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Task doesn't stop [duplicate]

In a thread, I create some System.Threading.Task and start each task.
When I do a .Abort() to kill the thread, the tasks are not aborted.
How can I transmit the .Abort() to my tasks ?

You can't. Tasks use background threads from the thread pool. Also canceling threads using the Abort method is not recommended. You may take a look at the following blog post which explains a proper way of canceling tasks using cancellation tokens. Here's an example:
class Program
{
static void Main()
{
var ts = new CancellationTokenSource();
CancellationToken ct = ts.Token;
Task.Factory.StartNew(() =>
{
while (true)
{
// do some heavy work here
Thread.Sleep(100);
if (ct.IsCancellationRequested)
{
// another thread decided to cancel
Console.WriteLine("task canceled");
break;
}
}
}, ct);
// Simulate waiting 3s for the task to complete
Thread.Sleep(3000);
// Can't wait anymore => cancel this task
ts.Cancel();
Console.ReadLine();
}
}

Like this post suggests, this can be done in the following way:
int Foo(CancellationToken token)
{
Thread t = Thread.CurrentThread;
using (token.Register(t.Abort))
{
// compute-bound work here
}
}
Although it works, it's not recommended to use such approach. If you can control the code that executes in task, you'd better go with proper handling of cancellation.

Aborting a Task is easily possible if you capture the thread in which the task is running in. Here is an example code to demonstrate this:
void Main()
{
Thread thread = null;
Task t = Task.Run(() =>
{
//Capture the thread
thread = Thread.CurrentThread;
//Simulate work (usually from 3rd party code)
Thread.Sleep(1000);
//If you comment out thread.Abort(), then this will be displayed
Console.WriteLine("Task finished!");
});
//This is needed in the example to avoid thread being still NULL
Thread.Sleep(10);
//Cancel the task by aborting the thread
thread.Abort();
}
I used Task.Run() to show the most common use-case for this - using the comfort of Tasks with old single-threaded code, which does not use the CancellationTokenSource class to determine if it should be canceled or not.

This sort of thing is one of the logistical reasons why Abort is deprecated. First and foremost, do not use Thread.Abort() to cancel or stop a thread if at all possible. Abort() should only be used to forcefully kill a thread that is not responding to more peaceful requests to stop in a timely fashion.
That being said, you need to provide a shared cancellation indicator that one thread sets and waits while the other thread periodically checks and gracefully exits. .NET 4 includes a structure designed specifically for this purpose, the CancellationToken.

I use a mixed approach to cancel a task.
Firstly, I'm trying to Cancel it politely with using the Cancellation.
If it's still running (e.g. due to a developer's mistake), then misbehave and kill it using an old-school Abort method.
Checkout an example below:
private CancellationTokenSource taskToken;
private AutoResetEvent awaitReplyOnRequestEvent = new AutoResetEvent(false);
void Main()
{
// Start a task which is doing nothing but sleeps 1s
LaunchTaskAsync();
Thread.Sleep(100);
// Stop the task
StopTask();
}
/// <summary>
/// Launch task in a new thread
/// </summary>
void LaunchTaskAsync()
{
taskToken = new CancellationTokenSource();
Task.Factory.StartNew(() =>
{
try
{ //Capture the thread
runningTaskThread = Thread.CurrentThread;
// Run the task
if (taskToken.IsCancellationRequested || !awaitReplyOnRequestEvent.WaitOne(10000))
return;
Console.WriteLine("Task finished!");
}
catch (Exception exc)
{
// Handle exception
}
}, taskToken.Token);
}
/// <summary>
/// Stop running task
/// </summary>
void StopTask()
{
// Attempt to cancel the task politely
if (taskToken != null)
{
if (taskToken.IsCancellationRequested)
return;
else
taskToken.Cancel();
}
// Notify a waiting thread that an event has occurred
if (awaitReplyOnRequestEvent != null)
awaitReplyOnRequestEvent.Set();
// If 1 sec later the task is still running, kill it cruelly
if (runningTaskThread != null)
{
try
{
runningTaskThread.Join(TimeSpan.FromSeconds(1));
}
catch (Exception ex)
{
runningTaskThread.Abort();
}
}
}

