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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

How to stop a thread if thread takes too long

I have a situation that i export data to a file and what i have been asked to do is to provide a cancel button which on click will stop the export if it takes too much time to export.
I started exporting to the file in a thread. And i try to abort the thread on the button click. But it do not work.
I searched on Google and i found that abort() is not recommended. But what else should I choose to achieve it?
My current code is:
private void ExportButtonClick(object param)
{
IList<Ur1R2_Time_Points> data = ct.T_UR.ToList();
DataTable dtData = ExportHelper.ToDataTable(data);
thread = new Thread(new ThreadStart(()=>ExportHelper.DataTableToCsv(dtData, "ExportFile.csv")));
thread.SetApartmentState(ApartmentState.STA);
thread.IsBackground = true;
thread.Name = "PDF";
thread.Start();
}
private void StopButtonClick(object param)
{
if (thread.Name == "PDF")
{
thread.Interrupt();
thread.Abort();
}
}

Aborting a thread is a bad idea, especially when dealing with files. You won't have a chance to clean up half-written files or clean-up inconsistent state.
It won't harm the .NET Runtime bat it can hurt your own application eg if the worker method leaves global state, files or database records in an inconsistent state.
It's always preferable to use cooperative cancellation - the thread periodically checks a coordination construct like a ManualResetEvent or CancellationToken. You can't use a simple variable like a Boolean flag, as this can lead to race conditions, eg if two or more threads try to set it at the same time.
You can read about cancellation in .NET in the Cancellation in Managed Threads section of MSDN.
The CancellationToken/CancellationTokenSource classes were added in .NET 4 to make cancellation easier that passing around events.
In your case, you should modify your DataTableToCsv to accept a CancellationToken. That token is generated by a CancellationTokenSource class.
When you call CancellationTokenSource.Cancel the token's IsCancellationRequested property becomes true. Your DataTableToCsv method should check this flag periodically. If it's set, it should exit any loops, delete any inconsistent files etc.
Timeouts are directly supported with CancelAfter. Essentially, CancelAfter starts a timer that will fire Cancel when it expires.
Your code could look like this:
CancellationTokenSource _exportCts = null;
private void ExportButtonClick(object param)
{
IList<Ur1R2_Time_Points> data = ct.T_UR.ToList();
DataTable dtData = ExportHelper.ToDataTable(data);
_exportCts=new CancellationTokenSource();
var token=_exportCts.Token;
thread = new Thread(new ThreadStart(()=>
ExportHelper.DataTableToCsv(dtData, "ExportFile.csv",token)));
thread.SetApartmentState(ApartmentState.STA);
thread.IsBackground = true;
thread.Name = "PDF";
_exportCts.CancelAfter(10000);
thread.Start();
}
private void StopButtonClick(object param)
{
if (_exportCts!=null)
{
_exportCts.Cancel();
}
}
DataTableToCsv should contain code similar to this:
foreach(var row in myTable)
{
if (token.IsCancellationRequested)
{
break;
}
//else continue with processing
var line=String.Join(",", row.ItemArray);
writer.WriteLine(line);
}
You can clean up your code quite a bit by using tasks instead of raw threads:
private async void ExportButtonClick(object param)
{
IList<Ur1R2_Time_Points> data = ct.T_UR.ToList();
DataTable dtData = ExportHelper.ToDataTable(data);
_exportCts=new CancellationTokenSource();
var token=_exportCts.Token;
_exportCts.CancelAfter(10000);
await Task.Run(()=> ExportHelper.DataTableToCsv(dtData, "ExportFile.csv",token)));
MessageBox.Show("Finished");
}
You could also speed it up by using asynchronous operations, eg to read data from the database or write to text files without blocking or using threads. Windows IO (both file and network) is asynchronous at the driver level. Methods like File.WriteLineAsync don't use threads to write to a file.
Your Export button handler could become :
private void ExportButtonClick(object param)
{
IList<Ur1R2_Time_Points> data = ct.T_UR.ToList();
DataTable dtData = ExportHelper.ToDataTable(data);
_exportCts=new CancellationTokenSource();
var token=_exportCts.Token;
_exportCts.CancelAfter(10000);
await Task.Run(async ()=> ExportHelper.DataTableToCsv(dtData, "ExportFile.csv",token)));
MessageBox.Show("Finished");
}
and DataTableToCsv :
public async Task DataTableToCsv(DataTable table, string file,CancellationToken token)
{
...
foreach(var row in myTable)
{
if (token.IsCancellationRequested)
{
break;
}
//else continue with processing
var line=String.Join(",", row.ItemArray);
await writer.WriteLineAsync(line);
}

