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Difference between Task.Run(()=> DoWorkAsync()) and new Thread(async()=> DoWorkAsync());

I recently came across some code which confused me heavily, I have always thought that you must use threads or Async tasks, not mix and match between them,
public async Task DoWork()
{
Task.Delay(1000);
}
Now I saw code calling this like so:
public void Main()
{
var thread = new Thread(async () => { await DoWorkAync(); })
{
Priority = ThreadPriority.Highest,
IsBackground = true
};
// Start thread
proccessThread.Start();
}
Now this magically seemed to NOT create a thread each time it was run, it seemed to be using the ThreadPool.
now what I am struggling to understand is the difference between the above and:
public void Main()
{
var task = Task.Run(DoWorkASync);
}
From my testing, it seems that C# Thread has a different functionality when passing in an Async Expression vs the standard method on which to run>

This construct:
var thread = new Thread(async () => { await DoWorkAync(); });
// Start thread
proccessThread.Start();
Calls Thread constructor overload accepting ThreadStart delegate, and ThreadStart delegate is () => void. So you have this:
var thread = new Thread(StuffYourThreadExecutes);
thread.Start();
static async void StuffYourThreadExecutes() {
await DoWorkAsync();
}
So you start new thread and it runs the code until first asynchronous operation begins. Then thread exists. After that first asynchronous operation completes - the rest executes on whatever thread task scheduler providers (usually thread pool thread). Any exceptions which happen during this process cannot be observed.
For example if DoWorkAsync is something like:
static async Task DoWorkAsync(){
await Task.Delay(1000);
}
Then thread starts and almost immediately exits, doing nothing useful.
Task.Run, when passing async delegate there, does what is stated in docs:
Queues the specified work to run on the thread pool and returns a
proxy for the task
So whole operation just runs on thread pool thread without creating threads for nothing. You can observe exceptions by awaiting task returned by Task.Run.

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..

What's the difference between Foo().Result and Task.Run(() => Foo()).Result in C#?

In C# what is the difference between these two statements? If I use the first one in my constructor in my test classes I get a deadlock, or something similar, and the tests never finish. With the second one the code works.
// Deadlock.
var r = MyMethod().Result;
// Works.
var r = Task.Run(() => MyMethod()).Result;
Update: There is a bit more context in this commit: https://github.com/webCRMdotcom/erp-integrations/pull/92/commits/dd8af89899ce1de837ef6e34f0688a685a5cea3b.

The difference is the starting thread context.
Here a simple sample
using System;
using System.Threading.Tasks;
public class Program
{
public static void Main()
{
string r;
OutputThreadInfo("Main");
r = MyMethod().Result;
r = Task.Run( () => MyMethod() ).Result;
}
public static async Task<string> MyMethod()
{
OutputThreadInfo("MyMethod");
await Task.Delay(50);
return "finished";
}
private static void OutputThreadInfo(string context)
{
Console.WriteLine("{0} {1}",context,System.Threading.Thread.CurrentThread.ManagedThreadId);
}
}
.net fiddle
which will output
Main 32
MyMethod 32
MyMethod 63
The first call of MyMethod will start at the same thread as Main and if started from a thread with a synchronization context it will block.
The second call of MyMethod will start from a different thread (worker thread from thread pool) as Main which does not have a synchronization context and therefor will not block.
PS You should keep in mind that Console applications do not have a synchronization context as default but WinForms, WPF, UWP application do have and so will behave somehow different on async/await

Task.Result and Task.Wait block the current thread you should use await for this to work without any problems. (Though they only block if not already completed).
The second line will create a task and will start it's execution on a available thread in the Thread Pool and that's why it doesn't block.
This is because the Task construct when used with async-await will generate a State Machine that keeps track of all the awaits used in the code block and when all finishes then it can return the result. Keep in mind thought that depending on the Synchronization Context you are in, the code after await may run on a different thread then the one the task started.
So what I do when I have to execute synchronous an async method I use a small piece of code like this:
private static readonly TaskFactory _tf = new TaskFactory(
CancellationToken.None, TaskCreationOptions.None,
TaskContinuationOptions.None, TaskScheduler.Default);
public static TResult RunSync<TResult>(Func<Task<TResult>> func)
{
return _tf.StartNew<Task<TResult>>((Func<Task<TResult>>) (() =>
{
return func();
})).Unwrap<TResult>().GetAwaiter().GetResult();
}
Keep in mind that, if needed, you have to use the same CultureInfo inside the RunSync StarNew task factory call so you won't have this kind of problems.

