I created a small wrapper around CancellationToken and CancellationTokenSource. The problem I have is that the CancelAsync method of CancellationHelper doesn't work as expected.
I'm experiencing the problem with the ItShouldThrowAExceptionButStallsInstead method. To cancel the running task, it calls await coordinator.CancelAsync();, but the task is not cancelled actually and doesn't throw an exception on task.Wait
ItWorksWellAndThrowsException seems to be working well and it uses coordinator.Cancel, which is not an async method at all.
The question why is the task is not cancelled when I call CancellationTokenSource's Cancel method in async method?
Don't let the waitHandle confuse you, it's only for not letting the task finish early.
Let the code speak for itself:
using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
namespace TestCancellation
{
class Program
{
static void Main(string[] args)
{
ItWorksWellAndThrowsException();
//ItShouldThrowAExceptionButStallsInstead();
}
private static void ItShouldThrowAExceptionButStallsInstead()
{
Task.Run(async () =>
{
var coordinator = new CancellationHelper();
var waitHandle = new ManualResetEvent(false);
var task = Task.Run(() =>
{
waitHandle.WaitOne();
//this works well though - it throws
//coordinator.ThrowIfCancellationRequested();
}, coordinator.Token);
await coordinator.CancelAsync();
//waitHandle.Set(); -- with or without this it will throw
task.Wait();
}).Wait();
}
private static void ItWorksWellAndThrowsException()
{
Task.Run(() =>
{
var coordinator = new CancellationHelper();
var waitHandle = new ManualResetEvent(false);
var task = Task.Run(() => { waitHandle.WaitOne(); }, coordinator.Token);
coordinator.Cancel();
task.Wait();
}).Wait();
}
}
public class CancellationHelper
{
private CancellationTokenSource cancellationTokenSource;
private readonly List<Task> tasksToAwait;
public CancellationHelper()
{
cancellationTokenSource = new CancellationTokenSource();
tasksToAwait = new List<Task>();
}
public CancellationToken Token
{
get { return cancellationTokenSource.Token; }
}
public void AwaitOnCancellation(Task task)
{
if (task == null) return;
tasksToAwait.Add(task);
}
public void Reset()
{
tasksToAwait.Clear();
cancellationTokenSource = new CancellationTokenSource();
}
public void ThrowIfCancellationRequested()
{
cancellationTokenSource.Token.ThrowIfCancellationRequested();
}
public void Cancel()
{
cancellationTokenSource.Cancel();
Task.WaitAll(tasksToAwait.ToArray());
}
public async Task CancelAsync()
{
cancellationTokenSource.Cancel();
try
{
await Task.WhenAll(tasksToAwait.ToArray());
}
catch (AggregateException ex)
{
ex.Handle(p => p is OperationCanceledException);
}
}
}
}
Cancellation in .NET is cooperative.
That means that the one holding the CancellationTokenSource signals cancellation and the one holding the CancellationToken needs to check whether cancellation was signaled (either by polling the CancellationToken or by registering a delegate to run when it is signaled).
In your Task.Run you use the CancellationToken as a parameter, but you don't check it inside the task itself so the task will only be cancelled if the token was signaled before the task had to a chance to start.
To cancel the task while it's running you need to check the CancellationToken:
var task = Task.Run(() =>
{
token.ThrowIfCancellationRequested();
}, token);
In your case you block on a ManualResetEvent so you wouldn't be able to check the CancellationToken. You can register a delegate to the CancellationToken that frees up the reset event:
token.Register(() => waitHandle.Set())
Related
I am trying to create a helper class for sending some information periodically to backend server.
Attaching the code below.
public class HeartBeatService
{
private CancellationToken _cancellationToken;
private CancellationTokenSource _cancellationTokenSource;
public void StartHeartBeatService(TimeSpan timeSpan)
{
_cancellationTokenSource = new CancellationTokenSource();
_cancellationToken = _cancellationTokenSource.Token;
Task.Run(async () =>
{
while (!_cancellationToken.IsCancellationRequested)
{
SendHeartBeatToAzure();
try
{
await Task.Delay(timeSpan, _cancellationToken);
}
catch
{
break;
}
}
});
}
public void SuspendHeartBeatService()
{
_cancellationTokenSource?.Cancel();
}
private async void SendHeartBeatToAzure()
{
var platformService = ServiceLocator.Get<IPlatformService>();
var location = await platformService?.GetPositionAsync();
if (!double.IsNaN(location.Item1) && !double.IsNaN(location.Item2))
{
Debug.WriteLine($"Triggering Heartbeat with location{location.Item1},{location.Item2}");
//TODO Invoke heartbeat api call.
