How to: Cancel a Task and Its Children
using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;
public class Example
{
public static void Main()
{
var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;
// Store references to the tasks so that we can wait on them and
// observe their status after cancellation.
Task t;
var tasks = new ConcurrentBag<Task>();
Console.WriteLine("Press any key to begin tasks...");
Console.WriteLine("To terminate the example, press 'c' to cancel and exit...");
Console.ReadKey();
Console.WriteLine();
// Request cancellation of a single task when the token source is canceled.
// Pass the token to the user delegate, and also to the task so it can
// handle the exception correctly.
t = Task.Factory.StartNew( () => DoSomeWork(1, token), token);
Console.WriteLine("Task {0} executing", t.Id);
tasks.Add(t);
// Request cancellation of a task and its children. Note the token is passed
// to (1) the user delegate and (2) as the second argument to StartNew, so
// that the task instance can correctly handle the OperationCanceledException.
t = Task.Factory.StartNew( () => {
// Create some cancelable child tasks.
Task tc;
for (int i = 3; i <= 10; i++) {
// For each child task, pass the same token
// to each user delegate and to StartNew.
tc = Task.Factory.StartNew( iteration => DoSomeWork((int)iteration, token), i, token);
Console.WriteLine("Task {0} executing", tc.Id);
tasks.Add(tc);
// Pass the same token again to do work on the parent task.
// All will be signaled by the call to tokenSource.Cancel below.
DoSomeWork(2, token);
}
},
token);
Console.WriteLine("Task {0} executing", t.Id);
tasks.Add(t);
// Request cancellation from the UI thread.
if (Console.ReadKey().KeyChar == 'c') {
tokenSource.Cancel();
Console.WriteLine("\nTask cancellation requested.");
// Optional: Observe the change in the Status property on the task.
// It is not necessary to wait on tasks that have canceled. However,
// if you do wait, you must enclose the call in a try-catch block to
// catch the TaskCanceledExceptions that are thrown. If you do
// not wait, no exception is thrown if the token that was passed to the
// StartNew method is the same token that requested the cancellation.
}
try {
Task.WaitAll(tasks.ToArray());
}
catch (AggregateException e) {
Console.WriteLine("\nAggregateException thrown with the following inner exceptions:");
// Display information about each exception.
foreach (var v in e.InnerExceptions) {
if (v is TaskCanceledException)
Console.WriteLine(" TaskCanceledException: Task {0}",
((TaskCanceledException) v).Task.Id);
else
Console.WriteLine(" Exception: {0}", v.GetType().Name);
}
Console.WriteLine();
}
// Display status of all tasks.
foreach (var task in tasks)
Console.WriteLine("Task {0} status is now {1}", task.Id, task.Status);
}
static void DoSomeWork(int taskNum, CancellationToken ct)
{
// Was cancellation already requested?
if (ct.IsCancellationRequested == true) {
Console.WriteLine("Task {0} was cancelled before it got started.",
taskNum);
ct.ThrowIfCancellationRequested();
}
int maxIterations = 100;
// NOTE!!! A "TaskCanceledException was unhandled
// by user code" error will be raised here if "Just My Code"
// is enabled on your computer. On Express editions JMC is
// enabled and cannot be disabled. The exception is benign.
// Just press F5 to continue executing your code.
for (int i = 0; i <= maxIterations; i++) {
// Do a bit of work. Not too much.
var sw = new SpinWait();
for (int j = 0; j <= 100; j++)
sw.SpinOnce();
if (ct.IsCancellationRequested) {
Console.WriteLine("Task {0} cancelled", taskNum);
ct.ThrowIfCancellationRequested();
}
}
}
}