How to Use CancellationToken in C# for Task Cancellation

Use CancellationToken in C# to signal cancellation requests to running tasks or async methods. Pass a CancellationToken to the method and check its IsCancellationRequested property or call ThrowIfCancellationRequested() to stop work gracefully.

Syntax

The CancellationToken is usually obtained from a CancellationTokenSource. You pass the token to methods that support cancellation. Inside those methods, check if cancellation was requested and stop work if needed.

• CancellationTokenSource: Creates and controls the token.
• CancellationToken: Passed to tasks or async methods to observe cancellation.
• IsCancellationRequested: Property to check if cancellation was requested.
• ThrowIfCancellationRequested(): Throws an exception to stop the operation immediately.

csharp

var cts = new CancellationTokenSource();
CancellationToken token = cts.Token;

// Pass token to a method
SomeMethod(token);

void SomeMethod(CancellationToken cancellationToken)
{
    if (cancellationToken.IsCancellationRequested)
    {
        // Clean up and stop work
        return;
    }
    // Do work
}

Example

This example shows how to start a task that counts numbers and supports cancellation. When the user presses a key, the cancellation is requested and the task stops gracefully.

csharp

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

class Program
{
    static async Task Main()
    {
        var cts = new CancellationTokenSource();
        CancellationToken token = cts.Token;

        Task countingTask = CountNumbersAsync(token);

        Console.WriteLine("Press any key to cancel...");
        Console.ReadKey(true);

        cts.Cancel();

        try
        {
            await countingTask;
        }
        catch (OperationCanceledException)
        {
            Console.WriteLine("Counting was canceled.");
        }
    }

    static async Task CountNumbersAsync(CancellationToken cancellationToken)
    {
        int count = 0;
        while (true)
        {
            cancellationToken.ThrowIfCancellationRequested();
            Console.WriteLine(count++);
            await Task.Delay(500, cancellationToken);
        }
    }
}

Output

Press any key to cancel...
0
1
2
3
... (numbers printed every 0.5 seconds until key press)
Counting was canceled.

Common Pitfalls

Common mistakes when using CancellationToken include:

• Not passing the token to all async methods or tasks that support cancellation.
• Failing to check IsCancellationRequested or call ThrowIfCancellationRequested(), so the task never stops.
• Ignoring OperationCanceledException which is thrown when cancellation is requested.
• Calling Cancel() on the CancellationTokenSource too late or not at all.

Always handle cancellation exceptions and clean up resources properly.

csharp

/* Wrong way: ignoring cancellation token */
async Task DoWorkAsync()
{
    while (true)
    {
        await Task.Delay(1000); // No cancellation token passed
        Console.WriteLine("Working...");
    }
}

/* Right way: pass token and check cancellation */
async Task DoWorkAsync(CancellationToken token)
{
    while (true)
    {
        token.ThrowIfCancellationRequested();
        await Task.Delay(1000, token);
        Console.WriteLine("Working...");
    }
}

Quick Reference

CancellationToken Cheat Sheet:

Term	Description
CancellationTokenSource	Creates and controls cancellation tokens.
CancellationToken	Passed to tasks/methods to observe cancellation.
IsCancellationRequested	True if cancellation was requested.
ThrowIfCancellationRequested()	Throws OperationCanceledException to stop work.
Cancel()	Method on CancellationTokenSource to request cancellation.

Term	Description
CancellationTokenSource	Creates and controls cancellation tokens.
CancellationToken	Passed to tasks/methods to observe cancellation.
IsCancellationRequested	True if cancellation was requested.
ThrowIfCancellationRequested()	Throws OperationCanceledException to stop work.
Cancel()	Method on CancellationTokenSource to request cancellation.

Key Takeaways

Use CancellationTokenSource to create and control cancellation tokens.

Pass CancellationToken to async methods and check for cancellation regularly.

Call ThrowIfCancellationRequested() to stop work immediately when requested.

Handle OperationCanceledException to clean up after cancellation.

Always pass the token to all cancellable async operations to ensure proper cancellation.