Simple question, to repeat the title:
Does closing the WinForms application stops all active BackgroundWorkers?
Yes, it does.
BackgroundWorker.RunWorkerAsync simply calls BeginInvoke on a internal delegate, which in turn queues the request to the ThreadPool. Since all ThreadPool threads are background, yes, it will end when the application ends.
However, keep in mind that:
By "closing the WinForms application" I am presuming closing the main Form instance (that's generally the one passed to Application.Run in the Program class autogenerated by Visual Studio). If you have a child window with a background worker, it will not stop its BackgroundWorker automatically.
Letting a background thread be aborted on application exit is not the recommended way to end the thread, as you have no guarantees where it will be aborted. A much better way would be to signal the worker before closing, wait for it to finish gracefully, and then exit.
More info: Delegate.BeginInvoke, MSDN on Thread Pooling, Thread.IsBackground
The only way a thread can go on executing after your main (UI) thread has stopped is if it has been created explicitely, by creating a new Thread instance and setting the IsBackground to false. If you don't (or if you use the ThreadPool which spawns background threads - or the BackgroundWorker which also uses the ThreadPool internally) your thread will be a background thread and will be terminated when the main thread ends.
BackgroundWorker threads are background threads (ThreadPool threads), which die when the application dies.
Yes, it will. I wrote this simple form, and closing the form exits the application:
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
this.backgroundWorker1.RunWorkerAsync();
}
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
while (true)
{
Thread.Sleep(100);
}
}
}
If the application completely closes (as in nothing is preventing it from shutting down) your background workers will also be gone.
Once the process is gone all associated threads are gone as well.
First of all, just to make this answer simple:
When a process has closed, all of its threads have terminated. There's no question about this.
The question, as I interpret it, thus becomes:
Will still-running BackgroundWorker instances prevent an application from closing?
The answer to that question is: No, they won't.
I think yes. Because threads are associated with process and if the process is closed all threads has to end.
Related
I am facing an issue with communication between threads in a C#.NET application.
Hope someone will guide me in the right direction about the possible solutions.
I have an application in C#.NET.It is a windows form application.
My application has two threads - One thread is the main thread (UI thread) and the other one is the child thread. Lets call the child thread the "workerThread"
There is only one form used in the application.Lets call this form the "MainForm"
The child thread is started when the MainForm loads (used the form's "Load" event handler to start the thread)
In the MainForm class, I have a variable named "stopWork" which is a public boolean variable and it serves as a flag to indicate whether the child thread should continue working or should it stop
I have another class (besides the MainForm class) which contains the method that I execute in the the child thread. Lets call this second class the "WorkerClass".
I pass a reference to the current form (the MainForm) into the constructor of the "WorkerClass"
I have a button "stop" in the main form which sets "stopWork" to "true" if its clicked and then calls "workerThread.Join()" to wait for the child thread to finish excecution.
In the child thread, the method "doWork" keeps checking the status of "parentForm.stopWork" inside a for loop. If "stopWork" is set to "true" then the loop breaks and subsequently the method ends.
Now, the issue is, once I am clicking the "stop" button ,the application hangs.
I am pasting parts of the code below so that it is easier to understand :
public partial class MainForm : Form
{
Thread workerThread = null;
ThreadStart workerThreadStart = null;
WorkerClass workerClass = null;
public bool stopWork = true;
/*.......... some code ............*/
private void MainForm_Load(object sender, EventArgs e)
{
workerThreadStart = new ThreadStart(startWork);
workerThread = new Thread(workerThreadStart);
stopWork = false;
workerThread.Start();
}
private void startWork()
{
workerClass = new WorkerClass(this);
}
private void buttonStop_Click(object sender, EventArgs e) //"stop" button
{
if (workerThread != null)
{
if (workerThread.IsAlive == true)
{
stopWork = true;
workerThread.Join();
}
}
}
/*.......... some more code ............*/
}
public class WorkerClass
{
MainForm parentForm=null;
/*......... some variables and code ........*/
public WorkerClass(MainForm parentForm)
{
this.parentForm=parentForm;
}
/* .............. some more code ...........*/
public void doWork()
{
/*.......... some variables and code ...........*/
for(int i=0;i<100000;i++)
{
// ** Here is the check to see if parentForm has set stopWork to true **
if(parentForm.stopWork==true)
break;
/*......... do some work in the loop ..........*/
}
}
/********* and more code .........*/
}
I think I may know where the problem lies.
The problem is in the "doWork" method in the child thread trying to access "stopWork" variable in the parent form when already the parent form is blocked by calling the "workerThread.Join()" method. So ,I think this is a "deadlock" problem.
Am I right in identifying the problem ? Or am I wrong and the problem lies somewhere else ?