To answer Prerak K's question about how to use CancellationTokens when not using an anonymous method in Task.Factory.StartNew(), you pass the CancellationToken as a parameter into the method you're starting with StartNew(), as shown in the MSDN example here.
e.g.
var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;
Task.Factory.StartNew( () => DoSomeWork(1, token), token);
static void DoSomeWork(int taskNum, CancellationToken ct)
{
// Do work here, checking and acting on ct.IsCancellationRequested where applicable,
}

You should not try to do this directly. Design your tasks to work with a CancellationToken, and cancel them this way.
In addition, I would recommend changing your main thread to function via a CancellationToken as well. Calling Thread.Abort() is a bad idea - it can lead to various problems that are very difficult to diagnose. Instead, that thread can use the same Cancellation that your tasks use - and the same CancellationTokenSource can be used to trigger the cancellation of all of your tasks and your main thread.
This will lead to a far simpler, and safer, design.

Tasks have first class support for cancellation via cancellation tokens. Create your tasks with cancellation tokens, and cancel the tasks via these explicitly.

You can use a CancellationToken to control whether the task gets cancelled. Are you talking about aborting it before it's started ("nevermind, I already did this"), or actually interrupting it in middle? If the former, the CancellationToken can be helpful; if the latter, you will probably need to implement your own "bail out" mechanism and check at appropriate points in the task execution whether you should fail fast (you can still use the CancellationToken to help you, but it's a little more manual).
MSDN has an article about cancelling Tasks:
http://msdn.microsoft.com/en-us/library/dd997396.aspx

Task are being executed on the ThreadPool (at least, if you are using the default factory), so aborting the thread cannot affect the tasks. For aborting tasks, see Task Cancellation on msdn.

I tried CancellationTokenSource but i can't do this. And i did do this with my own way. And it works.
namespace Blokick.Provider
{
public class SignalRConnectProvider
{
public SignalRConnectProvider()
{
}
public bool IsStopRequested { get; set; } = false; //1-)This is important and default `false`.
public async Task<string> ConnectTab()
{
string messageText = "";
for (int count = 1; count < 20; count++)
{
if (count == 1)
{
//Do stuff.
}
try
{
//Do stuff.
}
catch (Exception ex)
{
//Do stuff.
}
if (IsStopRequested) //3-)This is important. The control of the task stopping request. Must be true and in inside.
{
return messageText = "Task stopped."; //4-) And so return and exit the code and task.
}
if (Connected)
{
//Do stuff.
}
if (count == 19)
{
//Do stuff.
}
}
return messageText;
}
}
}
And another class of the calling the method:
namespace Blokick.Views
{
[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class MessagePerson : ContentPage
{
SignalRConnectProvider signalR = new SignalRConnectProvider();
public MessagePerson()
{
InitializeComponent();
signalR.IsStopRequested = true; // 2-) And this. Make true if running the task and go inside if statement of the IsStopRequested property.
if (signalR.ChatHubProxy != null)
{
signalR.Disconnect();
}
LoadSignalRMessage();
}
}
}

You can abort a task like a thread if you can cause the task to be created on its own thread and call Abort on its Thread object. By default, a task runs on a thread pool thread or the calling thread - neither of which you typically want to abort.
To ensure the task gets its own thread, create a custom scheduler derived from TaskScheduler. In your implementation of QueueTask, create a new thread and use it to execute the task. Later, you can abort the thread, which will cause the task to complete in a faulted state with a ThreadAbortException.
Use this task scheduler:
class SingleThreadTaskScheduler : TaskScheduler
{
public Thread TaskThread { get; private set; }
protected override void QueueTask(Task task)
{
TaskThread = new Thread(() => TryExecuteTask(task));
TaskThread.Start();
}
protected override IEnumerable<Task> GetScheduledTasks() => throw new NotSupportedException(); // Unused
protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued) => throw new NotSupportedException(); // Unused
}
Start your task like this:
var scheduler = new SingleThreadTaskScheduler();
var task = Task.Factory.StartNew(action, cancellationToken, TaskCreationOptions.LongRunning, scheduler);
Later, you can abort with:
scheduler.TaskThread.Abort();
Note that the caveat about aborting a thread still applies:
The Thread.Abort method should be used with caution. Particularly when you call it to abort a thread other than the current thread, you do not know what code has executed or failed to execute when the ThreadAbortException is thrown, nor can you be certain of the state of your application or any application and user state that it is responsible for preserving. For example, calling Thread.Abort may prevent static constructors from executing or prevent the release of unmanaged resources.