You can use a boolean flag. Use a volatile boolean for that.
In the helper do something like:
this.aborted = false;
while(!finished && !aborted) {
//process one row
}
Whenever you want to cancel the operation, you call a method to set aborted to true:
public void Abort() {
this.aborted = true;
}

Have a read here: https://msdn.microsoft.com/en-us/library/system.threading.threadabortexception(v=vs.110).aspx
When a call is made to the Abort method to destroy a thread, the common language runtime throws a ThreadAbortException. ThreadAbortException is a special exception that can be caught, but it will automatically be raised again at the end of the catch block. When this exception is raised, the runtime executes all the finally blocks before ending the thread. Because the thread can do an unbounded computation in the finally blocks or call Thread.ResetAbort to cancel the abort, there is no guarantee that the thread will ever end. If you want to wait until the aborted thread has ended, you can call the Thread.Join method. Join is a blocking call that does not return until the thread actually stops executing.
Since Thread.Abort() is executed by another thread, it can happen anytime and when it happens ThreadAbortException is thrown on target thread.
Inside ExportHelper.DataTableToCsv:
catch(ThreadAbortException e) {
Thread.ResetAbort();
}
On StopButtonClick
if (thread.Name == "PDF")
{
thread.Interrupt();
thread.Join();
}

To Stop a thread you have one option of Thread.Abort.However because this method thrown ThreadAbortException on the target thread when it executed by another thead.
Which is not recommended.
The second option to stop a thread is by using shared variable that both your target and your calling thread can access.
See the Example ::
public static class Program
{
public static void ThreadMethod(object o)
{
for (int i = 0; i < (int)o; i++)
{
Console.WriteLine("ThreadProc: { 0}", i);
Thread.Sleep(0);
}
}
public static void Main()
{
bool stopped = false;
Thread t = new Thread(new ThreadStart(() =>
{
while (!stopped)
{
Console.WriteLine("Running...");
Thread.Sleep(1000);
}
}));
t.Start();
Console.WriteLine("Press any key to exit");
Console.ReadKey();
stopped = true;
t.Join();
}
}
//Source :: Book --> Programming in c#

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);
}
}

TaskContinuationOptions.RunContinuationsAsynchronously and Stack Dives

In this blog post, Stephan Toub describes a new feature that will be included in .NET 4.6 which adds another value to the TaskCreationOptions and TaskContinuationOptions enums called RunContinuationsAsynchronously.
He explains:
"I talked about a ramification of calling {Try}Set* methods on
TaskCompletionSource, that any synchronous continuations off
of the TaskCompletionSource’s Task could run synchronously as
part of the call. If we were to invoke SetResult here while holding
the lock, then synchronous continuations off of that Task would be run
while holding the lock, and that could lead to very real problems.
So, while holding the lock we grab the TaskCompletionSource to
be completed, but we don’t complete it yet, delaying doing so until
the lock has been released"
And gives the following example to demonstrate:
private SemaphoreSlim _gate = new SemaphoreSlim(1, 1);
private async Task WorkAsync()
{
await _gate.WaitAsync().ConfigureAwait(false);
try
{
// work here
}
finally { _gate.Release(); }
}
Now imagine that you have lots of calls to WorkAsync:
await Task.WhenAll(from i in Enumerable.Range(0, 10000) select WorkAsync());
We've just created 10,000 calls to WorkAsync that will be
appropriately serialized on the semaphore. One of the tasks will
enter the critical region, and the others will queue up on the
WaitAsync call, inside SemaphoreSlim effectively enqueueing the task
to be completed when someone calls Release. If Release completed that
Task synchronously, then when the first task calls Release, it'll
synchronously start executing the second task, and when it calls
Release, it'll synchronously start executing the third task, and so
on. If the "//work here" section of code above didn't include any
awaits that yielded, then we're potentially going to stack dive here
and eventually potentially blow out the stack.
I'm having a hard time grasping the part where he talks about executing the continuation synchronously.
Question
How could this possibly cause a stack dive? More so, And what is RunContinuationsAsynchronously effectively going to do in order to solve that problem?

