CancellationToken

Syntax

CancellationTokenSource cts = new CancellationTokenSource();\nawait SomeAsyncMethod(cts.Token);

Examples

async Task CountAsync(CancellationToken token)
{
    for (int i = 1; i <= 10; i++)
    {
        token.ThrowIfCancellationRequested();
        Console.WriteLine(i);
        await Task.Delay(200, token);
    }
}

CancellationTokenSource cts = new CancellationTokenSource();

Task countTask = CountAsync(cts.Token);
await Task.Delay(500);
cts.Cancel();  // request cancellation after 500ms

try
{
    await countTask;
}
catch (OperationCanceledException)
{
    Console.WriteLine("Counting was cancelled");
}

async Task<string> FetchDataAsync(CancellationToken token)
{
    await Task.Delay(3000, token);  // simulate a slow operation
    return "data";
}

using CancellationTokenSource cts = new CancellationTokenSource();
cts.CancelAfter(TimeSpan.FromSeconds(1));  // auto-cancel after 1 second

try
{
    string result = await FetchDataAsync(cts.Token);
    Console.WriteLine(result);
}
catch (OperationCanceledException)
{
    Console.WriteLine("Request timed out");
}

void ProcessItems(List<int> items, CancellationToken token)
{
    foreach (int item in items)
    {
        if (token.IsCancellationRequested)
        {
            Console.WriteLine("Stopping early due to cancellation");
            return;
        }
        Console.WriteLine($"Processing {item}");
    }
}

Best practices

• Accept a CancellationToken as the last parameter of any async method that might run for a while, so callers can opt into cancellation support
• Pass the token through to every nested async call (Task.Delay(ms, token), HttpClient calls, etc.) so cancellation actually propagates, not just at the top level
• Use CancelAfter() for straightforward timeout scenarios instead of manually managing a separate timer
• Always dispose a CancellationTokenSource (via using) once you are done with it, to release its underlying resources

At a glance

Purpose

Applications on the .NET platform

File extension

.cs

Runs in

.NET runtime

Usually used with

.NET SDK and libraries

Specifications & further reading

• C# language reference

Related C# documentation