}
}
}
The code for sending the information to server is working fine.
But there is some issue with CancellationToken which is not working/it is not cancelling.
not sure what's wrong with the implementation.
Change the signature of the SendHeartBeatToAzure to return a Task, so that it can be awaited:
private async Task SendHeartBeatToAzure()
Then await the task returned by the method inside the loop. To achieve a stable and consisted heartbeat, it is a good idea to create the Task.Delay task before calling the method:
Task.Run(async () =>
{
while (true)
{
var delayTask = Task.Delay(timeSpan, _cancellationToken);
await SendHeartBeatToAzure();
await delayTask;
}
});
As a side note, you should probably store the task returned by Task.Run as a readonly property of the HeartBeatService class, so that the status of the task can be monitored.
I have a method that contains an infinite loop. I call that method from main method and I want to cancel it if it takes more that 2 seconds.
I tried these approaches
Task.WhenAny with cancellation of the non completed tasks and timeout
How can I cancel Task.WhenAll?
How to cancel a task using a CancellationToken and await Task.WhenAny
and lots of other links, but non has worked.
here is my code
static void Main(string[] args)
{
var cts = new CancellationTokenSource(2000);
var task = Task.Run(() => RunTask(), cts.Token);
await task;
}
public static void RunTask()
{
while (true)
{
Console.WriteLine("in while");
}
}
I also tried to cancel my token but it didn't work.
It is not possible to Dispose the task.
I don't have access to RunTask source code. I just can run it.
I cannot send CancellationToken to my method. How can I terminate RunTask() method?
Cancellation tokens are a way to request cancellation. It's up to the running code to accept that request and actually stop the work. It's not like killing a process. For example, if you had something like:
public static void RunTask(CancellationToken cancellationToken)
{
while (!cancellationToken.IsCancellationRequested)
{
Console.WriteLine("in while");
}
}
Then, when it's canceled, the loop will end. However, given that the current code doesn't offer any method of canceling and you cannot modify it, then there is no way to cancel it.
What about this approach:
static void Main(string[] args)
{
var cts = new CancellationTokenSource(2000);
var task = Task.Run(() => RunTask(cts.Token), cts.Token);
await task;
}
public static void RunTask(CancellationToken token)
{
while (!token.IsCancellationRequested)
{
Console.WriteLine("in while");
}
}
This is not a duplicate of
Task not garbage collected. The symptoms are similar though.
The code below is a console app that creates an STA thread for use with WinForms. Tasks are posted to that thread via a custom task scheduler obtained with TaskScheduler.FromCurrentSynchronizationContext, which just implicitly wraps an instance of WindowsFormsSynchronizationContext here.
Depending on what causes this STA thread to end, the final task var terminatorTask = Run(() => Application.ExitThread()), scheduled in the WinformsApartment.Dispose method, may not always be getting a chance to execute. Regardless of that, I believe this task still should be getting garbage-collected, but it isn't. Why?
Here's a self-contained example illustrating that (s_debugTaskRef.IsAlive is true at the finish), tested with .NET 4.8, both Debug and Release:
using System;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace ConsoleTest
{
class Program
{
// entry point
static async Task Main(string[] args)
{
try
{
using (var apartment = new WinformsApartment(() => new Form()))
{
await Task.Delay(1000);
await apartment.Run(() => Application.ExitThread());
}
}
catch (Exception ex)
{
Console.WriteLine($"Error: {ex.Message}");
Environment.Exit(-1);
}
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
GC.WaitForPendingFinalizers();
Console.WriteLine($"IsAlive: {WinformsApartment.s_debugTaskRef.IsAlive}");
Console.ReadLine();
}
}
public class WinformsApartment : IDisposable
{
readonly Thread _thread; // the STA thread
readonly TaskScheduler _taskScheduler; // the STA thread's task scheduler
readonly Task _threadEndTask; // to keep track of the STA thread completion
readonly object _lock = new object();
public TaskScheduler TaskScheduler { get { return _taskScheduler; } }
public Task AsTask { get { return _threadEndTask; } }
/// <summary>MessageLoopApartment constructor</summary>
public WinformsApartment(Func<Form> createForm)
{
var schedulerTcs = new TaskCompletionSource<TaskScheduler>();
var threadEndTcs = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
// start an STA thread and gets a task scheduler
_thread = new Thread(_ =>
{
try
{
// handle Application.Idle just once
// to make sure we're inside the message loop
// and the proper synchronization context has been correctly installed
void onIdle(object s, EventArgs e) {
Application.Idle -= onIdle;
// make the task scheduler available
schedulerTcs.SetResult(TaskScheduler.FromCurrentSynchronizationContext());
};
Application.Idle += onIdle;
Application.Run(createForm());
threadEndTcs.TrySetResult(true);
}
catch (Exception ex)
{
threadEndTcs.TrySetException(ex);
}
});
async Task waitForThreadEndAsync()
{
// we use TaskCreationOptions.RunContinuationsAsynchronously
// to make sure thread.Join() won't try to join itself
Debug.Assert(Thread.CurrentThread != _thread);