In case this is indeed a deadlock, what are the possible solutions to solve this ?
I did a bit of googling and found lots of resources on thread synchronisation and how to avoid deadlocks. But I could not understand how to apply them specifically to my problem.
I would really appreciate any help or guidance on resolving this issue.
Yes, the code you wrote is highly vulnerable to deadlock. The BackgroundWorker class is especially prone to cause this kind of deadlock.
The problem is located in code we can't see in your snippet, the WorkerClass. You are surely doing something there that affects the UI in one way or another, always the primary reason to consider creating a thread in the first place. You probably use Control.Invoke() to have some code run on the UI thread and update a control. Perhaps also to signal that the worker thread is completed and, say, set the Enable property of a button back to true.
That's deadlock city, such code cannot run until the UI thread goes idle, back to pumping its message loop. It will never be idle in your case, it is stuck in Thread.Join(). The worker thread can't complete because the UI thread won't go idle, the UI thread can't go idle because the worker thread isn't finishing. Deadlock.
BackgroundWorker has this problem too, the RunWorkerCompleted event cannot run unless the UI thread is idle. What you need to do is not block the UI thread. Easier said than done, BGW can help you get this right because it runs an event when it completes. You can have this event do whatever you now do in the code past the Thread.Join() call. You'll need a boolean flag in your class to indicate that you are in the 'waiting for completion' state. This answer has relevant code.
Use a BackgroundWorker for this task instead. When you want to stop the task's execution, call the background worker's CancelAsync method.
Generally speaking, rolling your own threading code (on any platform) is a recipe for disaster if you don't have an expert-level understanding of multithreading (and even then it's still dangerous).
A button on the parent form is used to start the thread.
If the parent form is closed in the development environment the thread keeps running in the background preventing edits to the source code on a 64 bit Windows 7 platform.
The thread has to be killed by Menu > Debug > Stop Debugging.
What is the proper way to programmatically kill the thread when the parent Form is closed?
private void buttonW_Click(object sender, EventArgs e)
{
Thread t = new Thread(Main.MyThread);
t.Start();
}
private static void MyThread()
{
...
}
If you want the app to exit when the main thread has finished, you can just make the new thread a background thread:
Thread t = new Thread(Main.MyThread);
t.IsBackground = true;
t.Start();
Basically the process will exit when all the foreground threads have exited.
Note that this could be bad news if the background thread is writing a file when the form is closed, or something similar...
Environment.Exit(0) terminates this process and gives the underlying operating system the specified exit code. It is more generally used in console applications.
Environment.Exit(0);
The following code does not exit the application. How can I exit the application and make sure all the running threads are closed?
foreach (Form form in Application.OpenForms)
{
form.Close();
}
Application.Exit();
You don't show the use of any threads in your code, but let's suppose you do have threads in it. To close all your threads you should set all of them to background threads before you start them, then they will be closed automatically when the application exits, e.g.:
Thread myThread = new Thread(...);
myThread.IsBackground = true; // <-- Set your thread to background
myThread.Start(...);
A "HOWTO: Stop Multiple Threads" article from microsoft: http://msdn.microsoft.com/en-us/library/aa457093.aspx
You can try the following code:
Environment.Exit(Environment.ExitCode);
I went through a similar issue in my software, but unfortunately just making the threads to work in background didn't solve the issue. In fact while the thread brings back data (the main software is data driven) and if I close the application, it results to Windows Error, giving rise to a debugging message.
So what actually worked for me:
Step 1: Made all threads run in background such as
Thread aThread = new Thread(MethodName);
aThread.IsBackground = true; //<-- Set the thread to work in background
aThread.Start();
Step 2: In the closing action of the form/application call the Environment.Exit method, such as
Environment.Exit(Environment.ExitCode);
This kept the memory managed perfectly, with no memory leak.
Hope this helps.
This should work for all threads you opened.
protected override void OnExiting(Object sender, EventArgs args)
{
base.OnExiting(sender, args);
Environment.Exit(Environment.ExitCode);
}
This is the best way to make sure that your application closes (forcefully):
(Process.GetCurrentProcess()).Kill()
The problem with Environment.Exit is that it does not work unless it is on the main thread. Also, it sometimes thread locks.
The main reason that your program is not closing properly is because the form is not able to dispose itself (and thus all object that it created). Fixing this is much more difficult. I would recommend running the above code after waiting a while for any possible dispose handlers to get called first.
This got the job done for me:
Instead of using:
Application.Exit()
which leaves other threads open, try using:
Environment.Exit(Environment.ExitCode);
I puzzled with this for a while as the common answer is to make the threads you call mythread.IsBackground = true;
The common answer to run a thread is to use a while loop with a flag
while(!exitflag)
{
// thread running
}
When User presses the eXit forms button Visual studio still is running the threads but the form closes. I used the Form Closing event to set the exitcode flag and then wait for the threads to close.