You can use this class..:
It works for all typs of returned Values..
using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
namespace CarNUChargeTester
{
public class TimeOutTaskRunner<T>
{
private Func<T> func;
private int sec;
private T result;
public TimeOutTaskRunner(Func<T> func, int sec)
{
this.func = func;
this.sec = sec;
}
public bool run()
{
var scheduler = new SingleThreadTaskScheduler();
Task<T> task = Task<T>.Factory.StartNew(func, (new CancellationTokenSource()).Token, TaskCreationOptions.LongRunning, scheduler);
if (!task.Wait(TimeSpan.FromSeconds(sec)))
{
scheduler.TaskThread.Abort();
return false;
}
result = task.Result;
return true;
}
public T getResult() { return result; }
}
class SingleThreadTaskScheduler : TaskScheduler
{
public Thread TaskThread { get; private set; }
protected override void QueueTask(Task task)
{
TaskThread = new Thread(() => TryExecuteTask(task));
TaskThread.Start();
}
protected override IEnumerable<Task> GetScheduledTasks() => throw new NotSupportedException();
protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued) => throw new NotSupportedException();
}
}
To use it you can write:
TimeOutTaskRunner<string> tr = new TimeOutTaskRunner<string>(f, 10); // 10 sec to run f
if (!tr.run())
errorMsg("TimeOut"); !! My func
tr.getResult() // get the results if it done without timeout..

How Do I Create a Looping Service inside an C# Async/Await application?

I have written a class with a method that runs as a long-running Task in the thread pool. The method is a monitoring service to periodically make a REST request to check on the status of another system. It's just a while() loop with a try()catch() inside so that it can handle its own exceptions and and gracefully continuing if something unexpected happens.
Here's an example:
public void LaunchMonitorThread()
{
Task.Run(() =>
{
while (true)
{
try
{
//Check system status
Thread.Sleep(5000);
}
catch (Exception e)
{
Console.WriteLine("An error occurred. Resuming on next loop...");
}
}
});
}
It works fine, but I want to know if there's another pattern I could use that would allow the Monitor method to run as regular part of a standard Async/Await application, instead of launching it with Task.Run() -- basically I'm trying to avoid fire-and-forget pattern.
So I tried refactoring the code to this:
public async Task LaunchMonitorThread()
{
while (true)
{
try
{
//Check system status
//Use task.delay instead of thread.sleep:
await Task.Delay(5000);
}
catch (Exception e)
{
Console.WriteLine("An error occurred. Resuming on next loop...");
}
}
}
But when I try to call the method in another async method, I get the fun compiler warning:
"Because this call is not awaited, execution of the current method continues before the call is completed."
Now I think this is correct and what I want. But I have doubts because I'm new to async/await. Is this code going to run the way I expect or is it going to DEADLOCK or do something else fatal?

What you are really looking for is the use of a Timer. Use the one in the System.Threading namespace. There is no need to use Task or any other variation thereof (for the code sample you have shown).
private System.Threading.Timer timer;
void StartTimer()
{
timer = new System.Threading.Timer(TimerExecution, null, TimeSpan.FromSeconds(5), TimeSpan.FromSeconds(5));
}
void TimerExecution(object state)
{
try
{
//Check system status
}
catch (Exception e)
{
Console.WriteLine("An error occurred. Resuming on next loop...");
}
}
From the documentation
Provides a mechanism for executing a method on a thread pool thread at specified intervals
You could also use System.Timers.Timer but you might not need it. For a comparison between the 2 Timers see also System.Timers.Timer vs System.Threading.Timer.