The key concept here is that a task's continuation may run synchronously on the same thread that completed the antecedent task.
Let's imagine that this is SemaphoreSlim.Release's implementation (it's actually Toub's AsyncSemphore's):
public void Release()
{
TaskCompletionSource<bool> toRelease = null;
lock (m_waiters)
{
if (m_waiters.Count > 0)
toRelease = m_waiters.Dequeue();
else
++m_currentCount;
}
if (toRelease != null)
toRelease.SetResult(true);
}
We can see that it synchronously completes a task (using TaskCompletionSource).
In this case, if WorkAsync has no other asynchronous points (i.e. no awaits at all, or all awaits are on an already completed task) and calling _gate.Release() may complete a pending call to _gate.WaitAsync() synchronously on the same thread you may reach a state in which a single thread sequentially releases the semaphore, completes the next pending call, executes // work here and then releases the semaphore again etc. etc.
This means that the same thread goes deeper and deeper in the stack, hence stack dive.
RunContinuationsAsynchronously makes sure the continuation doesn't run synchronously and so the thread that releases the semaphore moves on and the continuation is scheduled for another thread (which one depends on the other continuation parameters e.g. TaskScheduler)
This logically resembles posting the completion to the ThreadPool:
public void Release()
{
TaskCompletionSource<bool> toRelease = null;
lock (m_waiters)
{
if (m_waiters.Count > 0)
toRelease = m_waiters.Dequeue();
else
++m_currentCount;
}
if (toRelease != null)
Task.Run(() => toRelease.SetResult(true));
}

