We could abort a Thread like this:
Thread thread = new Thread(SomeMethod);
.
.
.
thread.Abort();
But can I abort a Task (in .Net 4.0) in the same way not by cancellation mechanism. I want to kill the Task immediately.
The guidance on not using a thread abort is controversial. I think there is still a place for it but in exceptional circumstance. However you should always attempt to design around it and see it as a last resort.
Example;
You have a simple windows form application that connects to a blocking synchronous web service. Within which it executes a function on the web service within a Parallel loop.
CancellationTokenSource cts = new CancellationTokenSource();
ParallelOptions po = new ParallelOptions();
po.CancellationToken = cts.Token;
po.MaxDegreeOfParallelism = System.Environment.ProcessorCount;
Parallel.ForEach(iListOfItems, po, (item, loopState) =>
{
Thread.Sleep(120000); // pretend web service call
});
Say in this example, the blocking call takes 2 mins to complete. Now I set my MaxDegreeOfParallelism to say ProcessorCount. iListOfItems has 1000 items within it to process.
The user clicks the process button and the loop commences, we have 'up-to' 20 threads executing against 1000 items in the iListOfItems collection. Each iteration executes on its own thread. Each thread will utilise a foreground thread when created by Parallel.ForEach. This means regardless of the main application shutdown, the app domain will be kept alive until all threads have finished.
However the user needs to close the application for some reason, say they close the form.
These 20 threads will continue to execute until all 1000 items are processed. This is not ideal in this scenario, as the application will not exit as the user expects and will continue to run behind the scenes, as can be seen by taking a look in task manger.
Say the user tries to rebuild the app again (VS 2010), it reports the exe is locked, then they would have to go into task manager to kill it or just wait until all 1000 items are processed.
I would not blame you for saying, but of course! I should be cancelling these threads using the CancellationTokenSource object and calling Cancel ... but there are some problems with this as of .net 4.0. Firstly this is still never going to result in a thread abort which would offer up an abort exception followed by thread termination, so the app domain will instead need to wait for the threads to finish normally, and this means waiting for the last blocking call, which would be the very last running iteration (thread) that ultimately gets to call po.CancellationToken.ThrowIfCancellationRequested.
In the example this would mean the app domain could still stay alive for up to 2 mins, even though the form has been closed and cancel called.
Note that Calling Cancel on CancellationTokenSource does not throw an exception on the processing thread(s), which would indeed act to interrupt the blocking call similar to a thread abort and stop the execution. An exception is cached ready for when all the other threads (concurrent iterations) eventually finish and return, the exception is thrown in the initiating thread (where the loop is declared).
I chose not to use the Cancel option on a CancellationTokenSource object. This is wasteful and arguably violates the well known anti-patten of controlling the flow of the code by Exceptions.
Instead, it is arguably 'better' to implement a simple thread safe property i.e. Bool stopExecuting. Then within the loop, check the value of stopExecuting and if the value is set to true by the external influence, we can take an alternate path to close down gracefully. Since we should not call cancel, this precludes checking CancellationTokenSource.IsCancellationRequested which would otherwise be another option.
Something like the following if condition would be appropriate within the loop;
if (loopState.ShouldExitCurrentIteration || loopState.IsExceptional || stopExecuting) {loopState.Stop(); return;}
The iteration will now exit in a 'controlled' manner as well as terminating further iterations, but as I said, this does little for our issue of having to wait on the long running and blocking call(s) that are made within each iteration (parallel loop thread), since these have to complete before each thread can get to the option of checking if it should stop.
In summary, as the user closes the form, the 20 threads will be signaled to stop via stopExecuting, but they will only stop when they have finished executing their long running function call.
We can't do anything about the fact that the application domain will always stay alive and only be released when all foreground threads have completed. And this means there will be a delay associated with waiting for any blocking calls made within the loop to complete.
Only a true thread abort can interrupt the blocking call, and you must mitigate leaving the system in a unstable/undefined state the best you can in the aborted thread's exception handler which goes without question. Whether that's appropriate is a matter for the programmer to decide, based on what resource handles they chose to maintain and how easy it is to close them in a thread's finally block. You could register with a token to terminate on cancel as a semi workaround i.e.
CancellationTokenSource cts = new CancellationTokenSource();
ParallelOptions po = new ParallelOptions();
po.CancellationToken = cts.Token;
po.MaxDegreeOfParallelism = System.Environment.ProcessorCount;
Parallel.ForEach(iListOfItems, po, (item, loopState) =>
{
using (cts.Token.Register(Thread.CurrentThread.Abort))
{
Try
{
Thread.Sleep(120000); // pretend web service call
}
Catch(ThreadAbortException ex)
{
// log etc.