await threadEndTcs.Task.ConfigureAwait(false);
_thread.Join();
}
_thread.SetApartmentState(ApartmentState.STA);
_thread.IsBackground = true;
_thread.Start();
_taskScheduler = schedulerTcs.Task.Result;
_threadEndTask = waitForThreadEndAsync();
}
// TODO: it's here for debugging leaks
public static readonly WeakReference s_debugTaskRef = new WeakReference(null);
/// <summary>shutdown the STA thread</summary>
public void Dispose()
{
lock(_lock)
{
if (Thread.CurrentThread == _thread)
throw new InvalidOperationException();
if (!_threadEndTask.IsCompleted)
{
// execute Application.ExitThread() on the STA thread
var terminatorTask = Run(() => Application.ExitThread());
s_debugTaskRef.Target = terminatorTask; // TODO: it's here for debugging leaks
_threadEndTask.GetAwaiter().GetResult();
}
}
}
/// <summary>Task.Factory.StartNew wrappers</summary>
public Task Run(Action action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler);
}
public Task<TResult> Run<TResult>(Func<TResult> action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler);
}
public Task Run(Func<Task> action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler).Unwrap();
}
public Task<TResult> Run<TResult>(Func<Task<TResult>> action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler).Unwrap();
}
}
}
I suspect this might be a .NET Framework bug. I'm currently investigating it and I'll post what I may find, but maybe someone could provide an explanation right away.
Ok so it appears the WindowsFormsSynchronizationContext doesn't get properly disposed of here. Not sure if it's a bug or a "feature", but the following change does fix it:
SynchronizationContext syncContext = null;
void onIdle(object s, EventArgs e) {
Application.Idle -= onIdle;
syncContext = SynchronizationContext.Current;
// make the task scheduler available
schedulerTcs.SetResult(TaskScheduler.FromCurrentSynchronizationContext());
};
Application.Idle += onIdle;
Application.Run(createForm());
SynchronizationContext.SetSynchronizationContext(null);
(syncContext as IDisposable)?.Dispose();
Now IsAlive is false and the task gets properly GC'ed. Comment out (syncContext as IDisposable)?.Dispose() above, and IsAlive is back to true.
Updated, if anyone uses a similar pattern (I myself use it for automation), I'd now recommend controlling the lifetime and disposal of WindowsFormsSynchronizationContext explicitly:
public class WinformsApartment : IDisposable
{
readonly Thread _thread; // the STA thread
readonly TaskScheduler _taskScheduler; // the STA thread's task scheduler
readonly Task _threadEndTask; // to keep track of the STA thread completion
readonly object _lock = new object();
public TaskScheduler TaskScheduler { get { return _taskScheduler; } }
public Task AsTask { get { return _threadEndTask; } }
/// <summary>MessageLoopApartment constructor</summary>
public WinformsApartment(Func<Form> createForm)
{
var schedulerTcs = new TaskCompletionSource<TaskScheduler>();
var threadEndTcs = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
// start an STA thread and gets a task scheduler
_thread = new Thread(_ =>
{
try
{
// handle Application.Idle just once
// to make sure we're inside the message loop
// and the proper synchronization context has been correctly installed
void onIdle(object s, EventArgs e)
{
Application.Idle -= onIdle;
// make the task scheduler available
schedulerTcs.SetResult(TaskScheduler.FromCurrentSynchronizationContext());
};
Application.Idle += onIdle;
Application.Run(createForm());
threadEndTcs.TrySetResult(true);
}
catch (Exception ex)
{
threadEndTcs.TrySetException(ex);
}
});
async Task waitForThreadEndAsync()
{
// we use TaskCreationOptions.RunContinuationsAsynchronously
// to make sure thread.Join() won't try to join itself
Debug.Assert(Thread.CurrentThread != _thread);
try
{
await threadEndTcs.Task.ConfigureAwait(false);
}
finally
{
_thread.Join();
}
}
_thread.SetApartmentState(ApartmentState.STA);
_thread.IsBackground = true;
_thread.Start();
_taskScheduler = schedulerTcs.Task.Result;
_threadEndTask = waitForThreadEndAsync();
}
// TODO: it's here for debugging leaks
public static readonly WeakReference s_debugTaskRef = new WeakReference(null);
/// <summary>shutdown the STA thread</summary>
public void Dispose()
{
lock (_lock)
{
if (Thread.CurrentThread == _thread)
throw new InvalidOperationException();
if (!_threadEndTask.IsCompleted)
{
// execute Application.ExitThread() on the STA thread
var terminatorTask = Run(() => Application.ExitThread());
s_debugTaskRef.Target = terminatorTask; // TODO: it's here for debugging leaks
_threadEndTask.GetAwaiter().GetResult();
}
}
}
/// <summary>Task.Factory.StartNew wrappers</summary>
public Task Run(Action action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler);
}
public Task<TResult> Run<TResult>(Func<TResult> action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler);
}
public Task Run(Func<Task> action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler).Unwrap();
}
public Task<TResult> Run<TResult>(Func<Task<TResult>> action, CancellationToken token = default(CancellationToken))
{
return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler).Unwrap();
}
}
I'm implementing a worker engine with an upper limit to concurrency. I'm using a semaphore to wait until concurrency drops below the maximum, then use Task.Factory.StartNew to wrap the async handler in a try/catch, with a finally which releases the semaphore.
