Develop Reference

How to terminate a thread when the worker can't check the termination string

I have the following code running in a Windows form. The method it is calling takes about 40 seconds to complete, and I need to allow the user the ability to click an 'Abort' button to stop the thread running.
Normally I would have the Worker() method polling to see if the _terminationMessage was set to "Stop" but I can't do this here because the long running method, ThisMethodMightReturnSomethingAndICantChangeIt() is out of my control.
How do I implement this user feature please ?
Here is my thread code.
private const string TerminationValue = "Stop";
private volatile string _terminationMessage;
private bool RunThread()
{
try
{
var worker = new Thread(Worker);
_terminationMessage = "carry on";
_successful = false;
worker.Start();
worker.Join();
finally
{
return _successful;
}
}
private void Worker()
{
ThisMethodMightReturnSomethingAndICantChangeIt();
_successful = true;
}

Well, the simple answer would be "you can't". There's no real thread abort that you can use to cancel any processing that's happenning.
Thread.Abort will allow you to abort a managed thread, running managed code at the moment, but it's really just a bad idea. It's very easy to end up in an inconsistent state just because you were just now running a singleton constructor or something. In the end, there's quite a big chance you're going to blow something up.
A bit orthogonal to the question, but why are you still using threading code like this? Writing multi-threaded code is really hard, so you want to use as many high-level features as you can. The complexity can easily be seen already in your small snippet of code - you're Joining the newly created thread, which means that you're basically gaining no benefit whatsoever from starting the Worker method on a new thread - you start it, and then you just wait. It's just like calling Worker outright, except you'll save an unnecessary thread.
try will not catch exceptions that pop up in a separate thread. So any exception that gets thrown inside of Worker will simply kill your whole process. Not good.
The only way to implement reliable cancellation is through cooperative aborts. .NET has great constructs for this since 4.0, CancellationToken. It's easy to use, it's thread-safe (unlike your solution), and it can be propagated through all the method chain so that you can implement cancellation at depth. Sadly, if you simply can't modify the ThisMethodMightReturnSomethingAndICantChangeIt method, you're out of luck.
The only "supported" "cancellation" pattern that just works is Process.Kill. You'd have to launch the processing method in a wholy separate process, not just a separate thread. That can be killed, and it will not hurt your own process. Of course, it means you have to separate that call into a new process - that's usually quite tricky, and it's not a very good design (though it seems like you have little choice).
So if the method doesn't support some form of cancellation, just treat it like so. It can't be aborted, period. Any way that does abort it is a dirty hack.

Well, here's my solution so far. I will definitely read up on newer .NET higher level features as you suggest. Thanks for the pointers in the right direction
private void RunThread()
{
try
{
var worker = new Thread(Worker);
SetFormEnabledStatus(false);
_usuccessful = false;
worker.Start();
// give up if no response before timeout
worker.Join(60000); // TODO - Add timeout to config
worker.Abort();
}
finally
{
SetFormEnabledStatus(true);
}
}
private void Worker()
{
try
{
_successful= false;
ThisMethodMightReturnSomethingAndICantChangeIt();
_successful = true;
}
catch (ThreadAbortException ex)
{
// nlog.....
}
catch (Exception ex)
{
// nlog...
}
}

Weird InvokeRequired issue

I have a UserControl with a TreeView control called mTreeView on it. I can get data updates from multiple different threads, and these cause the TreeView to be updated. To do this, I've devised the following pattern:
all data update event handlers must acquire a lock and then check for InvokeRequired; if so, do the work by calling Invoke. Here's the relevant code:
public partial class TreeViewControl : UserControl
{
object mLock = new object();
void LockAndInvoke(Control c, Action a)
{
lock (mLock)
{
if (c.InvokeRequired)
{
c.Invoke(a);
}
else
{
a();
}
}
}
public void DataChanged(object sender, NewDataEventArgs e)
{
LockAndInvoke(mTreeView, () =>
{
// get the data
mTreeView.BeginUpdate();
// perform update
mTreeView.EndUpdate();
});
}
}
My problem is, sometimes, upon startup, I will get an InvalidOperationException on mTreeView.BeginUpdate(), saying mTreeView is being updated from a thread different than the one it was created. I go back in the call stack to my LockAndInvoke, and lo and behold, c.InvokeRequired is true but the else branch was taken! It's as if InvokeRequired had been set to true on a different thread after the else branch was taken.
Is there anything wrong with my approach, and what can I do to prevent this?
EDIT: my colleague tells me that the problem is that InvokeRequired is false until the control is created, so this is why it happens on startup. He's not sure what to do about it though. Any ideas?