}
Finally
{
// clean up here
}
}
});
but this will still result in an exception in the declaring thread.
All things considered, interrupt blocking calls using the parallel.loop constructs could have been a method on the options, avoiding the use of more obscure parts of the library. But why there is no option to cancel and avoid throwing an exception in the declaring method strikes me as a possible oversight.
But can I abort a Task (in .Net 4.0) in the same way not by
cancellation mechanism. I want to kill the Task immediately.
Other answerers have told you not to do it. But yes, you can do it. You can supply Thread.Abort() as the delegate to be called by the Task's cancellation mechanism. Here is how you could configure this:
class HardAborter
{
public bool WasAborted { get; private set; }
private CancellationTokenSource Canceller { get; set; }
private Task<object> Worker { get; set; }
public void Start(Func<object> DoFunc)
{
WasAborted = false;
// start a task with a means to do a hard abort (unsafe!)
Canceller = new CancellationTokenSource();
Worker = Task.Factory.StartNew(() =>
{
try
{
// specify this thread's Abort() as the cancel delegate
using (Canceller.Token.Register(Thread.CurrentThread.Abort))
{
return DoFunc();
}
}
catch (ThreadAbortException)
{
WasAborted = true;
return false;
}
}, Canceller.Token);
}
public void Abort()
{
Canceller.Cancel();
}
}
disclaimer: don't do this.
Here is an example of what not to do:
var doNotDoThis = new HardAborter();
// start a thread writing to the console
doNotDoThis.Start(() =>
{
while (true)
{
Thread.Sleep(100);
Console.Write(".");
}
return null;
});
// wait a second to see some output and show the WasAborted value as false
Thread.Sleep(1000);
Console.WriteLine("WasAborted: " + doNotDoThis.WasAborted);
// wait another second, abort, and print the time
Thread.Sleep(1000);
doNotDoThis.Abort();
Console.WriteLine("Abort triggered at " + DateTime.Now);
// wait until the abort finishes and print the time
while (!doNotDoThis.WasAborted) { Thread.CurrentThread.Join(0); }
Console.WriteLine("WasAborted: " + doNotDoThis.WasAborted + " at " + DateTime.Now);
Console.ReadKey();
You shouldn't use Thread.Abort()
Tasks can be Cancelled but not aborted.
The Thread.Abort() method is (severely) deprecated.
Both Threads and Tasks should cooperate when being stopped, otherwise you run the risk of leaving the system in a unstable/undefined state.
If you do need to run a Process and kill it from the outside, the only safe option is to run it in a separate AppDomain.
This answer is about .net 3.5 and earlier.
Thread-abort handling has been improved since then, a.o. by changing the way finally blocks work.
But Thread.Abort is still a suspect solution that you should always try to avoid.
And in .net Core (.net 5+) Thread.Abort() will now throw a PlatformNotSupportedException .
Kind of underscoring the 'deprecated' point.
Everyone knows (hopefully) its bad to terminate thread. The problem is when you don't own a piece of code you're calling. If this code is running in some do/while infinite loop , itself calling some native functions, etc. you're basically stuck. When this happens in your own code termination, stop or Dispose call, it's kinda ok to start shooting the bad guys (so you don't become a bad guy yourself).
So, for what it's worth, I've written those two blocking functions that use their own native thread, not a thread from the pool or some thread created by the CLR. They will stop the thread if a timeout occurs:
// returns true if the call went to completion successfully, false otherwise
public static bool RunWithAbort(this Action action, int milliseconds) => RunWithAbort(action, new TimeSpan(0, 0, 0, 0, milliseconds));
public static bool RunWithAbort(this Action action, TimeSpan delay)
{
if (action == null)
throw new ArgumentNullException(nameof(action));
var source = new CancellationTokenSource(delay);
var success = false;
var handle = IntPtr.Zero;
var fn = new Action(() =>
{
using (source.Token.Register(() => TerminateThread(handle, 0)))
{
action();
success = true;
}
});
handle = CreateThread(IntPtr.Zero, IntPtr.Zero, fn, IntPtr.Zero, 0, out var id);
WaitForSingleObject(handle, 100 + (int)delay.TotalMilliseconds);
CloseHandle(handle);
return success;
}
// returns what's the function should return if the call went to completion successfully, default(T) otherwise
public static T RunWithAbort<T>(this Func<T> func, int milliseconds) => RunWithAbort(func, new TimeSpan(0, 0, 0, 0, milliseconds));
public static T RunWithAbort<T>(this Func<T> func, TimeSpan delay)
{
if (func == null)
throw new ArgumentNullException(nameof(func));
var source = new CancellationTokenSource(delay);
var item = default(T);
var handle = IntPtr.Zero;
var fn = new Action(() =>
{
using (source.Token.Register(() => TerminateThread(handle, 0)))
{
item = func();
}
});
handle = CreateThread(IntPtr.Zero, IntPtr.Zero, fn, IntPtr.Zero, 0, out var id);
WaitForSingleObject(handle, 100 + (int)delay.TotalMilliseconds);
CloseHandle(handle);
return item;
}
[DllImport("kernel32")]
private static extern bool TerminateThread(IntPtr hThread, int dwExitCode);
[DllImport("kernel32")]
private static extern IntPtr CreateThread(IntPtr lpThreadAttributes, IntPtr dwStackSize, Delegate lpStartAddress, IntPtr lpParameter, int dwCreationFlags, out int lpThreadId);
[DllImport("kernel32")]
private static extern bool CloseHandle(IntPtr hObject);
[DllImport("kernel32")]
private static extern int WaitForSingleObject(IntPtr hHandle, int dwMilliseconds);
While it's possible to abort a thread, in practice it's almost always a very bad idea to do so. Aborthing a thread means the thread is not given a chance to clean up after itself, leaving resources undeleted, and things in unknown states.