How can I prevent synchronous continuations on a Task?

I have some library (socket networking) code that provides a Task-based API for pending responses to requests, based on TaskCompletionSource<T>. However, there's an annoyance in the TPL in that it seems to be impossible to prevent synchronous continuations. What I would like to be able to do is either:
tell a TaskCompletionSource<T> that is should not allow callers to attach with TaskContinuationOptions.ExecuteSynchronously, or
set the result (SetResult / TrySetResult) in a way that specifies that TaskContinuationOptions.ExecuteSynchronously should be ignored, using the pool instead
Specifically, the issue I have is that the incoming data is being processed by a dedicated reader, and if a caller can attach with TaskContinuationOptions.ExecuteSynchronously they can stall the reader (which affects more than just them). Previously, I have worked around this by some hackery that detects whether any continuations are present, and if they are it pushes the completion onto the ThreadPool, however this has significant impact if the caller has saturated their work queue, as the completion will not get processed in a timely fashion. If they are using Task.Wait() (or similar), they will then essentially deadlock themselves. Likewise, this is why the reader is on a dedicated thread rather than using workers.
So; before I try and nag the TPL team: am I missing an option?
Key points:
I don't want external callers to be able to hijack my thread
I can't use the ThreadPool as an implementation, as it needs to work when the pool is saturated
The example below produces output (ordering may vary based on timing):
Continuation on: Main thread
Press [return]
Continuation on: Thread pool
The problem is the fact that a random caller managed to get a continuation on "Main thread". In the real code, this would be interrupting the primary reader; bad things!
Code:
using System;
using System.Threading;
using System.Threading.Tasks;
static class Program
{
static void Identify()
{
var thread = Thread.CurrentThread;
string name = thread.IsThreadPoolThread
? "Thread pool" : thread.Name;
if (string.IsNullOrEmpty(name))
name = "#" + thread.ManagedThreadId;
Console.WriteLine("Continuation on: " + name);
}
static void Main()
{
Thread.CurrentThread.Name = "Main thread";
var source = new TaskCompletionSource<int>();
var task = source.Task;
task.ContinueWith(delegate {
Identify();
});
task.ContinueWith(delegate {
Identify();
}, TaskContinuationOptions.ExecuteSynchronously);
source.TrySetResult(123);
Console.WriteLine("Press [return]");
Console.ReadLine();
}
}

New in .NET 4.6:
.NET 4.6 contains a new TaskCreationOptions: RunContinuationsAsynchronously.
Since you're willing to use Reflection to access private fields...
You can mark the TCS's Task with the TASK_STATE_THREAD_WAS_ABORTED flag, which would cause all continuations not to be inlined.
const int TASK_STATE_THREAD_WAS_ABORTED = 134217728;
var stateField = typeof(Task).GetField("m_stateFlags", BindingFlags.NonPublic | BindingFlags.Instance);
stateField.SetValue(task, (int) stateField.GetValue(task) | TASK_STATE_THREAD_WAS_ABORTED);
Edit:
Instead of using Reflection emit, I suggest you use expressions. This is much more readable and has the advantage of being PCL-compatible:
var taskParameter = Expression.Parameter(typeof (Task));
const string stateFlagsFieldName = "m_stateFlags";
var setter =
Expression.Lambda<Action<Task>>(
Expression.Assign(Expression.Field(taskParameter, stateFlagsFieldName),
Expression.Or(Expression.Field(taskParameter, stateFlagsFieldName),
Expression.Constant(TASK_STATE_THREAD_WAS_ABORTED))), taskParameter).Compile();
Without using Reflection:
If anyone's interested, I've figured out a way to do this without Reflection, but it is a bit "dirty" as well, and of course carries a non-negligible perf penalty:
try
{
Thread.CurrentThread.Abort();
}
catch (ThreadAbortException)
{
source.TrySetResult(123);
Thread.ResetAbort();
}

