How do I abort cancel TPL Tasks

Question

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

User · Answer

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.

User · Answer

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      gt  TryExecuteTask task            TaskThread Start               protected override IEnumerable lt Task gt  GetScheduledTasks     gt  throw new NotSupportedException       Unused     protected override bool TryExecuteTaskInline Task task  bool taskWasPreviouslyQueued    gt  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

User · Answer

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      gt                   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

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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

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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      gt                        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   gt  cancel this task          ts Cancel            Console ReadLine

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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            lt summary gt          Launch task in a new thread      lt  summary gt  void LaunchTaskAsync         taskToken   new CancellationTokenSource        Task Factory StartNew      gt                        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           lt summary gt          Stop running task      lt  summary gt  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

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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 lt string gt  ConnectTab                         string messageText                   for  int count   1  count  lt  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

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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       gt  DoSomeWork 1  token   token    static void DoSomeWork int taskNum  CancellationToken ct           Do work here  checking and acting on ct IsCancellationRequested where applicable

[c#] How do I abort/cancel TPL Tasks?

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