In practice, if you abort a thread, you should only do so in conjunction with killing the process. Sadly, all too many people think ThreadAbort is a viable way of stopping something and continuing on, it's not.
Since Tasks run as threads, you can call ThreadAbort on them, but as with generic threads you almost never want to do this, except as a last resort.
I faced a similar problem with Excel's Application.Workbooks.
If the application is busy, the method hangs eternally. My approach was simply to try to get it in a task and wait, if it takes too long, I just leave the task be and go away (there is no harm "in this case", Excel will unfreeze the moment the user finishes whatever is busy).
In this case, it's impossible to use a cancellation token. The advantage is that I don't need excessive code, aborting threads, etc.
public static List<Workbook> GetAllOpenWorkbooks()
{
//gets all open Excel applications
List<Application> applications = GetAllOpenApplications();
//this is what we want to get from the third party library that may freeze
List<Workbook> books = null;
//as Excel may freeze here due to being busy, we try to get the workbooks asynchronously
Task task = Task.Run(() =>
{
try
{
books = applications
.SelectMany(app => app.Workbooks.OfType<Workbook>()).ToList();
}
catch { }
});
//wait for task completion
task.Wait(5000);
return books; //handle outside if books is null
}
This is my implementation of an idea presented by #Simon-Mourier, using the dotnet thread, short and simple code:
public static bool RunWithAbort(this Action action, int milliseconds)
{
if (action == null) throw new ArgumentNullException(nameof(action));
var success = false;
var thread = new Thread(() =>
{
action();
success = true;
});
thread.IsBackground = true;
thread.Start();
thread.Join(milliseconds);
thread.Abort();
return success;
}
You can "abort" a task by running it on a thread you control and aborting that thread. This causes the task to complete in a faulted state with a ThreadAbortException. You can control thread creation with a custom task scheduler, as described in this answer. Note that the caveat about aborting a thread applies.
(If you don't ensure the task is created on its own thread, aborting it would abort either a thread-pool thread or the thread initiating the task, neither of which you typically want to do.)
using System;
using System.Threading;
using System.Threading.Tasks;
...
var cts = new CancellationTokenSource();
var task = Task.Run(() => { while (true) { } });
Parallel.Invoke(() =>
{
task.Wait(cts.Token);
}, () =>
{
Thread.Sleep(1000);
cts.Cancel();
});
This is a simple snippet to abort a never-ending task with CancellationTokenSource.
I would like to use this solution to call Console.ReadLine() with a timeout:
delegate string ReadLineDelegate();
string ReadLine(int timeoutms)
{
string resultstr = null;
ReadLineDelegate d = Console.ReadLine;
IAsyncResult result = d.BeginInvoke(null, null);
result.AsyncWaitHandle.WaitOne(timeoutms);//timeout e.g. 15000 for 15 secs
if (result.IsCompleted)
{
resultstr = d.EndInvoke(result);
Console.WriteLine("Read: " + resultstr);
}
else
{
Console.WriteLine("Timed out!");
// Bug? resource leak? No d.EndInvoke(), which blocks until Console.ReadLine() returns
}
result.AsyncWaitHandle.Close();
return resultstr;
}
but commenters warned:
every ReadLine you call sits there waiting for input.
If you call it 100 times, it creates 100 threads
which don't all go away until you hit Enter 100 times!