How could this possibly cause a stack dive? More so, And what is
RunContinuationsAsynchronously effectively going to do in order to
solve that problem?
i3arnon provides a very good explanation of the reasons behind introducing RunContinuationsAsynchronously. My answer is rather orthogonal to his; in fact, I'm writing this for my own reference as well (I myself ain't gonna remember any subtleties of this in half a year from now :)
First of all, let's see how TaskCompletionSource's RunContinuationsAsynchronously option is different from Task.Run(() => tcs.SetResult(result)) or the likes. Let's try a simple console application:
using System;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApplications
{
class Program
{
static void Main(string[] args)
{
ThreadPool.SetMinThreads(100, 100);
Console.WriteLine("start, " + new { System.Environment.CurrentManagedThreadId });
var tcs = new TaskCompletionSource<bool>();
// test ContinueWith-style continuations (TaskContinuationOptions.ExecuteSynchronously)
ContinueWith(1, tcs.Task);
ContinueWith(2, tcs.Task);
ContinueWith(3, tcs.Task);
// test await-style continuations
ContinueAsync(4, tcs.Task);
ContinueAsync(5, tcs.Task);
ContinueAsync(6, tcs.Task);
Task.Run(() =>
{
Console.WriteLine("before SetResult, " + new { System.Environment.CurrentManagedThreadId });
tcs.TrySetResult(true);
Thread.Sleep(10000);
});
Console.ReadLine();
}
// log
static void Continuation(int id)
{
Console.WriteLine(new { continuation = id, System.Environment.CurrentManagedThreadId });
Thread.Sleep(1000);
}
// await-style continuation
static async Task ContinueAsync(int id, Task task)
{
await task.ConfigureAwait(false);
Continuation(id);
}
// ContinueWith-style continuation
static Task ContinueWith(int id, Task task)
{
return task.ContinueWith(
t => Continuation(id),
CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
}
}
}
Note how all continuations run synchronously on the same thread where TrySetResult has been called:
start, { CurrentManagedThreadId = 1 }
before SetResult, { CurrentManagedThreadId = 3 }
{ continuation = 1, CurrentManagedThreadId = 3 }
{ continuation = 2, CurrentManagedThreadId = 3 }
{ continuation = 3, CurrentManagedThreadId = 3 }
{ continuation = 4, CurrentManagedThreadId = 3 }
{ continuation = 5, CurrentManagedThreadId = 3 }
{ continuation = 6, CurrentManagedThreadId = 3 }
Now what if we don't want this to happen, and we want each continuation to run asynchronously (i.e., in parallel with other continuations and possibly on another thread, in the absence of any synchronization context)?
There's a trick that could do it for await-style continuations, by installing a fake temporary synchronization context (more details here):
public static class TaskExt
{
class SimpleSynchronizationContext : SynchronizationContext
{
internal static readonly SimpleSynchronizationContext Instance = new SimpleSynchronizationContext();
};
public static void TrySetResult<TResult>(this TaskCompletionSource<TResult> #this, TResult result, bool asyncAwaitContinuations)
{
if (!asyncAwaitContinuations)
{
#this.TrySetResult(result);
return;
}
var sc = SynchronizationContext.Current;
SynchronizationContext.SetSynchronizationContext(SimpleSynchronizationContext.Instance);
try
{
#this.TrySetResult(result);
}
finally
{
SynchronizationContext.SetSynchronizationContext(sc);
}
}
}
Now, using tcs.TrySetResult(true, asyncAwaitContinuations: true) in our test code:
start, { CurrentManagedThreadId = 1 }
before SetResult, { CurrentManagedThreadId = 3 }
{ continuation = 1, CurrentManagedThreadId = 3 }
{ continuation = 2, CurrentManagedThreadId = 3 }
{ continuation = 3, CurrentManagedThreadId = 3 }
{ continuation = 4, CurrentManagedThreadId = 4 }
{ continuation = 5, CurrentManagedThreadId = 5 }
{ continuation = 6, CurrentManagedThreadId = 6 }
Note how await continuations now run in parallel (albeit, still after all synchronous ContinueWith continuations).
This asyncAwaitContinuations: true logic is a hack and it works for await continuations only. The new RunContinuationsAsynchronously makes it work consistently for any kind of continuations, attached to TaskCompletionSource.Task.
Another nice aspect of RunContinuationsAsynchronously is that any await-style continuations scheduled to be resumed on specific synchronization context will run on that context asynchronously (using SynchronizationContext.Post, even if TCS.Task completes on the same context (unlike the current behavior of TCS.SetResult). ContinueWith-style continuations will be also be run asynchronously by their corresponding task schedulers (most often, TaskScheduler.Default or TaskScheduler.FromCurrentSynchronizationContext). They won't be inlined via TaskScheduler.TryExecuteTaskInline. I believe Stephen Toub has clarified that in the comments to his blog post, and it also can be seen here in CoreCLR's Task.cs.
Why should we be worrying about imposing asynchrony on all continuations?
I usually need it when I deal with async methods which execute cooperatively (co-routines).
A simple example is a pause-able asynchronous processing: one async process pauses/resumes the execution of another. Their execution workflow synchronizes at certain await points, and TaskCompletionSource is used for such kind of synchronization, directly or indirectly.
