how cancel token work in these cases

Question

how cancel token work in these cases

rajesh yadav 291

following code is from https://learn.microsoft.com/en-us/dotnet/csharp/asynchronous-programming/cancel-an-async-task-or-a-list-of-tasks

i could not understand when it will show following two messages from last main code.

Console.WriteLine("Download task has been cancelled.");

Console.WriteLine("Download task completed before cancel request was processed.");

 Task finishedTask = await Task.WhenAny(new[] { cancelTask, sumPageSizesTask });
    if (finishedTask == cancelTask)
    {
        // wait for the cancellation to take place:
        try
        {
            await sumPageSizesTask;
            Console.WriteLine("Download task completed before cancel request was processed.");
        }
        catch (TaskCanceledException)
        {
            Console.WriteLine("Download task has been cancelled.");
        }
    }

    Console.WriteLine("Application ending.");
}

Accepted answer

3 additional answers

Your answer

Answer 1

Danny Nguyen (WICLOUD CORPORATION) 800 Microsoft External Staff

Hi @rajesh yadav , Since others have given great insights, I will offer what I've found regarding this subject.

The two messages depend entirely on which task reaches a terminal state first and whether the cancellation token actually interrupts the download.

"Download task has been cancelled." You pressed Enter before all the downloads finished, and the cancellation token stopped the work mid-way. A download was still in progress when the cancel request hit, so sumPageSizesTask threw a TaskCanceledException and ended in the Canceled state.
"Download task completed before cancel request was processed." You pressed Enter before all the downloads finished, but they were so close to completion that the task finished normally before the cancellation token took effect. This case is rare, because most of the time the HTTP calls will observe the cancellation and throw before completing.

Though through testing, I've found that sumPageSizesTask always complete before cancelTask, therefore finishedTask gets the value of sumPageSizesTask:

Task finishedTask = await Task.WhenAny(new[] { cancelTask, sumPageSizesTask });

Thus, this part is probably going to get skipped over:

if (finishedTask == cancelTask)
{
    // wait for the cancellation to take place:
    try
    {
        await sumPageSizesTask;
        Console.WriteLine("Download task completed before cancel request was processed.");
    }
    catch (TaskCanceledException)
    {
        Console.WriteLine("Download task has been cancelled.");
    }
}

You should also check out these documentations for more information on Task cancellation:CancellationTokenSource.Cancel Method (System.Threading) | Microsoft Learn

Task Cancellation - .NET | Microsoft Learn

Hope this clears things up

SurferOnWww 4,811 Reputation points

2025-08-12T01:29:47.64+00:00

"Download task completed before cancel request was processed." You pressed Enter before all the downloads finished, ...

All the download must be finished. Otherwise, an OperationCanceledException is thrown and Console.WriteLine("Download task completed before cancel request was processed."); is skipped.

Though through testing, I've found that sumPageSizesTask always complete before cancelTask, therefore finishedTask gets the value of sumPageSizesTask:

Although I do not know how you tested, it is not the case in my environment as shown below:
Danny Nguyen (WICLOUD CORPORATION) 800 Reputation points Microsoft External Staff

2025-08-12T02:34:13.2166667+00:00

That's interesting. In my case sumPageSizesTask always wins in testing:

The only case I was able to consistently get cancelTask to win is to press Enter early. In that case sumPageSizesTask status would still be WaitingForActivation.

Maybe this is environment or performance specific.

You're correct on "Download task completed before cancel request was processed." though, thanks for that.
rajesh yadav 291 Reputation points

2025-08-12T10:13:29.02+00:00

Mr danny ,

what i have understood is, it did enter the try cache of

if (finishedTask == cancelTask) {}

but sumPageSizesTask status was still be WaitingForActivation.

so it could not get into the

catch (TaskCanceledException) { Console.WriteLine("Download task has been cancelled."); }

and there was not question of printing.

Console.WriteLine("Download task completed before cancel request was processed.");

so it only printed

Application started.

Press the ENTER key to cancel...

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ENTER key pressed: cancelling downloads.

Application ending.
Danny Nguyen (WICLOUD CORPORATION) 800 Reputation points Microsoft External Staff

2025-08-12T11:00:00.2533333+00:00
@rajesh yadav The if condition if (finishedTask == cancelTask) definitely took place.

In my case:

sumPageSizesTask finished first.

Task finishedTask = await Task.WhenAny(new[] { cancelTask, sumPageSizesTask }); would referenced sumPageSizesTask since it finished first.