If you need fire-and-forget operation, it is fine. I'd suggest to improve it with CancellationToken
public async Task LaunchMonitorThread(CancellationToken token)
{
while (!token.IsCancellationRequested)
{
try
{
//Check system status
//Use task.delay instead of thread.sleep:
await Task.Delay(5000, token);
}
catch (Exception e)
{
Console.WriteLine("An error occurred. Resuming on next loop...");
}
}
}
besides that, you can use it like
var cancellationToken = new CancellationToken();
var monitorTask = LaunchMonitorThread(cancellationToken);
and save task and/or cancellationToken to interrupt monitor wherever you want

The method Task.Run that you use to fire is perfect to start long-running async functions from a non-async method.
You are right: the forget part is not correct. If for instance your process is going to close, it would be neater if you kindly asked the started thread to finish its task.
The proper way to do this would be to use a CancellationTokenSource. If you order the CancellationTokenSource to Cancel, then all procedures that were started using Tokens from this CancellationTokenSource will stop neatly within reasonable time.
So let's create a class LongRunningTask, that will create a long running Task upon construction and Cancel this task using the CancellationTokenSource upon Dispose().
As both the CancellationTokenSource as the Task implement IDisposable the neat way would be to Dispose these two when the LongRunningTask object is disposed
class LongRunningTask : IDisposable
{
public LongRunningTask(Action<CancellationToken> action)
{ // Starts a Task that will perform the action
this.cancellationTokenSource = new CancellationTokenSource();
this.longRunningTask = Task.Run( () => action (this.cancellationTokenSource.Token));
}
private readonly CancellationTokenSource cancellationTokenSource;
private readonly Task longRunningTask;
private bool isDisposed = false;
public async Task CancelAsync()
{ // cancel the task and wait until the task is completed:
if (this.isDisposed) throw new ObjectDisposedException();
this.cancellationTokenSource.Cancel();
await this.longRunningTask;
}
// for completeness a non-async version:
public void Cancel()
{ // cancel the task and wait until the task is completed:
if (this.isDisposed) throw new ObjectDisposedException();
this.cancellationTokenSource.Cancel();
this.longRunningTask.Wait;
}
}
Add a standard Dispose Pattern
public void Dispose()
{
this.Dispose(true);
GC.SuppressFinalize(this);
}
protected void Dispose(bool disposing)
{
if (disposing && !this.isDisposed)
{ // cancel the task, and wait until task completed:
this.Cancel();
this.IsDisposed = true;
}
}
Usage:
var longRunningTask = new LongRunningTask( (token) => MyFunction(token)
...
// when application closes:
await longRunningTask.CancelAsync(); // not necessary but the neat way to do
longRunningTask.Dispose();
The Action {...} has a CancellationToken as input parameter, your function should regularly check it
async Task MyFunction(CancellationToken token)
{
while (!token.IsCancellationrequested)
{
// do what you have to do, make sure to regularly (every second?) check the token
// when calling other tasks: pass the token
await Task.Delay(TimeSpan.FromSeconds(5), token);
}
}
Instead of checking for Token, you could call token.ThrowIfCancellationRequested. This will throw an exception that you'll have to catch

