How to Safely Cancel an Async Operation in C#

Asynchronous programming is essential for building responsive applications, but it comes with challenges - particularly when you need to cancel operations.

Here's how to safely implement cancellation in C#.

Using CancellationTokenSource

The key to proper cancellation is the CancellationTokenSource class. This provides a token that can be passed to async methods and monitored for cancellation requests.

// Create a cancellation source with timeout
var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
var token = cts.Token;

try
{
    // Pass token to async operations
    await DoLongRunningTaskAsync(token);
}
catch (OperationCanceledException)
{
    // Handle cancellation gracefully
    Console.WriteLine("Operation was canceled");
}
finally
{
    // Always dispose the CancellationTokenSource
    cts.Dispose();
}

Implementing Cancellation in Your Methods

When writing cancellable async methods, check for cancellation at appropriate points:

async Task DoLongRunningTaskAsync(CancellationToken token)
{
    // Check before starting expensive work
    token.ThrowIfCancellationRequested();
    
    for (int i = 0; i < 100; i++)
    {
        // Periodically check during loops
        if (token.IsCancellationRequested)
        {
            // Clean up resources if needed
            CleanupResources();
            
            // Then throw the standard exception
            throw new OperationCanceledException(token);
        }
        
        await Task.Delay(100, token); // Built-in methods accept tokens
    }
}

Best Practices

1. Always dispose of CancellationTokenSource objects
2. Use token.ThrowIfCancellationRequested() for cleaner code
3. Check for cancellation before expensive operations
4. Pass the token to all nested async calls
5. Handle OperationCanceledException appropriately in your calling code

By following these patterns, you can ensure your async operations respond promptly to cancellation requests while maintaining clean, resource-efficient code.