[c#] How to use the CancellationToken property?

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(() => 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.<>c__DisplayClass1.<Main>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(() =>
      {
         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();
      }
    }
  }
}

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 () => {     
  while(!token.IsCancellationRequested) {
      Console.Write("*");         
      await Task.Delay(1000);
  }
}, token);

Console.WriteLine("Press enter to stop the task"); 
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 () =>             
{                 
    while (!token.IsCancellationRequested) {
         Console.Write("*");                      
         await Task.Delay(1000);                
    }           
    token.ThrowIfCancellationRequested();               
}, token)
.ContinueWith(t =>
 {
      t.Exception?.Handle(e => true);
      Console.WriteLine("You have canceled the task");
 },TaskContinuationOptions.OnlyOnCanceled);  
 
Console.WriteLine("Press enter to stop the task");                 
Console.ReadLine();                 
cancellationTokenSource.Cancel();                 
task.Wait(); 

Hope this helps to understand better.

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 () => {
    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/

@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.

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(() => Work(cancelSource.Token), cancelSource.Token);
}

if (t.IsCancelled)
{
    Console.WriteLine("Canceled!");
}