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How to call async method in a thread but wait for it in c#

I have a thread which is responsible for calling a webapi from 4 websites exactly every 2 seconds. The Webapi call method should not be awaited because if a website is not available it will wait 5 second to get timeout and then the next website call will be delayed.
As HttpClient in .NET 4.7.2 has only async methods , it should be used with await, and if not , compiler gives warning and we may get unexpected behavior (as Microsoft says) .
So should I use Task.Run or call Threadpool.QueueUserWorkItem to make a webapi call in parallel.
Here is sudocode :
public class Test1
{
private AutoResetEvent waitEvent = new AutoResetEvent(false);
private volatile bool _terminated = false;
public void Start()
{
Thread T = new Thread(ProcThread);
T.Start();
}
private async void ProcThread()
{
while (!_terminated)
{
await CallWebApi(); <=========== this line
waitEvent.WaitOne(2000);
}
}
private async Task CallWebApi()
{
HttpClient client = new HttpClient();
.....
.....
}
}

So you have an async procedure that uses a HttpClient to fetch some information and process the fetched data:
async Task CallWebApiAsync() {...}
Improvement 1: it is good practice to suffix async methods with async. This is done to make it possible to let an async version exist next to a non-async version that does something similarly.
Inside this method you are using one of the HttpClient methods to fetch the information. As CallWebApiAsync is awaitable, I assume the async methods are used (GetAsync, GetStreamAsync, etc), and that the method only awaits when it needs the result of the async method.
The nice thing about this is, that as a user of CallWebApiAsync, as long as you don't await the call, you are free to do other things, even if the website isn't reacting. The problem is: after 2 seconds, you want to call the method again. But what to do if the method hasn't finished yet.
Improvement 2 Because you want to be able to start a new Task, while the previous one has not finished: remember the started tasks, and throw them away when finished.
HashSet<Task> activeTasks = new HashSet<Task>(); // efficient add, lookup, and removal
void TaskStarted(Task startedTask)
{
// remember the startedTask
activeTasks.Add(startedTask);
}
void TaskCompleted(Task completedTask)
{
// If desired: log or process the results
LogFinishedTask(completedTask);
// Remove the completedTask from the set of ActiveTasks:
activeTasks.Remove(completedTask);
}
It might be handy to remove all completed tasks at once:
void RemoveCompletedTasks()
{
var completedTasks = activeTasks.Where(task => task.IsCompleted).ToList();
foreach (var task in completedTasks)
{
TaskCompleted(completedTask);
}
}
Now we can adjust your ProcThread.
Improvement 3: in async-await always return Task instead of void and Task<TResult> instead of TResult. Only exception: eventhandlers return void.
async Task ProcThread()
{
// Repeatedly: start a task; remember it, and wait 2 seconds
TimeSpan waitTime = TimeSpan.FromSeconds(2);
while (!terminationRequested)
{
Task taskWebApi = CallWebApiAsync();
// You didn't await, so you are free to do other things
// Remember the task that you started.
this.TaskStarted(taskWebApi);
// wait a while before you start new task:
await Task.Delay(waitTime);
// before starting a new task, remove all completed tasks
this.RemoveCompletedTasks();
}
}
Improvement 4: Use TimeSpan.
TimeSpan.FromSeconds(2) is much easier to understand what it represents than a value 2000.
How to stop?
The problem is of course, after you request termination there might still be some tasks running. You'll have to wait for them to finish. But even then: some tasks might not finish at all within reasonable time.
Improvement 5: use CancellationToken to request cancellation.
To cancel tasks in a neat way, class CancellationToken is invented. Users who start a task create a CancellationTokenSource object, and ask this object for a CancellationToken. This token is passed to all async methods. As soon as the user wants to cancel all tasks that were started using this CancellationTokenSource, he requests the CancellationTokenSource to cancel.
All tasks that have a token from this source have promised to regularly check the token to see if cancellation is requested. If so, the task does some cleanup (if needed) and returns.
Everything summarized in one class:
class Test1
{
private HttpClient httpClient = new HttpClient(...);
private HashSet<TTask> activeTasks = new HashSet<TTask>();
public async Task StartAsync(CancellationToken cancellationToken)
{
// repeated CallWebApiAsync until cancellation is requested
TimeSpan waitTime = TimeSpan.FromSeconds(2);
// repeat the following until OperationCancelled
try
{
while (true))
{
// stop if cancellation requested
cancellationToken.ThrowIfCancellationRequested();
var taskWebApi = this.CallWebApiAsync(cancellationToken);
this.activeTasks.Add(taskWebApi);
await Task.Delay(waitTime, cancellationToken);
// remove all completed tasks:
activeTasks.RemoveWhere(task => task.IsCompleted);
}
}
catch (OperationCanceledException exception)
{
// caller requested to cancel. Wait until all tasks are finished.
await Task.WhenAll(this.activeTasks);
// if desired do some logging for all tasks that were not completed.
}
}
And the adjusted CallWebApiAsync:
private async Task CallWebApiAsync(CancellationToken cancellationToken)
{
const string requestUri = ...
var httpResponseMessage = await this.httpClient.GetAsync(requestUri, cancellationToken);
// if here: cancellation not requested
this.ProcessHttpResponse(httpResponseMessage);
}
private void ProcessHttpRespons(HttpResponseMessage httpResponseMessage)
{
...
}
}
Usage:
CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();
Test1 test = new Test1();
Task taskCallWebApiRepeatedly = test.StartAsync(cancellationTokenSource.Token);
// because you didn't await, you are free to do other things, while WebApi is called
// every 2 seconds
DoSomethingElse();
// you get bored. Request cancellation:
cancellationTokenSource.Cancel();
// of course you need to await until all tasks are finished:
await Task.Wait(taskCallWebApiRepeatedly);
Because everyone promises to check regularly if cancellation is requested, you are certain that within reasonable time all tasks are finished, and have cleaned up their mess. The definition or "reasonable time" is arbitrary, but let's say, less than 100 msec?