I realise this creates threads on the thread pool - but my question is, when one of those task-running threads actually awaits (on a real IO call or wait handle), is the thread returned to the pool, as I'd hope it would be?
If there's a better way to implement a task scheduler with limited concurrency where the work handler is an async method (returns Task), I'd love to hear it too. Or, let's say ideally, if there's a way to queue up an async method (again, it's a Task-returning async method) that feels less dodgy than wrapping it in a synchronous delegate and passing it to Task.Factory.StartNew, that would seem perfect..?
(This also makes me think that there are two kinds of parallelism here: how many tasks are being processed overall, but also how many continuations are running on different threads concurrently. Might be cool to have configurable options for both, though not a fixed requirement..)
Edit: snippet:
concurrencySemaphore.Wait(cancelToken);
deferRelease = false;
try
{
var result = GetWorkItem();
if (result == null)
{ // no work, wait for new work or exit signal
signal = WaitHandle.WaitAny(signals);
continue;
}
deferRelease = true;
tasks.Add(Task.Factory.StartNew(() =>
{
try
{
DoWorkHereAsync(result); // guess I'd think to .GetAwaiter().GetResult() here.. not run this yet
}
finally
{
concurrencySemaphore.Release();
}
}, cancelToken));
}
finally
{
if (!deferRelease)
{
concurrencySemaphore.Release();
}
}
Here an example of a TaskWorker, that will not produce countless worker threads.
The magic is done by awaiting SemaphoreSlim.WaitAsync() which is an IO task (and there is no thread).
class TaskWorker
{
private readonly SemaphoreSlim _semaphore;
public TaskWorker(int maxDegreeOfParallelism)
{
if (maxDegreeOfParallelism <= 0)
{
throw new ArgumentOutOfRangeException(nameof(maxDegreeOfParallelism));
}
_semaphore = new SemaphoreSlim(maxDegreeOfParallelism, maxDegreeOfParallelism);
}
public async Task RunAsync(Func<Task> taskFactory, CancellationToken cancellationToken = default(CancellationToken))
{
// No ConfigureAwait(false) here to keep the SyncContext if any
// for the real task
await _semaphore.WaitAsync(cancellationToken);
try
{
await taskFactory().ConfigureAwait(false);
}
finally
{
_semaphore.Release(1);
}
}
public async Task<T> RunAsync<T>(Func<Task<T>> taskFactory, CancellationToken cancellationToken = default(CancellationToken))
{
await _semaphore.WaitAsync(cancellationToken);
try
{
return await taskFactory().ConfigureAwait(false);
}
finally
{
_semaphore.Release(1);
}
}
}
and a simple console app to test
class Program
{
static void Main(string[] args)
{
var worker = new TaskWorker(1);
var cts = new CancellationTokenSource();
var token = cts.Token;
var tasks = Enumerable.Range(1, 10)
.Select(e => worker.RunAsync(() => SomeWorkAsync(e, token), token))
.ToArray();
Task.WhenAll(tasks).GetAwaiter().GetResult();
}
static async Task SomeWorkAsync(int id, CancellationToken cancellationToken)
{
Console.WriteLine($"Some Started {id}");
await Task.Delay(2000, cancellationToken).ConfigureAwait(false);
Console.WriteLine($"Some Finished {id}");
}
}
Update
TaskWorker implementing IDisposable
class TaskWorker : IDisposable
{
private readonly CancellationTokenSource _cts = new CancellationTokenSource();
private readonly SemaphoreSlim _semaphore;
private readonly int _maxDegreeOfParallelism;
public TaskWorker(int maxDegreeOfParallelism)
{
if (maxDegreeOfParallelism <= 0)
{
throw new ArgumentOutOfRangeException(nameof(maxDegreeOfParallelism));
}
_maxDegreeOfParallelism = maxDegreeOfParallelism;
_semaphore = new SemaphoreSlim(maxDegreeOfParallelism, maxDegreeOfParallelism);
}