It is a standard threading race. You are starting the thread too soon, before the TreeView is created. So your code sees InvokeRequired as false and fails when a split second later the native control gets created. Fix this by only starting the thread when the form's Load event fires, the first event that guarantees that all the control handles are valid.
Some mis-conceptions in the code btw. Using lock is unnecessary, both InvokeRequired and Begin/Invoke are thread-safe. And InvokeRequired is an anti-pattern. You almost always know that the method is going to be called by a worker thread. So use InvokeRequired only to throw an exception when it is false. Which would have allowed diagnosing this problem early.

When you marshal back to the UI thread, it's one thread--it can do only one thing at at time. You don't need any locks when you call Invoke.
The problem with Invoke is that it blocks the calling thread. That calling thread usually doesn't care about what get's completed on the UI thread. In that case I recommend using BeginInvoke to marshal the action back to the UI thread asynchronously. There are circumstances where the background thread can be blocked on Invoke while the UI thread can be waiting for the background thread to complete something and you end up with a deadlock: For example:
private bool b;
public void EventHandler(object sender, EventArgs e)
{
while(b) Thread.Sleep(1); // give up time to any other waiting threads
if(InvokeRequired)
{
b = true;
Invoke((MethodInvoker)(()=>EventHandler(sender, e)), null);
b = false;
}
}
... the above will deadlock on the while loop while because Invoke won't return until the call to EventHandler returns and EventHandler won't return until b is false...
Note my use of a bool to stop certain sections of code from running. This is very similar to lock. So, yes, you can end up having a deadlock by using lock.
Simply do this:
public void DataChanged(object sender, NewDataEventArgs e)
{
if(InvokeRequired)
{
BeginInvoke((MethodInvoker)(()=>DataChanged(sender, e)), null);
return;
}
// get the data
mTreeView.BeginUpdate();
// perform update
mTreeView.EndUpdate();
}
This simply re-invokes the DataChanged method asynchronously on the UI thread.

The pattern as you have shown it above looks 100% fine to me (albeit with some extra unnecessary locking, however I can't see how this would cause the problem you have described).
As David W points out, the only difference between what you are doing and this extension method is that you directly access mTreeView on the UI thread instead of passing it in as an argument to your action, however this will only make a difference if the value of mTreeView changes, and in any case you would have to try fairly hard to get this to cause the problem you have described.
Which means that the problem must be something else.
The only thing that I can think of is that you may have created mTreeView on a thread other than the UI thread - if this is the case then accessing the tree view will be 100% safe, however if you try and add that tree view to a form which was created on a different thread then it will go bang with an exception similar to the one that you describe.

How to kill a thread instantly in C#?

I am using the thread.Abort method to kill the thread, but it not working. Is there any other way of terminating the thread?
private void button1_Click(object sender, EventArgs e)
{
if (Receiver.IsAlive == true)
{
MessageBox.Show("Alive");
Receiver.Abort();
}
else
{
MessageBox.Show("Dead");
Receiver.Start();
}
}
I am using this but every time I get the Alive status, Receiver is my global thread.

The reason it's hard to just kill a thread is because the language designers want to avoid the following problem: your thread takes a lock, and then you kill it before it can release it. Now anyone who needs that lock will get stuck.
What you have to do is use some global variable to tell the thread to stop. You have to manually, in your thread code, check that global variable and return if you see it indicates you should stop.

