How to use the CancellationToken property

Question

Compared to the preceding code for class RulyCanceler  I wanted to run code using  CancellationTokenSource   How do I use it as mentioned in Cancellation Tokens  i e  without throwing catching an exception    Can I use the IsCancellationRequested property     I attempted to use it like this     cancelToken ThrowIfCancellationRequested      and  try     new Thread      gt  Work cancelSource Token   Start      catch  OperationCanceledException      Console WriteLine  Canceled         but this gave a run-time error on cancelToken ThrowIfCancellationRequested    in method Work CancellationToken cancelToken      System OperationCanceledException was unhandled   Message The operation was canceled    Source mscorlib   StackTrace         at System Threading CancellationToken ThrowIfCancellationRequested          at  7CancellationTokens Token Work CancellationToken cancelToken  in C  xxx Token cs line 33        at  7CancellationTokens Token  lt  gt c  DisplayClass1  lt Main gt b  0   in C  xxx Token cs line 22        at System Threading ThreadHelper ThreadStart Context Object state         at System Threading ExecutionContext Run ExecutionContext executionContext  ContextCallback callback  Object state  Boolean ignoreSyncCtx         at System Threading ExecutionContext Run ExecutionContext executionContext  ContextCallback callback  Object state         at System Threading ThreadHelper ThreadStart     InnerException    The code that I successfully ran caught the OperationCanceledException in the new thread   using System  using System Threading  namespace  7CancellationTokens     internal class Token         private static void Main               var cancelSource   new CancellationTokenSource          new Thread      gt                   try                       Work cancelSource Token       Start                        catch  OperationCanceledException                         Console WriteLine  Canceled                            Start           Thread Sleep 1000         cancelSource Cancel       Safely cancel worker        Console ReadLine              private static void Work CancellationToken cancelToken              while  true                  Console Write  345            cancelToken ThrowIfCancellationRequested

User · Answer

You can use ThrowIfCancellationRequested without handling the exception   The use of ThrowIfCancellationRequested is meant to be used from within a Task  not a Thread   When used within a Task  you do not have to handle the exception yourself  and get the Unhandled Exception error   It will result in leaving the Task  and the Task IsCancelled property will be True  No exception handling needed   In your specific case  change the Thread to a Task   Task t   null  try       t   Task Run      gt  Work cancelSource Token   cancelSource Token      if  t IsCancelled        Console WriteLine  Canceled

User · Answer

BrainSlugs83  You shouldn t blindly trust everything posted on stackoverflow  The comment in Jens code is incorrect  the parameter doesn t control whether exceptions are thrown or not    MSDN is very clear what that parameter controls  have you read it  http   msdn microsoft com en-us library dd321703 v vs 110  aspx     If throwOnFirstException is true  an exception will immediately   propagate out of the call to Cancel  preventing the remaining   callbacks and cancelable operations from being processed  If   throwOnFirstException is false  this overload will aggregate any   exceptions thrown into an AggregateException  such that one callback   throwing an exception will not prevent other registered callbacks from   being executed    The variable name is also wrong because Cancel is called on CancellationTokenSource not the token itself and the source changes state of each token it manages

User · Answer

You can create a Task with cancellation token  when you app goto background you can cancel this token   You can do this in PCL https   developer xamarin com guides xamarin-forms application-fundamentals app-lifecycle  var cancelToken   new CancellationTokenSource    Task Factory StartNew async      gt        await Task Delay 10000          call web API    cancelToken Token      this stops the Task  cancelToken Cancel false     Anther solution is user Timer in Xamarin Forms  stop timer when app goto background https   xamarinhelp com xamarin-forms-timer

User · Answer

You have to pass the CancellationToken to the Task  which will periodically monitors the token to see whether cancellation is requested     CancellationTokenSource provides the token and have authority to cancel the token CancellationTokenSource  cancellationTokenSource    new  CancellationTokenSource    CancellationToken  token     cancellationTokenSource Token        Task need to be cancelled with CancellationToken  Task  task     Task Run async       gt                while  token IsCancellationRequested          Console Write  quot   quot                           await Task Delay 1000          token    Console WriteLine  quot Press  enter  to  stop  the  task quot     Console ReadLine     cancellationTokenSource Cancel      In this case  the operation will end when cancellation is requested and the Task will have a RanToCompletion state  If you want to be acknowledged that your task has been cancelled  you have to use ThrowIfCancellationRequested to throw an OperationCanceledException exception  Task  task     Task Run async       gt                                                                  while    token IsCancellationRequested             Console Write  quot   quot                                                       await Task Delay 1000                                                                 token ThrowIfCancellationRequested                                   token   ContinueWith t   gt           t Exception  Handle e   gt  true         Console WriteLine  quot You have canceled the task quot       TaskContinuationOptions OnlyOnCanceled       Console WriteLine  quot Press enter to stop the task quot                     Console ReadLine                     cancellationTokenSource Cancel                     task Wait      Hope this helps to understand better

User · Answer

You can implement your work method as follows   private static void Work CancellationToken cancelToken        while  true                if cancelToken IsCancellationRequested                        return                    Console Write  345              That s it  You always need to handle cancellation by yourself - exit from method when it is appropriate time to exit  so that your work and data is in consistent state   UPDATE  I prefer not writing while   cancelToken IsCancellationRequested  because often there are few exit points where you can stop executing safely across loop body  and loop usually have some logical condition to exit  iterate over all items in collection etc    So I believe it s better not to mix that conditions as they have different intention   Cautionary note about avoiding CancellationToken ThrowIfCancellationRequested      Comment in question by Eamon Nerbonne          replacing ThrowIfCancellationRequested with a bunch of checks for IsCancellationRequested exits gracefully  as this answer says  But that s not just an implementation detail  that affects observable behavior  the task will no longer end in the cancelled state  but in RanToCompletion  And that can affect not just explicit state checks  but also  more subtly  task chaining with e g  ContinueWith  depending on the TaskContinuationOptions used  I d say that avoiding ThrowIfCancellationRequested is dangerous advice

[c#] How to use the CancellationToken property?

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