public async Task RunAsync(Func<Task> taskFactory, CancellationToken cancellationToken = default(CancellationToken))
{
ThrowIfDisposed();
using (var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, _cts.Token))
{
// No ConfigureAwait(false) here to keep the SyncContext if any
// for the real task
await _semaphore.WaitAsync(cts.Token);
try
{
await taskFactory().ConfigureAwait(false);
}
finally
{
_semaphore.Release(1);
}
}
}
public async Task<T> RunAsync<T>(Func<Task<T>> taskFactory, CancellationToken cancellationToken = default(CancellationToken))
{
ThrowIfDisposed();
using (var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, _cts.Token))
{
await _semaphore.WaitAsync(cts.Token);
try
{
return await taskFactory().ConfigureAwait(false);
}
finally
{
_semaphore.Release(1);
}
}
}
private void ThrowIfDisposed()
{
if (disposedValue)
{
throw new ObjectDisposedException(this.GetType().FullName);
}
}
#region IDisposable Support
private bool disposedValue = false;
protected virtual void Dispose(bool disposing)
{
if (!disposedValue)
{
if (disposing)
{
_cts.Cancel();
// consume all semaphore slots
for (int i = 0; i < _maxDegreeOfParallelism; i++)
{
_semaphore.WaitAsync().GetAwaiter().GetResult();
}
_semaphore.Dispose();
_cts.Dispose();
}
disposedValue = true;
}
}
public void Dispose()
{
Dispose(true);
}
#endregion
}
You can think that thread is returned to a ThreadPool even thought it is not actauly a return. The thread simply picks next queued item when async operation starts.
I would suggest you to look at Task.Run instead of Task.Factory.StartNew Task.Run vs Task.Factory.StartNew.
And also have a look at TPL DataFlow. I think it will match your requirements.
I have some methods returning Task<T> on which I can await at will. I'd like to have those Tasks executed on a custom TaskScheduler instead of the default one.
var task = GetTaskAsync ();
await task;
I know I can create a new TaskFactory (new CustomScheduler ()) and do a StartNew () from it, but StartNew () takes an action and create the Task, and I already have the Task (returned behind the scenes by a TaskCompletionSource)
How can I specify my own TaskScheduler for await ?
I think what you really want is to do a Task.Run, but with a custom scheduler. StartNew doesn't work intuitively with asynchronous methods; Stephen Toub has a great blog post about the differences between Task.Run and TaskFactory.StartNew.
So, to create your own custom Run, you can do something like this:
private static readonly TaskFactory myTaskFactory = new TaskFactory(
CancellationToken.None, TaskCreationOptions.DenyChildAttach,
TaskContinuationOptions.None, new MyTaskScheduler());
private static Task RunOnMyScheduler(Func<Task> func)
{
return myTaskFactory.StartNew(func).Unwrap();
}
private static Task<T> RunOnMyScheduler<T>(Func<Task<T>> func)
{
return myTaskFactory.StartNew(func).Unwrap();
}
private static Task RunOnMyScheduler(Action func)
{
return myTaskFactory.StartNew(func);
}
private static Task<T> RunOnMyScheduler<T>(Func<T> func)
{
return myTaskFactory.StartNew(func);
}
Then you can execute synchronous or asynchronous methods on your custom scheduler.
The TaskCompletionSource<T>.Task is constructed without any action and the scheduler
is assigned on the first call to ContinueWith(...) (from Asynchronous Programming with the Reactive Framework and the Task Parallel Library — Part 3).
Thankfully you can customize the await behavior slightly by implementing your own class deriving from INotifyCompletion and then using it in a pattern similar to await SomeTask.ConfigureAwait(false) to configure the scheduler that the task should start using in the OnCompleted(Action continuation) method (from await anything;).