...especially because I want to call this repeatedly in a forever-loop.
I understand that every BeginInvoke() needs a EndInvoke() but I don't want a blocking EndInvoke call in the else branch. Somehow we need to abort the running Console.ReadLine() call rather than let it run to completion, because it may never complete.
So all this (complex) code helped me to get Console.ReadLine to return at a timeout, but does not end the Console.ReadLine to quit or otherwise go away.
How can we make this to work correctly, without running into resource leaks?
NB: I added the AsyncWaitHandle.Close() as advised by MS Calling Sync calls asynchronously
After reading a lot of comments on several similar questions, as mentioned, I come to believe there is no real solution here. The Microsoft way with Begin/EndInvoke is
rather complex, and:
not adequate
A more straightforward method is to run the synchronous call in another thread, use a timing method to keep track of the timeout, and use Thread.Abort() to get rid of the timed-out synchronous call.
Caveat:
The synchronous call may or may not support to be aborted. For example, Console.ReadLine() will be aborted OK, but if you restart the thread, no data will be read from the Console anymore.
The accepted solution on the original question on top of my posting above uses a second thread, and a timing method. However, it does not kill the sychronous call but keeps it running because it is needed for subsequent async calls, which is a fine hack.
The code for using a second thread is actually straighforward:
public class MySyncProc
{
/// <summary>
/// Value returned from the process after completion
/// </summary>
public string Result = null;
...other shared data...
public MySyncProc() { }
public void Run()
{
Result = LengthyProcess(...);
return;
}
}
public string Run(int TimeoutMs)
{
MySyncProc SyncP = new MySyncProc() { arg1 = ..., etc };
//
Thread T = new Thread(SyncP.Run);
T.Start();
//
DateTime StopTime = DateTime.Now.AddMilliseconds(TimeoutMs);
while (DateTime.Now < StopTime && SyncP.Result == null)
{
Thread.Sleep(200);
}
if (T.IsAlive)
{
T.Abort();
Console.WriteLine("Aborted thread for: {0}", Name);
}
return SyncP.Result;
}
If you don't like the polling, use the slightly more complex AutoResetEvent as in the mentioned accepted solution.
I am working on an application that talks to a motion controller over ethernet.
To connect to the controller I use a library provided by the supplier, to connect you create an instance of the controller than then tell it to connect, this has the chance to block for a few seconds (with no controllable timeout) if there is no controller present. This cause freeze ups in the UI.
To avoid this I thought I would be able to use Tasks to run the connection in a different thread.
ConnectionTask = Task.Factory.StartNew(() =>
{
try
{
RMCLink rmc = RMCLink.CreateEthernetLink(DeviceType.RMC70, "192.168.0.55");
RMC.Connect();
}
catch
{
this.logger.Log("Failed to connect");
}
}, TaskCreationOptions.LongRunning);
This has no effect whatsoever and the UI still locks up.
I think I am using them properly as if I replace it with the below code the UI is fine even though the separate thread takes a few seconds before the message comes out.
ConnectionTask = Task.Factory.StartNew(() =>
{
int x = 1;
while (x != 0) x++;
this.logger.Log("Failed to connect");
}, TaskCreationOptions.LongRunning);
Is there any way I can identify what is going on and prevent calls that I do not know anything about their inner workings from locking the UI thread.
Use async/await, something along the lines of:
public async void MyButton_Click(object sender, EventArgs e)
{
await CreateEthernetLink();
this.logger.Log("Connected!");
}
private async Task CreateEthernetLink()
{
var task = Task.Run(() => {
try
{
RMCLink rmc = RMCLink.CreateEthernetLink(DeviceType.RMC70, "192.168.0.55");
rmc.Connect();
}
catch
{
this.logger.Log("Failed to connect");
}});
await task;
}
The await will capture the current thread (or SynchronizationContext - in this case the UI thread which is being blocked) and restore it after the async work has been completed.
So the threading is all handled for you behind the scenes and you should notice no difference in your application other than the fact that your application no longer freezes when performing connections.
EDIT: I also noticed in your code your initializing rmc but calling connect on RMC. I don't think this is correct.
I have a Thread (STAThread) in a Windows Service, which performs a big amount of work. When the windows service is restarted I want to stop this thread gracefully.
I know of a couple of ways
A volatile boolean
ManualResetEvent
CancellationToken
As far as I have found out Thread.Abort is a no go...
What is the best practice ?
The work is perfomed in another class than the one where the thread is started, so it is necessary to either introduce a cancellationToken parameter in a constructor or for example have a volatile variable. But I just can't figure out what is smartest.
