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 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
ThrowIfCancellationRequestedwith a bunch of checks forIsCancellationRequestedexits 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 inRanToCompletion. And that can affect not just explicit state checks, but also, more subtly, task chaining with e.g.ContinueWith, depending on theTaskContinuationOptionsused. I'd say that avoidingThrowIfCancellationRequestedis dangerous advice.
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/
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 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!");
}