Here is the usage:
TaskCompletionSource<object> source = new TaskCompletionSource<object>();
public async Task Foo() {
// Force await to schedule the task on the supplied scheduler
await SomeAsyncTask().ConfigureScheduler(scheduler);
}
public Task SomeAsyncTask() { return source.Task; }
Here is a simple implementation of ConfigureScheduler using a Task extension method with the important part in OnCompleted:
public static class TaskExtension {
public static CustomTaskAwaitable ConfigureScheduler(this Task task, TaskScheduler scheduler) {
return new CustomTaskAwaitable(task, scheduler);
}
}
public struct CustomTaskAwaitable {
CustomTaskAwaiter awaitable;
public CustomTaskAwaitable(Task task, TaskScheduler scheduler) {
awaitable = new CustomTaskAwaiter(task, scheduler);
}
public CustomTaskAwaiter GetAwaiter() { return awaitable; }
public struct CustomTaskAwaiter : INotifyCompletion {
Task task;
TaskScheduler scheduler;
public CustomTaskAwaiter(Task task, TaskScheduler scheduler) {
this.task = task;
this.scheduler = scheduler;
}
public void OnCompleted(Action continuation) {
// ContinueWith sets the scheduler to use for the continuation action
task.ContinueWith(x => continuation(), scheduler);
}
public bool IsCompleted { get { return task.IsCompleted; } }
public void GetResult() { }
}
}
Here's a working sample that will compile as a console application:
using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;
namespace Example {
class Program {
static TaskCompletionSource<object> source = new TaskCompletionSource<object>();
static TaskScheduler scheduler = new CustomTaskScheduler();
static void Main(string[] args) {
Console.WriteLine("Main Started");
var task = Foo();
Console.WriteLine("Main Continue ");
// Continue Foo() using CustomTaskScheduler
source.SetResult(null);
Console.WriteLine("Main Finished");
}
public static async Task Foo() {
Console.WriteLine("Foo Started");
// Force await to schedule the task on the supplied scheduler
await SomeAsyncTask().ConfigureScheduler(scheduler);
Console.WriteLine("Foo Finished");
}
public static Task SomeAsyncTask() { return source.Task; }
}
public struct CustomTaskAwaitable {
CustomTaskAwaiter awaitable;
public CustomTaskAwaitable(Task task, TaskScheduler scheduler) {
awaitable = new CustomTaskAwaiter(task, scheduler);
}
public CustomTaskAwaiter GetAwaiter() { return awaitable; }
public struct CustomTaskAwaiter : INotifyCompletion {
Task task;
TaskScheduler scheduler;
public CustomTaskAwaiter(Task task, TaskScheduler scheduler) {
this.task = task;
this.scheduler = scheduler;
}
public void OnCompleted(Action continuation) {
// ContinueWith sets the scheduler to use for the continuation action
task.ContinueWith(x => continuation(), scheduler);
}
public bool IsCompleted { get { return task.IsCompleted; } }
public void GetResult() { }
}
}
public static class TaskExtension {
public static CustomTaskAwaitable ConfigureScheduler(this Task task, TaskScheduler scheduler) {
return new CustomTaskAwaitable(task, scheduler);
}
}
public class CustomTaskScheduler : TaskScheduler {
protected override IEnumerable<Task> GetScheduledTasks() { yield break; }
protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued) { return false; }
protected override void QueueTask(Task task) {
TryExecuteTask(task);
}
}
}
There is no way to embed rich async features into a custom TaskScheduler. This class was not designed with async/await in mind. The standard way to use a custom TaskScheduler is as an argument to the Task.Factory.StartNew method. This method does not understand async delegates. It is possible to provide an async delegate, but it is treated as any other delegate that returns some result. To get the actual awaited result of the async delegate one must call Unwrap() to the task returned.