Windows Service running Async code not waiting on work to complete

In Brief
I have a Windows Service that executes several jobs as async Tasks in parallel. However, when the OnStop is called, it seems that these are all immediately terminated instead of being allowed to stop in a more gracious manner.
In more detail
Each job represents an iteration of work, so having completed its work the job then needs to run again.
To accomplish this, I am writing a proof-of-concept Windows Service that:
runs each job as an awaited async TPL Task (these are all I/O bound tasks)
each job is run iteratively within a loop
each job's loop is run in parallel
When I run the Service, I see everything executing as I expect. However, when I Stop the service, it seems that everything stops dead.
Okay - so how is this working?
In the Service I have a cancellation token, and a TaskCompletion Source:
private static CancellationTokenSource _cancelSource = new CancellationTokenSource();
private TaskCompletionSource<bool> _jobCompletion = new TaskCompletionSource<bool>();
private Task<bool> AllJobsCompleted { get { return _finalItems.Task; } }
The idea is that when every Job has gracefully stopped, then the Task AllJobsCompleted will be marked as completed.
The OnStart simply starts running these jobs:
protected override async void OnStart(string[] args)
{
_cancelSource = new CancellationTokenSource();
var jobsToRun = GetJobsToRun(); // details of jobs not relevant
Task.Run(() => this.RunJobs(jobsToRun, _cancelSource.Token).ConfigureAwait(false), _cancelSource.Token);
}
The Task RunJobs will run each job in a parallel loop:
private async Task RunModules(IEnumerable<Jobs> jobs, CancellationToken cancellationToken)
{
var parallelOptions = new ParallelOptions { CancellationToken = cancellationToken };
int jobsRunningCount = jobs.Count();
object lockObject = new object();
Parallel.ForEach(jobs, parallelOptions, async (job, loopState) =>
{
try
{
do
{
await job.DoWork().ConfigureAwait(false); // could take 5 seconds
parallelOptions.CancellationToken.ThrowIfCancellationRequested();
}while(true);
}
catch(OperationCanceledException)
{
lock (lockObject) { jobsRunningCount --; }
}
});
do
{
await Task.Delay(TimeSpan.FromSeconds(5));
} while (modulesRunningCount > 0);
_jobCompletion.SetResult(true);
}
So, what should be happening is that when each job finishes its current iteration, it should see that the cancellation has been signalled and it should then exit the loop and decrement the counter.
Then, when jobsRunningCount reaches zero, then we update the TaskCompletionSource. (There may be a more elegant way of achieving this...)
So, for the OnStop:
protected override async void OnStop()
{
this.RequestAdditionalTime(100000); // some large number
_cancelSource.Cancel();
TraceMessage("Task cancellation requested."); // Last thing traced
try
{
bool allStopped = await this.AllJobsCompleted;
TraceMessage(string.Format("allStopped = '{0}'.", allStopped));
}
catch (Exception e)
{
TraceMessage(e.Message);
}
}
What what I expect is this:
Click [STOP] on the Service
The Service should take sometime to stop
I should see a trace statement "Task cancellation requested."
I should see a trace statement saying either "allStopped = true", or the exception message
And when I debug this using a WPF Form app, I get this.
However, when I install it as a service:
Click [STOP] on the Service
The Service stops almost immediately
I only see the trace statement "Task cancellation requested."
What do I need to do to ensure the OnStop doesn't kill off my parallel async jobs and waits for the TaskCompletionSource?

Your problem is that OnStop is async void. So, when it does await this.AllJobsCompleted, what actually happens is that it returns from OnStop, which the SCM interprets as having stopped, and terminates the process.
This is one of the rare scenarios where you'd need to block on a task, because you cannot allow OnStop to return until after the task completes.
This should do it:
protected override void OnStop()
{
this.RequestAdditionalTime(100000); // some large number
_cancelSource.Cancel();
TraceMessage("Task cancellation requested."); // Last thing traced
try
{
bool allStopped = this.AllJobsCompleted.GetAwaiter().GetResult();
TraceMessage(string.Format("allStopped = '{0}'.", allStopped));
}
catch (Exception e)
{
TraceMessage(e.Message);
}
}

TPL inside Windows Service

I need to perform few tasks inside a Windows Service I am writing in parallel. I am using VS2013, .NET 4.5 and this thread Basic design pattern for using TPL inside windows service for C# shows that TPL is the way to go.
Below is my implementation. I was wondering if anyone can tell me if I have done it correctly!
public partial class FtpLink : ServiceBase
{
private readonly CancellationTokenSource _cancellationTokenSource = new CancellationTokenSource();
private readonly ManualResetEvent _runCompleteEvent = new ManualResetEvent(false);
public FtpLink()
{
InitializeComponent();
// Load configuration
WebEnvironment.Instance.Initialise();
}
protected override void OnStart(string[] args)
{
Trace.TraceInformation("DatabaseToFtp is running");
try
{
RunAsync(_cancellationTokenSource.Token).Wait();
}
finally
{
_runCompleteEvent.Set();
}
}
protected override void OnStop()
{
Trace.TraceInformation("DatabaseToFtp is stopping");
_cancellationTokenSource.Cancel();
_runCompleteEvent.WaitOne();
Trace.TraceInformation("DatabaseToFtp has stopped");
}
private async Task RunAsync(CancellationToken cancellationToken)
{
while (!cancellationToken.IsCancellationRequested)
{
Trace.TraceInformation("Working");
// Do the actual work
var tasks = new List<Task>
{
Task.Factory.StartNew(() => new Processor().ProcessMessageFiles(), cancellationToken),
Task.Factory.StartNew(() => new Processor().ProcessFirmware(), cancellationToken)
};
Task.WaitAll(tasks.ToArray(), cancellationToken);
// Delay the loop for a certain time
await Task.Delay(WebEnvironment.Instance.DatabasePollInterval, cancellationToken);
}
}
}

