CancellationToken and async requests

Cancellation is a signal the operation must observe. Passing a token to async I/O lets the caller request that work stop.

The idea

The caller owns the CancellationTokenSource and passes its token to the operation. Calling Cancel sends a cooperative signal. HttpClient observes the token and ends the request by throwing OperationCanceledException; the task then completes as cancelled.

Code

using var cts = new CancellationTokenSource();
Task<string> task = GetDataAsync(cts.Token);
cts.Cancel(); // signal cancellation while the request is running
try { await task; } catch (OperationCanceledException) { /* expected here */ }

static async Task<string> GetDataAsync(CancellationToken token) {
    try {
        return await httpClient.GetStringAsync(url, token);
    }
    catch (OperationCanceledException) {
        Console.WriteLine("Request cancelled");
        throw;
    }
}

Tips

• Cancellation does not forcibly stop a thread. The operation must accept and observe the token.
• Pass the same token through each layer of async work, and avoid catching OperationCanceledException as though it were an ordinary success.

In the .NET library

ASP.NET Core supplies a request-aborted token for incoming requests. Passing it to database and HTTP calls can stop unnecessary work when the client disconnects.