This is not the problem though. The problem is that the TaskScheduler infrastructure does not treat the async delegate as a single unit of work. Each task is split into multiple mini-tasks (using every await as a separator), and each mini-task is processed individually. This severely restricts the asynchronous functionality that can be implemented on top of this class. As an example here is a custom TaskScheduler that is intended to queue the supplied tasks one at a time (to limit the concurrency in other words):
public class MyTaskScheduler : TaskScheduler
{
private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1);
protected async override void QueueTask(Task task)
{
await _semaphore.WaitAsync();
try
{
await Task.Run(() => base.TryExecuteTask(task));
await task;
}
finally
{
_semaphore.Release();
}
}
protected override bool TryExecuteTaskInline(Task task,
bool taskWasPreviouslyQueued) => false;
protected override IEnumerable<Task> GetScheduledTasks() { yield break; }
}
The SemaphoreSlim should ensure that only one Task would run at a time. Unfortunately it doesn't work. The semaphore is released prematurely, because the Task passed in the call QueueTask(task) is not the task that represents the whole work of the async delegate, but only the part until the first await. The other parts are passed to the TryExecuteTaskInline method. There is no way to correlate these task-parts, because no identifier or other mechanism is provided. Here is what happens in practice:
var taskScheduler = new MyTaskScheduler();
var tasks = Enumerable.Range(1, 5).Select(n => Task.Factory.StartNew(async () =>
{
Console.WriteLine($"{DateTime.Now:HH:mm:ss.fff} Item {n} Started");
await Task.Delay(1000);
Console.WriteLine($"{DateTime.Now:HH:mm:ss.fff} Item {n} Finished");
}, default, TaskCreationOptions.None, taskScheduler))
.Select(t => t.Unwrap())
.ToArray();
Task.WaitAll(tasks);
Output:
05:29:58.346 Item 1 Started
05:29:58.358 Item 2 Started
05:29:58.358 Item 3 Started
05:29:58.358 Item 4 Started
05:29:58.358 Item 5 Started
05:29:59.358 Item 1 Finished
05:29:59.374 Item 5 Finished
05:29:59.374 Item 4 Finished
05:29:59.374 Item 2 Finished
05:29:59.374 Item 3 Finished
Disaster, all tasks are queued at once.
Conclusion: Customizing the TaskScheduler class is not the way to go when advanced async features are required.
Update: Here is another observation, regarding custom TaskSchedulers in the presence of an ambient SynchronizationContext. The await mechanism by default captures the current SynchronizationContext, or the current TaskScheduler, and invokes the continuation on either the captured context
or the scheduler. If both are present, the current SynchronizationContext is preferred, and the current TaskScheduler is ignored. Below is a demonstration of this behavior, in a WinForms application¹:
private async void Button1_Click(object sender, EventArgs e)
{
await Task.Factory.StartNew(async () =>
{
MessageBox.Show($"{Thread.CurrentThread.ManagedThreadId}, {TaskScheduler.Current}");
await Task.Delay(1000);
MessageBox.Show($"{Thread.CurrentThread.ManagedThreadId}, {TaskScheduler.Current}");
}, default, TaskCreationOptions.None,
TaskScheduler.FromCurrentSynchronizationContext()).Unwrap();
}
Clicking the button causes two messages to popup sequentially, with this information:
1, System.Threading.Tasks.SynchronizationContextTaskScheduler
1, System.Threading.Tasks.ThreadPoolTaskScheduler
This experiment shows that only the first part of the asynchronous delegate, the part before the first await, was scheduled on the non-default scheduler.
This behavior limits even further the practical usefulness of custom TaskSchedulers in an async/await-enabled environment.
¹ Windows Forms applications have a WindowsFormsSynchronizationContext installed automatically, when the Application.Run method is called.
Can you fit for this method call:
await Task.Factory.StartNew(
() => { /* to do what you need */ },
CancellationToken.None, /* you can change as you need */
TaskCreationOptions.None, /* you can change as you need */
customScheduler);
After the comments it looks like you want to control the scheduler on which the code after the await is run.
The compile creates a continuation from the await that runs on the current SynchronizationContext by default. So your best shot is to set up the SynchronizationContext before calling await.
There are some ways to await a specific context. See Configure Await from Jon Skeet, especially the part about SwitchTo, for more information on how to implement something like this.
EDIT:
The SwitchTo method from TaskEx has been removed, as it was too easy to misuse. See the MSDN Forum for reasons.
Faced with same issue, tried to use LimitedConcurrencyLevelTaskScheduler, but it does not support async tasks. So...
Just wrote my own small simple Scheduler, that allow to run async Tasks based on global ThreadPool (and Task.Run method) with ability to limit current max degree of parallelism. It is enough for my exact purposes, maybe will also help you, guys.
Main demo code (console app, dotnet core 3.1) :
static async Task Main(string[] args)
{
//5 tasks to run per time
int concurrentLimit = 5;
var scheduler = new ThreadPoolConcurrentScheduler(concurrentLimit);
//catch all errors in separate event handler
scheduler.OnError += Scheduler_OnError;
// just monitor "live" state and output to console
RunTaskStateMonitor(scheduler);
// simulate adding new tasks "on the fly"
SimulateAddingTasksInParallel(scheduler);
Console.WriteLine("start adding 50 tasks");
//add 50 tasks
for (var i = 1; i <= 50; i++)
{
scheduler.StartNew(myAsyncTask);
}
Console.WriteLine("50 tasks added to scheduler");
Thread.Sleep(1000000);
}
Supporting code (place it in the same place) :
private static void Scheduler_OnError(Exception ex)
{
Console.WriteLine(ex.ToString());
}
private static int currentTaskFinished = 0;
//your sample of async task
static async Task myAsyncTask()
{
Console.WriteLine("task started ");
using (HttpClient httpClient = new HttpClient())
{
//just make http request to ... wikipedia!