There are a few things i would do differently:
OnStart should execute in a timely fashion. Common practice is to defer work to a background thread which is in charge of doing the actual work. You're actually doing that but blocking that thread with a call to Task.Wait, which kind of makes the offloading to a background thread useless, because execution becomes synchronous again.
You're using the sync over async anti-pattern, this should be mostly avoided. Let the calling method invoke the work in parallel.
I think you might be using the ManualResetEvent the other way around. You're wrapping your RunAsync method in a try-finally block, but you're only calling WaitOne from OnStop. I'm not really sure you need a lock here at all, it doesn't seem (from your current code) that this code is being invoked in parallel. Instead, you can store the Task returned by RunAsync in a field and wait on it to complete.
You're using the blocking version, WaitAll. Instead, you could use the asynchronous version, Task.WhenAll, which can be asynchronously waited.

Cooperative multitasking using TPL

We are porting modeling application, which uses IronPython scripts for custom actions in modeling process. The existing application executes each Python script in separate thread and uses cooperative model for this. Now we want to port it to TPL, but first we want to measure context switching
.
Basically, what we have right now:
Tasks queue
each Task from this queue executes one IronPython script
Inside IronPython script we call for C# class method, which is synchronization point and should transfer Task (IronPython execution) to waiting state
What we want to do:
We want to make infinite loop, which will iterate through Tasks queue
when we get one Task we try to execute it
In PythonScript we want to call C# method and transfer this script to waiting state. but not remove it from the queue.
On next iteration when we get another Task we check is it in the waiting state. if so we wake it up and try to execute.
In each moment we should have only one active Task
And finally we want to measure how many Task we could execute per second
I don't really know is it something about cooperative multitasking?
We are thinking about custom TaskScheduler, is it good approach? Or does someone know better solution?
Thanks.
Updated:
Ok ,so for example, I have such code:
public class CooperativeScheduler : TaskScheduler, IDisposable
{
private BlockingCollection<Task> _tasks;
private Thread _thread;
private Task _currentTask;
public CooperativeScheduler()
{
this._tasks = new BlockingCollection<Task>();
this._thread = new Thread(() =>
{
foreach (Task task in this._tasks.GetConsumingEnumerable())
{
this._currentTask = task;
TryExecuteTask(this._currentTask);
}
}
);
this._thread.Name = "Cooperative scheduler thread";
this._thread.Start();
}
public void SleepCurrentTask()
{
if (this._currentTask != null)
{
// what to do here?
}
}
protected override IEnumerable<Task> GetScheduledTasks()
{
return this._tasks.ToArray<Task>();
}
protected override void QueueTask(Task task)
{
// No long task
this._tasks.Add(task);
}
protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
{
throw new NotImplementedException();
}
public void Dispose()
{
this._tasks.CompleteAdding();
this._thread.Join();
}
}
Custom Task Scheduler, it has one _thread for Task execution and _currentTask field for running task, also it has SleepCurrentTask in this method I want to suspend current Task execution, but I don't know how.
Client code is simple:
CancellationTokenSource tokenSource = new CancellationTokenSource();
Application app = Application.Create();
Task task = Task.Factory.StartNew(() =>
{
app.Scheduler.SleepCurrentTask();
},
tokenSource.Token, TaskCreationOptions.None, app.Scheduler);
}
Maybe someone has better ideas?

It sounds like you'd be well served to use the producer/consumer pattern .NET 4 has built in to a few collections.
Check out page 55 in this free PDF from Microsoft, Patterns of Parallel Programming