The following example shows how to register a delegate that will be invoked when a IsCancellationRequested property becomes true due to a call to Cancel on the object that created the token. Use this technique for cancelling asynchronous operations that do not natively support the unified cancellation framework, and for unblocking methods that might be waiting for an asynchronous operation to finish.
using System;
using System.Net;
using System.Threading;
using System.Threading.Tasks;
class CancelWithCallback
{
static void Main()
{
var cts = new CancellationTokenSource();
var token = cts.Token;
// Start cancelable task.
Task t = Task.Run( () => {
WebClient wc = new WebClient();
// Create an event handler to receive the result.
wc.DownloadStringCompleted += (obj, e) => {
// Check status of WebClient, not external token.
if (!e.Cancelled) {
Console.WriteLine("The download has completed:\n");
Console.WriteLine(e.Result + "\n\nPress any key.");
}
else {
Console.WriteLine("The download was canceled.");
}
};
// Do not initiate download if the external token
// has already been canceled.
if (!token.IsCancellationRequested) {
// Register the callback to a method that can unblock.
using (CancellationTokenRegistration ctr = token.Register(() => wc.CancelAsync()))
{
Console.WriteLine("Starting request\n");
wc.DownloadStringAsync(new Uri("http://www.contoso.com"));
}
}
}, token);
Console.WriteLine("Press 'c' to cancel.\n");
char ch = Console.ReadKey().KeyChar;
Console.WriteLine();
if (ch == 'c')
{
cts.Cancel();
}
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
cts.Dispose();
}
}