//sorry, Jimmy Wales! assume,guys, you will not DDOS wiki :)
var uri = new Uri("https://wikipedia.org/");
var response = await httpClient.GetAsync(uri);
string result = await response.Content.ReadAsStringAsync();
if (string.IsNullOrEmpty(result))
Console.WriteLine("error, await is not working");
else
Console.WriteLine($"task result : site length is {result.Length}");
}
//or simulate it using by sync sleep
//Thread.Sleep(1000);
//and for tesing exception :
//throw new Exception("my custom error");
Console.WriteLine("task finished ");
//just incrementing total ran tasks to output in console
Interlocked.Increment(ref currentTaskFinished);
}
static void SimulateAddingTasksInParallel(ThreadPoolConcurrentScheduler taskScheduler)
{
int runCount = 0;
Task.Factory.StartNew(() =>
{
while (true)
{
runCount++;
if (runCount > 5)
break;
//every 10 sec 5 times
Thread.Sleep(10000);
//adding new 5 tasks from outer task
Console.WriteLine("start adding new 5 tasks!");
for (var i = 1; i <= 5; i++)
{
taskScheduler.StartNew(myAsyncTask);
}
Console.WriteLine("new 5 tasks added!");
}
}, TaskCreationOptions.LongRunning);
}
static void RunTaskStateMonitor(ThreadPoolConcurrentScheduler taskScheduler)
{
int prev = -1;
int prevQueueSize = -1;
int prevFinished = -1;
Task.Factory.StartNew(() =>
{
while (true)
{
// getting current thread count in working state
var currCount = taskScheduler.GetCurrentWorkingThreadCount();
// getting inner queue state
var queueSize = taskScheduler.GetQueueTaskCount();
//just output overall state if something changed
if (prev != currCount || queueSize != prevQueueSize || prevFinished != currentTaskFinished)
{
Console.WriteLine($"Monitor : running tasks:{currCount}, queueLength:{queueSize}. total Finished tasks : " + currentTaskFinished);
prev = currCount;
prevQueueSize = queueSize;
prevFinished = currentTaskFinished;
}
// check it every 10 ms
Thread.Sleep(10);
}
}
, TaskCreationOptions.LongRunning);
}
Scheduler :
public class ThreadPoolConcurrentScheduler
{
private readonly int _limitParallelThreadsCount;
private int _threadInProgressCount = 0;
public delegate void onErrorDelegate(Exception ex);
public event onErrorDelegate OnError;
private ConcurrentQueue<Func<Task>> _taskQueue;
private readonly object _queueLocker = new object();
public ThreadPoolConcurrentScheduler(int limitParallelThreadsCount)
{
//set maximum parallel tasks to run
_limitParallelThreadsCount = limitParallelThreadsCount;
// thread-safe queue to store tasks
_taskQueue = new ConcurrentQueue<Func<Task>>();
}
//main method to start async task
public void StartNew(Func<Task> task)
{
lock (_queueLocker)
{
// checking limit
if (_threadInProgressCount >= _limitParallelThreadsCount)
{
//waiting new "free" threads in queue
_scheduleTask(task);
}
else
{
_startNewTask(task);
}
}
}
private void _startNewTask(Func<Task> task)
{
Interlocked.Increment(ref _threadInProgressCount);
Task.Run(async () =>
{
try
{
await task();
}
catch (Exception e)
{
//Console.WriteLine(e);
OnError?.Invoke(e);
}
}).ContinueWith(_onTaskEnded);
}
//will be called on task end
private void _onTaskEnded(Task task)
{
lock (_queueLocker)
{
Interlocked.Decrement(ref _threadInProgressCount);
//queue has more priority, so if thread is free - let's check queue first
if (!_taskQueue.IsEmpty)
{
if (_taskQueue.TryDequeue(out var result))
{
_startNewTask(result);
}
}
}
}
private void _scheduleTask(Func<Task> task)
{
_taskQueue.Enqueue(task);
}
//returning in progress task count
public int GetCurrentWorkingThreadCount()
{
return _threadInProgressCount;
}
//return number of tasks waiting to run
public int GetQueueTaskCount()
{
lock (_queueLocker) return _taskQueue.Count;
}
}
Few notes :
First - check comments to it, maybe it is the worst code ever!
Did not test in prod
Did not implement cancellation tokens and any other functionality, that should be there, but i'm too lazy. Sorry