
                {
                //Do stuff.
                }
                if (count == 19)
                {
                //Do stuff.
                }
            }
            return messageText;
        }
    }
}

And another class of the calling the method:

namespace Blokick.Views
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class MessagePerson : ContentPage
    {
        SignalRConnectProvider signalR = new SignalRConnectProvider();

        public MessagePerson()
        {
            InitializeComponent();

            signalR.IsStopRequested = true; // 2-) And this. Make true if running the task and go inside if statement of the IsStopRequested property.

            if (signalR.ChatHubProxy != null)
            {
                 signalR.Disconnect();
            }

            LoadSignalRMessage();
        }
    }
}
Hasan Tuna Oruç
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1

You can abort a task like a thread if you can cause the task to be created on its own thread and call Abort on its Thread object. By default, a task runs on a thread pool thread or the calling thread - neither of which you typically want to abort.

To ensure the task gets its own thread, create a custom scheduler derived from TaskScheduler. In your implementation of QueueTask, create a new thread and use it to execute the task. Later, you can abort the thread, which will cause the task to complete in a faulted state with a ThreadAbortException.

Use this task scheduler:

class SingleThreadTaskScheduler : TaskScheduler
{
    public Thread TaskThread { get; private set; }

    protected override void QueueTask(Task task)
    {
        TaskThread = new Thread(() => TryExecuteTask(task));
        TaskThread.Start();
    }

    protected override IEnumerable<Task> GetScheduledTasks() => throw new NotSupportedException(); // Unused
    protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued) => throw new NotSupportedException(); // Unused
}

Start your task like this:

var scheduler = new SingleThreadTaskScheduler();
var task = Task.Factory.StartNew(action, cancellationToken, TaskCreationOptions.LongRunning, scheduler);

Later, you can abort with:

scheduler.TaskThread.Abort();

Note that the caveat about aborting a thread still applies:

The Thread.Abort method should be used with caution. Particularly when you call it to abort a thread other than the current thread, you do not know what code has executed or failed to execute when the ThreadAbortException is thrown, nor can you be certain of the state of your application or any application and user state that it is responsible for preserving. For example, calling Thread.Abort may prevent static constructors from executing or prevent the release of unmanaged resources.

Edward Brey
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  • This code fails with a runtime exception: *System.InvalidOperationException: RunSynchronously may not be called on a task that was already started.* – Theodor Zoulias Mar 30 '20 at 16:00
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    @TheodorZoulias Good catch. Thanks. I fixed the code and generally improved the answer. – Edward Brey Mar 31 '20 at 00:18
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    Yeap, this fixed the bug. Another caveat that it should be probably mentioned is that `Thread.Abort` is not supported on .NET Core. Attempting to use it there results to an exception: *System.PlatformNotSupportedException: Thread abort is not supported on this platform.* A third caveat is that the `SingleThreadTaskScheduler` can not be used effectively with promise-style tasks, in other words with tasks created with `async` delegates. For example an embedded `await Task.Delay(1000)` runs in no thread, so it is unaffected be thread events. – Theodor Zoulias Mar 31 '20 at 04:35
  • 'SingleThreadTaskScheduler' does not implement inherited abstract member 'TaskScheduler.TryExecuteTaskInline(Task, bool) – Toolkit Jan 13 '21 at 15:22
  • @Toolkit Good catch. The last method had the wrong name. It was supposed to be `TryExecuteTaskInline`. I updated the code snippet. – Edward Brey Jan 13 '21 at 17:00
  • @EdwardBrey thank you, still `Abort()` doesn't really abort anything, it works as CancellationToken, can't break long operation – Toolkit Jan 13 '21 at 18:57
  • @Toolkit `Abort()` here is [Thread.Abort](https://learn.microsoft.com/en-us/dotnet/api/system.threading.thread.abort), which "Raises a ThreadAbortException in the thread on which it is invoked". It does so immediately, even amid a long operation. That's why you see all the caveats in the remarks. It is unrelated to `CancellationToken`. – Edward Brey Jan 13 '21 at 23:59
  • This is no longer supported in .NET 5.0+ You will want to look at alternatives to this. – Latency Jul 09 '21 at 23:24
-1

You can use this class..: It works for all typs of returned Values..

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace CarNUChargeTester
{
    public class TimeOutTaskRunner<T>
    {
        private Func<T> func;
        private int sec;
        private T result;
        public TimeOutTaskRunner(Func<T> func, int sec)
        {
            this.func = func;
            this.sec = sec;
        }

        public bool run()
        {
            var scheduler = new SingleThreadTaskScheduler();
            Task<T> task = Task<T>.Factory.StartNew(func, (new CancellationTokenSource()).Token, TaskCreationOptions.LongRunning, scheduler);
            if (!task.Wait(TimeSpan.FromSeconds(sec)))
            {
                scheduler.TaskThread.Abort();
                return false;
            }
            result = task.Result;
            return true;
        }
        public T getResult() { return result; }
    }
    class SingleThreadTaskScheduler : TaskScheduler
    {
        public Thread TaskThread { get; private set; }

        protected override void QueueTask(Task task)
        {
            TaskThread = new Thread(() => TryExecuteTask(task));
            TaskThread.Start();
        }

        protected override IEnumerable<Task> GetScheduledTasks() => throw new NotSupportedException();
        protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued) => throw new NotSupportedException();
    }
}

To use it you can write:

TimeOutTaskRunner<string> tr = new TimeOutTaskRunner<string>(f, 10); // 10 sec to run f
                if (!tr.run())
                    errorMsg("TimeOut"); !! My func
                tr.getResult() // get the results if it done without timeout..
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    As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community Nov 26 '22 at 00:47