The condition in if (finishedTask == cancelTask) would return False.

The catch block inside the it did not run for me.

I can verify this by checking the Id and Status of cancelTask, sumPageSizesTask and `finishedTask'.

I was assuming that sumPageSizesTask always finished first when cancelling mid downloads for everyone, which is not the case according to Surfer.

You can check this by logging the Status and Id of each task right before the If condition block.

Console.WriteLine($"[{DateTime.Now:HH:mm:ss.fff}] cancelTask Status: {cancelTask.Status}, ID: {cancelTask.Id}"); Console.WriteLine($"[{DateTime.Now:HH:mm:ss.fff}] sumPageSizesTask Status: {sumPageSizesTask.Status}, ID: {sumPageSizesTask.Id}");

Answer 2

There are two tasks cancelTask and sumPageSizesTask. If the cancelTask finishes first then s_cts token is canceled s_cts.Cancel(); The if (finishedTask == cancelTask) is true and the try...catch block is executed. The HttpClient.GetAsync or ReadAsByteArrayAsync methods, which are passed the cancellation token, will detect the cancellation request and throw an OperationCanceledException. The catch block executes Console.WriteLine("Download task has been cancelled.");

If sumPageSizesTask finishes first, then the if (finishedTask == cancelTask) is false and Console.WriteLine("Application ending."); is executed.

The message "Download task completed before cancel request was processed." is only displayed if the cancelTask finishes first, but the sumPageSizesTask also finishes on its own before the await sumPageSizesTask; statement is executed. In this specific and rare case, the await on an already-completed task will succeed without throwing an exception, and the code will then print this message.

Answer 3

SurferOnWww 4,811

i could not understand when it will show following two messages from last main code.

Console.WriteLine("Download task completed before cancel request was processed."); will never happen in any case. There is an exception as AgaveJoe stated in his answer. It is virtually impossible to print "Download task completed before cancel request was processed." with the sample code in the Microsoft document by manual operation. See my comment below If you want to test it.

Console.WriteLine("Download task has been cancelled."); occurs only when the Enter key is pressed before completion of SumPageSizesAsync method.

Why don't you use the debugger of Visual Studio? Put the break points as shown below and execute the application by clicking [Debug] => [Start Debugging]:

enter image description here

SurferOnWww 4,811

If you want to print "Download task completed before cancel request was processed", modify the code of SumPageSizesAsync method as shown below and press the Enter key within 3 seconds after "Download Complete" has been printed.

static async Task SumPageSizesAsync()
{
    var stopwatch = Stopwatch.StartNew();

    int total = 0;
    foreach (string url in s_urlList)
    {
        int contentLength = await ProcessUrlAsync(url, s_client, s_cts.Token);
        total += contentLength;
    }

    stopwatch.Stop();

    // Add the following 2 lines:
    Console.WriteLine("\nDownload Complete");
    await Task.Delay(3000);

    Console.WriteLine($"\nTotal bytes returned:  {total:#,#}");
    Console.WriteLine($"Elapsed time:          {stopwatch.Elapsed}\n");
}

The result is:

enter image description here

Answer 4

setting cancel task does not immediately stop the task. during processing the code must check the value of the cancel token and exit. in the sample code, the test is in http request code.

using System;
using System.Threading;
using System.Threading.Tasks;


var tokenSource = new CancellationTokenSource();
var ct = tokenSource.Token;

try
{
    await Task.WhenAll(
		Task.Run(() => 
		{
			for (var i = 0; i < 10; ++i)
			{
				// are we already canceled?
				ct.ThrowIfCancellationRequested();
                // do work
				Thread.Sl eep(1000); // space required to save code
			}
		}),
		Task.Run(() =>
		{
			// random reply between 1 and 11 seconds
			var rnd = new Random();
			var delay = rnd.Next(1, 11);

			Thread.Sleep(delay * 1000); 
			Console.WriteLine("Try canceling task...");
			tokenSource.Cancel();
		})
	);
    Console.WriteLine("Completed");
}
catch (OperationCanceledException)
{
   Console.WriteLine("task has been cancelled.");
}

notice the task checks for cancel before the work (sleep). if the cancel comes during the sleep and i < 9, then the next loop will throw the cancel error. but if the cancel is set when i == 9, then the work happens, and task completes before checking for the cancel.

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how cancel token work in these cases

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