Below is some ready-to-play-with sample code which uses an adaptation of Stephen Toub's PauseTokenSource. Here, one async method StartAndControlWorkAsync starts and periodically pauses/resumes another async method, DoWorkAsync. Try changing asyncAwaitContinuations: true to asyncAwaitContinuations: false and see the logic being completely broken:
using System;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApp
{
class Program
{
static void Main()
{
StartAndControlWorkAsync(CancellationToken.None).Wait();
}
// Do some work which can be paused/resumed
public static async Task DoWorkAsync(PauseToken pause, CancellationToken token)
{
try
{
var step = 0;
while (true)
{
token.ThrowIfCancellationRequested();
Console.WriteLine("Working, step: " + step++);
await Task.Delay(1000).ConfigureAwait(false);
Console.WriteLine("Before await pause.WaitForResumeAsync()");
await pause.WaitForResumeAsync();
Console.WriteLine("After await pause.WaitForResumeAsync()");
}
}
catch (Exception e)
{
Console.WriteLine("Exception: {0}", e);
throw;
}
}
// Start DoWorkAsync and pause/resume it
static async Task StartAndControlWorkAsync(CancellationToken token)
{
var pts = new PauseTokenSource();
var task = DoWorkAsync(pts.Token, token);
while (true)
{
token.ThrowIfCancellationRequested();
Console.WriteLine("Press enter to pause...");
Console.ReadLine();
Console.WriteLine("Before pause requested");
await pts.PauseAsync();
Console.WriteLine("After pause requested, paused: " + pts.IsPaused);
Console.WriteLine("Press enter to resume...");
Console.ReadLine();
Console.WriteLine("Before resume");
pts.Resume();
Console.WriteLine("After resume");
}
}
// Based on Stephen Toub's PauseTokenSource
// http://blogs.msdn.com/b/pfxteam/archive/2013/01/13/cooperatively-pausing-async-methods.aspx
// the main difference is to make sure that when the consumer-side code - which requested the pause - continues,
// the producer-side code has already reached the paused (awaiting) state.
// E.g. a media player "Pause" button is clicked, gets disabled, playback stops,
// and only then "Resume" button gets enabled
public class PauseTokenSource
{
internal static readonly Task s_completedTask = Task.Delay(0);
readonly object _lock = new Object();
bool _paused = false;
TaskCompletionSource<bool> _pauseResponseTcs;
TaskCompletionSource<bool> _resumeRequestTcs;
public PauseToken Token { get { return new PauseToken(this); } }
public bool IsPaused
{
get
{
lock (_lock)
return _paused;
}
}
// request a resume
public void Resume()
{
TaskCompletionSource<bool> resumeRequestTcs = null;
lock (_lock)
{
resumeRequestTcs = _resumeRequestTcs;
_resumeRequestTcs = null;
if (!_paused)
return;
_paused = false;
}
if (resumeRequestTcs != null)
resumeRequestTcs.TrySetResult(true, asyncAwaitContinuations: true);
}
// request a pause (completes when paused state confirmed)
public Task PauseAsync()
{
Task responseTask = null;
lock (_lock)
{
if (_paused)
return _pauseResponseTcs.Task;
_paused = true;
_pauseResponseTcs = new TaskCompletionSource<bool>();
responseTask = _pauseResponseTcs.Task;
_resumeRequestTcs = null;
}
return responseTask;
}
// wait for resume request
internal Task WaitForResumeAsync()
{
Task resumeTask = s_completedTask;
TaskCompletionSource<bool> pauseResponseTcs = null;
lock (_lock)
{
if (!_paused)
return s_completedTask;
_resumeRequestTcs = new TaskCompletionSource<bool>();
resumeTask = _resumeRequestTcs.Task;
pauseResponseTcs = _pauseResponseTcs;
_pauseResponseTcs = null;
}
if (pauseResponseTcs != null)
pauseResponseTcs.TrySetResult(true, asyncAwaitContinuations: true);
return resumeTask;
}
}
// consumer side
public struct PauseToken
{
readonly PauseTokenSource _source;
public PauseToken(PauseTokenSource source) { _source = source; }
public bool IsPaused { get { return _source != null && _source.IsPaused; } }
public Task WaitForResumeAsync()
{
return IsPaused ?
_source.WaitForResumeAsync() :
PauseTokenSource.s_completedTask;
}
}
}
public static class TaskExt
{
class SimpleSynchronizationContext : SynchronizationContext
{
internal static readonly SimpleSynchronizationContext Instance = new SimpleSynchronizationContext();
};
public static void TrySetResult<TResult>(this TaskCompletionSource<TResult> #this, TResult result, bool asyncAwaitContinuations)
{
if (!asyncAwaitContinuations)
{
#this.TrySetResult(result);
return;
}
var sc = SynchronizationContext.Current;
SynchronizationContext.SetSynchronizationContext(SimpleSynchronizationContext.Instance);
try
{
#this.TrySetResult(result);
}
finally
{
SynchronizationContext.SetSynchronizationContext(sc);
}
}
}
}
I didn't want to use Task.Run(() => tcs.SetResult(result)) here, because it would be redundant to push continuations to ThreadPool when they're already scheduled to run asynchronously on a UI thread with a proper synchronization context. At the same time, if both StartAndControlWorkAsync and DoWorkAsync run on the same UI synchronization context, we'd also have a stack dive (if tcs.SetResult(result) is used without Task.Run or SynchronizationContext.Post wrapping).
Now, RunContinuationsAsynchronously is probably the best solution to this problem.

How do I abort/cancel TPL Tasks?

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..