If all you want is to execute a method every two seconds, then a System.Timers.Timer is probably the most suitable tool to use:
public class Test1
{
private readonly HttpClient _client;
private readonly System.Timers.Timer _timer;
public Test1()
{
_client = new HttpClient();
_timer = new System.Timers.Timer();
_timer.Interval = 2000;
_timer.Elapsed += Timer_Elapsed;
}
private void Timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
{
var fireAndForgetTask = CallWebApiAsync();
}
private async Task CallWebApiAsync()
{
var html = await _client.GetStringAsync("http://example.com");
//...
}
public void Start() => _timer.Start();
public void Stop() => _timer.Stop();
}

something like this. BTW take this as pseudo code as I am typing sitting on my bed:)
List<Task> tasks = new List<Task>();
tasks.Add(CallWebApi());
while (! await Task.WhenAny(tasks))
{
tasks.Add(CallWebApi()); <=========== this line
await Task.Delay(2000);
}

Task doesn't stop [duplicate]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

What task is being returned and why is this task status RanToCompletion

I am rather new to task based programming and trying to determine how to return a task and verify that it has been started. The code that I got to work was not what I was expecting. The console application is as follows:
public static void Main(string[] args)
{
var mySimple = new Simple();
var cts = new CancellationTokenSource();
var task = mySimple.RunSomethingAsync(cts.Token);
while (task.Status != TaskStatus.RanToCompletion)
{
Console.WriteLine("Starting...");
Thread.Sleep(100);
}
Console.WriteLine("It is started");
Console.ReadKey();
cts.Cancel();
}
public class Simple
{
public async void RunSomething(CancellationToken token)
{
var count = 0;
while (true)
{
if (token.IsCancellationRequested)
{
break;
}
Console.WriteLine(count++);
await Task.Delay(TimeSpan.FromMilliseconds(1000), token).ContinueWith(task => { });
}
}
public Task RunSomethingAsync(CancellationToken token)
{
return Task.Run(() => this.RunSomething(token));
}
}
The output is:
Starting...
0
It is started
1
2
3
4
Why is the task that is being returned have a status as TaskStatus.RanToCompletion compared to TaskStatus.Running as we see that the while loop is still executing? Am I checking the status of the task of putting the RunSomething task on the threadpool rather than the RunSomething task itself?

RunSomething is an async void method, meaning it exposes no means of the caller ever determining when it finishes, they can only ever start the operation and then have no idea what happens next. You then wrap a call to it inside of Task.Run, this is schedluing a thread pool thread to start RunSomething. It will then complete as soon as it has finished starting that Task.
If RunSomething actually returned a Task, then the caller would be able to determine when it actually finished, and if you waited on it it wouldn't actually indicate that it was done until that asynchronous operation was actually finished (there would be no reason to use Task.Run to start it in another thead, you'd be better off just calling it directly and not wasting the effort of moving that to a thread pool thread).

Never use async void (https://msdn.microsoft.com/en-us/magazine/jj991977.aspx)
instead you should use async Task
If you need to call an async method from a non-async (such as from a static void main) you should do something like this:
mySimple.RunSomethingAsync(cts.Token).GetAwaiter().GetResult();
That will effectively make the method a synchronous call.

You can use async void, but only for events.

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..