Update
Just to clarify a little I have wrapped up a very simple example of what I'm talking about. As said earlier, this is being done in a windows service. Right now I'm thinking a volatile boolean that is checked on in the loop or a cancellationToken....
I cannot wait for the loop to finish, as stated below it can take several minutes, making the system administrators of the server believe that something is wrong with the service when they need to restart it.... I can without problems just drop all the work within the loop without problems, however I cannot do this with a Thread.Abort it is "evil" and furthermore a COM interface is called, so a small clean up is needed.
Class Scheduler{
private Thread apartmentThread;
private Worker worker;
void Scheduling(){
worker = new Worker();
apartmentThread = new Thread(Run);
apartmentThread.SetApartmentState(ApartmentState.STA);
apartmentThread.Start();
}
private void Run() {
while (!token.IsCancellationRequested) {
Thread.Sleep(pollInterval * MillisecondsToSeconds);
if (!token.IsCancellationRequested) {
worker.DoWork();
}
}
}
}
Class Worker{
//This will take several minutes....
public void DoWork(){
for(int i = 0; i < 50000; i++){
//Do some work including communication with a COM interface
//Communication with COM interface doesn't take long
}
}
}
UPDATE
Just examined performance, using a cancellationToken where the isCancelled state is "examined" in the code, is much faster than using a waitOne on a ManualResetEventSlim. Some quick figuers, an if on the cancellationToken iterating 100.000.000 times in a for loop costs me approx. 500 ms, where the WaitOne costs approx. 3 seconds. So performance in this scenario it is faster to use the cancellationToken.
You haven't posted enough of your implementation but I would highly recommend a CancellationToken if that is available to you. It's simple enough to use and understand from a maintainability standpoint. You can setup cooperative cancellation as well too if you decide to have more than one worker thread.
If you find yourself in a situation where this thread may block for long periods of time, it's best to setup your architecture so that this doesn't occur. You shouldn't be starting threads that won't play nice when you tell them to stop. If they don't stop when you ask them, the only real way is to tear down the process and let the OS kill them.
Eric Lippert posted a fantastic answer to a somewhat-related question here.
I tend to use a bool flag, a lock object and a Terminate() method, such as:
object locker = new object();
bool do_term = false;
Thread thread = new Thread(ThreadStart(ThreadProc));
thread.Start();
void ThreadProc()
{
while (true) {
lock (locker) {
if (do_term) break;
}
... do work...
}
}
void Terminate()
{
lock (locker) {
do_term = true;
}
}
Asides from Terminate() all the other fields and methods are private to the "worker" class.
Use a WaitHandle, most preferably a ManualResetEvent. Your best bet is to let whatever is in your loop finish. This is the safest way to accomplish your goal.
ManualResetEvent _stopSignal = new ManualResetEvent(false); // Your "stopper"
ManualResetEvent _exitedSignal = new ManualResetEvent(false);
void DoProcessing() {
try {
while (!_stopSignal.WaitOne(0)) {
DoSomething();
}
}
finally {
_exitedSignal.Set();
}
}
void DoSomething() {
//Some work goes here
}
public void Terminate() {
_stopSignal.Set();
_exitedSignal.WaitOne();
}
Then to use it:
Thread thread = new Thread(() => { thing.DoProcessing(); });
thread.Start();
//Some time later...
thing.Terminate();
If you have a particularly long-running process in your "DoSomething" implementation, you may want to call that asynchronously, and provide it with state information. That can get pretty complicated, though -- better to just wait until your process is finished, then exit, if you are able.
There are two situations in which you may find your thread:
Processing.
Blocking.
In the case where your thread is processing something, you must wait for your thread to finish processing in order for it to safely exit. If it's part of a work loop, then you can use a boolean flag to terminate the loop.
In the case where your thread is blocking, then you need to wake your thread and get it processing again. A thread may be blocking on a ManualResetEvent, a database call, a socket call or whatever else you could block on. In order to wake it up, you must call the Thread.Interrupt() method which will raise a ThreadInterruptedException.
It may look something like this:
private object sync = new object():
private bool running = false;
private void Run()
{
running = true;
while(true)
{
try
{
lock(sync)
{
if(!running)
{
break;
}
}
BlockingFunction();
}
catch(ThreadInterruptedException)
{
break;
}
}
}
public void Stop()
{
lock(sync)
{
running = false;
}
}
And here is how you can use it:
MyRunner r = new MyRunner();
Thread t = new Thread(()=>
{
r.Run();
});
t.IsBackground = true;
t.Start();
// To stop the thread
r.Stop();
// Interrupt the thread if it's in a blocking state
t.Interrupt();
// Wait for the thread to exit
t.Join();