I don't think there's anything in TPL which would provides explicit API control over TaskCompletionSource.SetResult continuations. I decided to keep my initial answer for controlling this behavior for async/await scenarios.
Here is another solution which imposes asynchronous upon ContinueWith, if the tcs.SetResult-triggered continuation takes place on the same thread the SetResult was called on:
public static class TaskExt
{
static readonly ConcurrentDictionary<Task, Thread> s_tcsTasks =
new ConcurrentDictionary<Task, Thread>();
// SetResultAsync
static public void SetResultAsync<TResult>(
this TaskCompletionSource<TResult> #this,
TResult result)
{
s_tcsTasks.TryAdd(#this.Task, Thread.CurrentThread);
try
{
#this.SetResult(result);
}
finally
{
Thread thread;
s_tcsTasks.TryRemove(#this.Task, out thread);
}
}
// ContinueWithAsync, TODO: more overrides
static public Task ContinueWithAsync<TResult>(
this Task<TResult> #this,
Action<Task<TResult>> action,
TaskContinuationOptions continuationOptions = TaskContinuationOptions.None)
{
return #this.ContinueWith((Func<Task<TResult>, Task>)(t =>
{
Thread thread = null;
s_tcsTasks.TryGetValue(t, out thread);
if (Thread.CurrentThread == thread)
{
// same thread which called SetResultAsync, avoid potential deadlocks
// using thread pool
return Task.Run(() => action(t));
// not using thread pool (TaskCreationOptions.LongRunning creates a normal thread)
// return Task.Factory.StartNew(() => action(t), TaskCreationOptions.LongRunning);
}
else
{
// continue on the same thread
var task = new Task(() => action(t));
task.RunSynchronously();
return Task.FromResult(task);
}
}), continuationOptions).Unwrap();
}
}
Updated to address the comment:
I don't control the caller - I can't get them to use a specific
continue-with variant: if I could, the problem would not exist in the
first place
I wasn't aware you don't control the caller. Nevertheless, if you don't control it, you're probably not passing the TaskCompletionSource object directly to the caller, either. Logically, you'd be passing the token part of it, i.e. tcs.Task. In which case, the solution might be even easier, by adding another extension method to the above:
// ImposeAsync, TODO: more overrides
static public Task<TResult> ImposeAsync<TResult>(this Task<TResult> #this)
{
return #this.ContinueWith(new Func<Task<TResult>, Task<TResult>>(antecedent =>
{
Thread thread = null;
s_tcsTasks.TryGetValue(antecedent, out thread);
if (Thread.CurrentThread == thread)
{
// continue on a pool thread
return antecedent.ContinueWith(t => t,
TaskContinuationOptions.None).Unwrap();
}
else
{
return antecedent;
}
}), TaskContinuationOptions.ExecuteSynchronously).Unwrap();
}
Use:
// library code
var source = new TaskCompletionSource<int>();
var task = source.Task.ImposeAsync();
// ...
// client code
task.ContinueWith(delegate
{
Identify();
}, TaskContinuationOptions.ExecuteSynchronously);
// ...
// library code
source.SetResultAsync(123);
This actually works for both await and ContinueWith (fiddle) and is free of reflection hacks.

What about instead of doing
var task = source.Task;
you do this instead
var task = source.Task.ContinueWith<Int32>( x => x.Result );
Thus you are always adding one continuation which will be executed asynchronously and then it doesn't matter if the subscribers want a continuation in the same context. It's sort of currying the task, isn't it?

The simulate abort approach looked really good, but led to the TPL hijacking threads in some scenarios.
I then had an implementation that was similar to checking the continuation object, but just checking for any continuation since there are actually too many scenarios for the given code to work well, but that meant that even things like Task.Wait resulted in a thread-pool lookup.
Ultimately, after inspecting lots and lots of IL, the only safe and useful scenario is the SetOnInvokeMres scenario (manual-reset-event-slim continuation). There are lots of other scenarios:
some aren't safe, and lead to thread hijacking
the rest aren't useful, as they ultimately lead to the thread-pool
So in the end, I opted to check for a non-null continuation-object; if it is null, fine (no continuations); if it is non-null, special-case check for SetOnInvokeMres - if it is that: fine (safe to invoke); otherwise, let the thread-pool perform the TrySetComplete, without telling the task to do anything special like spoofing abort. Task.Wait uses the SetOnInvokeMres approach, which is the specific scenario we want to try really hard not to deadlock.
Type taskType = typeof(Task);
FieldInfo continuationField = taskType.GetField("m_continuationObject", BindingFlags.Instance | BindingFlags.NonPublic);
Type safeScenario = taskType.GetNestedType("SetOnInvokeMres", BindingFlags.NonPublic);
if (continuationField != null && continuationField.FieldType == typeof(object) && safeScenario != null)
{
var method = new DynamicMethod("IsSyncSafe", typeof(bool), new[] { typeof(Task) }, typeof(Task), true);
var il = method.GetILGenerator();
var hasContinuation = il.DefineLabel();
il.Emit(OpCodes.Ldarg_0);
il.Emit(OpCodes.Ldfld, continuationField);
Label nonNull = il.DefineLabel(), goodReturn = il.DefineLabel();
// check if null
il.Emit(OpCodes.Brtrue_S, nonNull);
il.MarkLabel(goodReturn);
il.Emit(OpCodes.Ldc_I4_1);
il.Emit(OpCodes.Ret);
// check if is a SetOnInvokeMres - if so, we're OK
il.MarkLabel(nonNull);
il.Emit(OpCodes.Ldarg_0);
il.Emit(OpCodes.Ldfld, continuationField);
il.Emit(OpCodes.Isinst, safeScenario);
il.Emit(OpCodes.Brtrue_S, goodReturn);
il.Emit(OpCodes.Ldc_I4_0);
il.Emit(OpCodes.Ret);
IsSyncSafe = (Func<Task, bool>)method.CreateDelegate(typeof(Func<Task, bool>));