You can kill instantly doing it in that way:
private Thread _myThread = new Thread(SomeThreadMethod);
private void SomeThreadMethod()
{
// do whatever you want
}
[SecurityPermissionAttribute(SecurityAction.Demand, ControlThread = true)]
private void KillTheThread()
{
_myThread.Abort();
}
I always use it and works for me:)

You should first have some agreed method of ending the thread. For example a running_ valiable that the thread can check and comply with.
Your main thread code should be wrapped in an exception block that catches both ThreadInterruptException and ThreadAbortException that will cleanly tidy up the thread on exit.
In the case of ThreadInterruptException you can check the running_ variable to see if you should continue. In the case of the ThreadAbortException you should tidy up immediately and exit the thread procedure.
The code that tries to stop the thread should do the following:
running_ = false;
threadInstance_.Interrupt();
if(!threadInstance_.Join(2000)) { // or an agreed resonable time
threadInstance_.Abort();
}

thread will be killed when it finish it's work, so if you are using loops or something else you should pass variable to the thread to stop the loop after that the thread will be finished.

C# Thread.Abort is NOT guaranteed to abort the thread instantaneously. It will probably work when a thread calls Abort on itself but not when a thread calls on another.
Please refer to the documentation: http://msdn.microsoft.com/en-us/library/ty8d3wta.aspx
I have faced this problem writing tools that interact with hardware - you want immediate stop but it is not guaranteed. I typically use some flags or other such logic to prevent execution of parts of code running on a thread (and which I do not want to be executed on abort - tricky).

How can a background thread hang the UI thread?

I am using a background thread to initialize an instrument over USB. The UI hangs when I try to open the device. I would expect the background thread to pause when calling Open on the device, but not the UI thread. I am testing this with no UI interaction from the background thread. I don't know how to debug the problem, and it's too broad a question, but perhaps someone has seen something like this before. There is nothing wrong with the ActiveX interop as far as I know, the device works correctly. This is the general approach:
using System;
using FancyVoltmeterLibrary;
namespace SOQuestion
{
public class MeterClass
{
private FancyVoltmeter meter;
private Thread meterThread;
public MeterClass()
{
// Create instance of ActiveX/COM object.
meter = new FancyVoltmeter();
meterThread = new Thread(UpdateMeter);
meterThread.Name = "Meter Thread";
meterThread.Priority = ThreadPriority.Normal;
meterThread.IsBackground = true;
meterThread.Start();
}
private void UpdateMeter()
{
while(true)
{
Thread.Sleep(1000);
if(!meter.IsOpen())
{
// Meter may be powered off here.
// The call to Open takes about 1 second.
// UI hangs during the call???
meter.Open();
}
// code to read meter goes here.
}
}
}
}
Edit: Perhaps unclear what I meant. By 'hang' I should say 'freezes momentarily'.

Does meter require running in an STA? Is the call to Open() actually being marshalled back to the UI thread for this reason?
You can verify this is true by looking at the callstack of the hung UI thread in the debugger.

How long time does the instantiation of the FancyVoltmeter take? Could it be that it is not the Open method that causes the UI freeze, but creating the COM object (which is done on the UI thread)?
If that turns out to be the case, moving the creation of this object to happen on the new, separate worker thread should take care of the problem.
Edit: I saw now that you already found this out in your comment to Michael...

I would suggest you wrap the call to meter.open() in a separate method, and call that method from within the updateMeter() method using Invoke() or BeginInvoke() construct on the form or parent control. Doing this will marshal the action back on to the UI thread and should execute gracefully. I hope this helps.

Consider using a BackgroundWorker for this task.

Is it possible to put an event handler on a different thread to the caller?

Lets say I have a component called Tasking (that I cannot modify) which exposes a method “DoTask” that does some possibly lengthy calculations and returns the result in via an event TaskCompleted. Normally this is called in a windows form that the user closes after she gets the results.
In my particular scenario I need to associate some data (a database record) with the data returned in TaskCompleted and use that to update the database record.
I’ve investigated the use of AutoResetEvent to notify when the event is handled. The problem with that is AutoResetEvent.WaitOne() will block and the event handler will never get called. Normally AutoResetEvents is called be a separate thread, so I guess that means that the event handler is on the same thread as the method that calls.
Essentially I want to turn an asynchronous call, where the results are returned via an event, into a synchronous call (ie call DoSyncTask from another class) by blocking until the event is handled and the results placed in a location accessible to both the event handler and the method that called the method that started the async call.
public class SyncTask
{
TaskCompletedEventArgs data;
AutoResetEvent taskDone;
public SyncTask()
{
taskDone = new AutoResetEvent(false);
}
public string DoSyncTask(int latitude, int longitude)
{
Task t = new Task();
t.Completed = new TaskCompletedEventHandler(TaskCompleted);
t.DoTask(latitude, longitude);
taskDone.WaitOne(); // but something more like Application.DoEvents(); in WinForms.
taskDone.Reset();
return data.Street;
}
private void TaskCompleted(object sender, TaskCompletedEventArgs e)
{
data = e;
taskDone.Set(); //or some other mechanism to signal to DoSyncTask that the work is complete.
}
}
In a Windows App the following works correctly.
public class SyncTask
{
TaskCompletedEventArgs data;
public SyncTask()
{
taskDone = new AutoResetEvent(false);
}
public string DoSyncTask(int latitude, int longitude)
{
Task t = new Task();
t.Completed = new TaskCompletedEventHandler(TaskCompleted);
t.DoTask(latitude, longitude);
while (data == null) Application.DoEvents();
return data.Street;
}
private void TaskCompleted(object sender, TaskCompletedEventArgs e)
{
data = e;
}
}
I just need to replicate that behaviour in a window service, where Application.Run isn’t called and the ApplicationContext object isn’t available.

