Console.WriteLine causes CancellationToken to be ignored

Question

I was going through the MSDN article on CancellationToken
https://msdn.microsoft.com/en-us/library/dd997364(v=vs.110).aspx
If I add a Console.WriteLine() or a Thread.Sleep in the LongRunningFunc method, the cancellationtoken gets ignored and the method LongRunningFunc continues to execute.
using System;
using System.Net;
using System.Threading;
using System.Threading.Tasks;
class Example
EventHandler externalEvent;
void Example1()
CancellationTokenSource cts = new CancellationTokenSource();
externalEvent +=
(sender, obj) => { cts.Cancel(); }; //wire up an external requester
try {
int val = LongRunningFunc(cts.Token);
catch (OperationCanceledException) {
//cleanup after cancellation if required...
Console.WriteLine("Operation was canceled as expected.");
finally {
cts.Dispose();
private static int LongRunningFunc(CancellationToken token)
Console.WriteLine("Long running method");
int total = 0;
for (int i = 0; i < 100000; i++)
for (int j = 0; j < 100000; j++)
total++;
if (token.IsCancellationRequested)
{ // observe cancellation
Console.WriteLine("Cancellation observed.");
throw new OperationCanceledException(token); // acknowledge cancellation
Console.WriteLine("Done looping");
return total;
static void Main()
Example ex = new Example();
Thread t = new Thread(new ThreadStart(ex.Example1));
t.Start();
Console.WriteLine("Press 'c' to cancel.");
if (Console.ReadKey(true).KeyChar == 'c')
ex.externalEvent.Invoke(ex, new EventArgs());
Console.WriteLine("Press enter to exit.");
Console.ReadLine();
class CancelWaitHandleMiniSnippetsForOverviewTopic
static void CancelByCallback()
CancellationTokenSource cts = new CancellationTokenSource();
CancellationToken token = cts.Token;
WebClient wc = new WebClient();
// To request cancellation on the token
// will call CancelAsync on the WebClient.
token.Register(() => wc.CancelAsync());
Console.WriteLine("Starting request");
wc.DownloadStringAsync(new Uri("http://www.contoso.com"));
I changed the LongRunningFunc to add the bold lines as shown below
private static int LongRunningFunc(CancellationToken token)
Console.WriteLine("Long running method");
int total = 0;
for (int i = 0; i < 100000; i++)
for (int j = 0; j < 100000; j++)
total++;
Console.WriteLine("Total " + total);
Thread.Sleep(1000);
if (token.IsCancellationRequested)
{ // observe cancellation
Console.WriteLine("Cancellation observed.");
throw new OperationCanceledException(token); // acknowledge cancellation
Console.WriteLine("Done looping");
return total;
Why does Console.WriteLine or Thread.Sleep cause the cancellationtoken to be ignored?

Answer 1

The token isn't ignored, it just takes a long time (almost 28 hours) before it is checked. If you put Thread.Sleep in that for loop you should also move the cancellation check inside that loop, before Thread.Sleep. This way the delay in acknowledging cancellation
is at most 1 second.