if you can and are ready to use reflection, this should do it;
public static class MakeItAsync
{
static public void TrySetAsync<T>(this TaskCompletionSource<T> source, T result)
{
var continuation = typeof(Task).GetField("m_continuationObject", BindingFlags.NonPublic | BindingFlags.GetField | BindingFlags.Instance);
var continuations = (List<object>)continuation.GetValue(source.Task);
foreach (object c in continuations)
{
var option = c.GetType().GetField("m_options", BindingFlags.NonPublic | BindingFlags.GetField | BindingFlags.Instance);
var options = (TaskContinuationOptions)option.GetValue(c);
options &= ~TaskContinuationOptions.ExecuteSynchronously;
option.SetValue(c, options);
}
source.TrySetResult(result);
}
}

Updated, I posted a separate answer to deal with ContinueWith as opposed to await (because ContinueWith doesn't care about the current synchronization context).
You could use a dumb synchronization context to impose asynchrony upon continuation triggered by calling SetResult/SetCancelled/SetException on TaskCompletionSource. I believe the current synchronization context (at the point of await tcs.Task) is the criteria TPL uses to decide whether to make such continuation synchronous or asynchronous.
The following works for me:
if (notifyAsync)
{
tcs.SetResultAsync(null);
}
else
{
tcs.SetResult(null);
}
SetResultAsync is implemented like this:
public static class TaskExt
{
static public void SetResultAsync<T>(this TaskCompletionSource<T> tcs, T result)
{
FakeSynchronizationContext.Execute(() => tcs.SetResult(result));
}
// FakeSynchronizationContext
class FakeSynchronizationContext : SynchronizationContext
{
private static readonly ThreadLocal<FakeSynchronizationContext> s_context =
new ThreadLocal<FakeSynchronizationContext>(() => new FakeSynchronizationContext());
private FakeSynchronizationContext() { }
public static FakeSynchronizationContext Instance { get { return s_context.Value; } }
public static void Execute(Action action)
{
var savedContext = SynchronizationContext.Current;
SynchronizationContext.SetSynchronizationContext(FakeSynchronizationContext.Instance);
try
{
action();
}
finally
{
SynchronizationContext.SetSynchronizationContext(savedContext);
}
}
// SynchronizationContext methods
public override SynchronizationContext CreateCopy()
{
return this;
}
public override void OperationStarted()
{
throw new NotImplementedException("OperationStarted");
}
public override void OperationCompleted()
{
throw new NotImplementedException("OperationCompleted");
}
public override void Post(SendOrPostCallback d, object state)
{
throw new NotImplementedException("Post");
}
public override void Send(SendOrPostCallback d, object state)
{
throw new NotImplementedException("Send");
}
}
}
SynchronizationContext.SetSynchronizationContext is very cheap in terms of the overhead it adds. In fact, a very similar approach is taken by the implementation of WPF Dispatcher.BeginInvoke.
TPL compares the target synchronization context at the point of await to that of the point of tcs.SetResult. If the synchronization context is the same (or there is no synchronization context at both places), the continuation is called directly, synchronously. Otherwise, it's queued using SynchronizationContext.Post on the target synchronization context, i.e., the normal await behavior. What this approach does is always impose the SynchronizationContext.Post behavior (or a pool thread continuation if there's no target synchronization context).
Updated, this won't work for task.ContinueWith, because ContinueWith doesn't care about the current synchronization context. It however works for await task (fiddle). It also does work for await task.ConfigureAwait(false).
OTOH, this approach works for ContinueWith.

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..