I've had some trouble lately with making asynchronous calls and events at threads and returning them to the main thread.
I used SynchronizationContext to keep track of things. The (pseudo)code below shows what is working for me at the moment.
SynchronizationContext context;
void start()
{
//First store the current context
//to call back to it later
context = SynchronizationContext.Current;
//Start a thread and make it call
//the async method, for example:
Proxy.BeginCodeLookup(aVariable,
new AsyncCallback(LookupResult),
AsyncState);
//Now continue with what you were doing
//and let the lookup finish
}
void LookupResult(IAsyncResult result)
{
//when the async function is finished
//this method is called. It's on
//the same thread as the the caller,
//BeginCodeLookup in this case.
result.AsyncWaitHandle.WaitOne();
var LookupResult= Proxy.EndCodeLookup(result);
//The SynchronizationContext.Send method
//performs a callback to the thread of the
//context, in this case the main thread
context.Send(new SendOrPostCallback(OnLookupCompleted),
result.AsyncState);
}
void OnLookupCompleted(object state)
{
//now this code will be executed on the
//main thread.
}
I hope this helps, as it fixed the problem for me.

Maybe you could get DoSyncTask to start a timer object that checks for the value of your data variable at some appropriate interval. Once data has a value, you could then have another event fire to tell you that data now has a value (and shut the timer off of course).
Pretty ugly hack, but it could work... in theory.
Sorry, that's the best I can come up with half asleep. Time for bed...

I worked out a solution to the async to sync problem, at least using all .NET classes.
Link
It still doesn't work with COM. I suspect because of STA threading. The Event raised by the .NET component that hosts the COM OCX is never handled by my worker thread, so I get a deadlock on WaitOne().
someone else may appreciate the solution though :)

If Task is a WinForms component, it might be very aware of threading issues and Invoke the event handler on the main thread -- which seems to be what you're seeing.
So, it might be that it relies on a message pump happening or something. Application.Run has overloads that are for non-GUI apps. You might consider getting a thread to startup and pump to see if that fixes the issue.
I'd also recommend using Reflector to get a look at the source code of the component to figure out what it's doing.

You've almost got it. You need the DoTask method to run on a different thread so the WaitOne call won't prevent work from being done. Something like this:
Action<int, int> doTaskAction = t.DoTask;
doTaskAction.BeginInvoke(latitude, longitude, cb => doTaskAction.EndInvoke(cb), null);
taskDone.WaitOne();

My comment on Scott W's answer seems a little cryptic after I re-read it. So let me be more explicit:
while( !done )
{
taskDone.WaitOne( 200 );
Application.DoEvents();
}
The WaitOne( 200 ) will cause it to return control to your UI thread 5 times per second (you can adjust this as you wish). The DoEvents() call will flush the windows event queue (the one that handles all windows event handling like painting, etc.). Add two members to your class (one bool flag "done" in this example, and one return data "street" in your example).
That is the simplest way to get what you want done. (I have very similar code in an app of my own, so I know it works)

Your code is almost right... I just changed
t.DoTask(latitude, longitude);
for
new Thread(() => t.DoTask(latitude, longitude)).Start();
TaskCompleted will be executed in the same thread as DoTask does. This should work.