I have a single form using a Timer and two additional threads, One collects streaming data and the other does calculations on that data and saves to an sql table. Data collection is collected in a circular buffer. I use two static bool values as flags where the threads set the flag to true or false using mythread.Isalive as the Form Closing event does not have access to the thread handles.
private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
Debug.WriteLine("Form Closing Event");
exitFlag = true;
int ThreadsClosed = 0;
while (ThreadsClosed < 2)
{
ThreadsClosed = 2;
if (ProcessDataAlive) ThreadsClosed = 0;
if (StreamingDataAlive) ThreadsClosed = 0;
Application.DoEvents();
}
Debug.WriteLine("Threads are all closed");
Thread.Sleep(5000); // allow debug window open to read remove for release
}
I have an application which does some background task (network listening & reading) in a separate Thread. It seems however that the Thread is not being Terminated/Aborted when I close the application (click "x" button on titlebar).
Is that because the main Thread routine is while(true) {...} ?
What is the solution here? I was looking for some "interruption" flag for the Thread as the condition for "while" loop, but didn't found any.
The simplest way is to set the IsBackground property of the thread to true. This will prevent it from keeping the application open. An application terminates when all non-background threads terminate.
A more controlled way to stop the thread is to send it a message to shut down cleanly and ensure that it has terminated before letting your main thread terminate.
A method that I wouldn't recommend is to call Thread.Abort. This has a number of problems, one of which is that it is not guaranteed to terminate the thread. From the documentation:
Calling this method usually terminates the thread.
Emphasis mine.
You can always force the issue:
class Program
{
public static void Main()
{
// ... do stuff
Environment.Exit(Environment.ExitCode);
}
}
The better approach is to set the Thread.IsBackground property to true as Mark already mentioned.
You could improve the while(true) loop to
void DoWork() {
while(!ShouldIQuitManualResetEvent.WaitOne(0)) {
// do something
}
IDidQuitManualResetEvent.Set()
}
A bit more graceful, short from the identifier names.
Well, instead of while(true), maybe you should:
while(appIsRunning)
{
}
And, at the closing event for your form,
appIsRunning = false;
thread.Join(2000);
where the last line is just to make sure you wait for the thread to cleanly finish. There are many other ways to force an end to a thread, but the problem is just there: you don't want to be forcing things, you want them to happen as naturally as possible.
After the join, you can check the status of thread to see if it has finished. If it doesn't, then (and only then) force its finish with a abort, and maybe notify your user (or write a log record) that something has not ended the way it should.
You can start your thread as:
ThreadPool.QueueUserWorkItem(DoStuff, input)
And it will be abort automatically with application close.
I'm looking for a way to restart a thread that has been stopped by Abort()..
public partial class MyProgram : Form
{
private Thread MyThread = new Thread(MyFunction);
private System.Windows.Forms.Button startStopBtn = new System.Windows.Forms.Button();
public MyProgram()
{
MyThread.Start();
startStopBtn += new EventHandler(doStop);
startStopBtn.Text = "Stop";
}
private static void MyFunction()
{
// do something
}
private void doStop(object sender, EventArgs e)
{
MyThread.Abort();
startStopBtn -= new EventHandler(doStop);
startStopBtn += new EventHandler(doStart);
startStopBtn.Text = "Start";
}
private void doStart(object sender, EventArgs e)
{
MyThread.Start(); // << Error returned when clicking the button for 2nd time
startStopBtn -= new EventHandler(doStart);
startStopBtn += new EventHandler(doStop);
startStopBtn.Text = "Stop";
}
}
Any idea?
Once you have aborted your thread, you cannot start it again.
But your actual problem is that you are aborting your thread. You should never use Thread.Abort().
If your thread should be paused and continued several times, you should consider using other mechanisms (like AutoResetEvent, for example).
[EDIT]
The simplest solution to abort a thread, as mentioned by Ian Griffiths in the link above, is:
The approach I always recommend is dead simple. Have a volatile bool field that is visible both to your worker thread and your UI thread. If the user clicks cancel, set this flag. Meanwhile, on your worker thread, test the flag from time to time. If you see it get set, stop what you're doing.
The only thing that you need to do to make it work properly, is to rearrange your background method so that it runs in a loop - so that you can periodically check if your flag has been set by a different thread.
If you need to have pause and resume functionality for the same worker thread, instead of the simple volatile bool flag approach, you could go for a slightly more complex approach, a synchronizing construct such as AutoResetEvent. These classes also provide a way to put the worker thread to sleep for a specified (or indefinite) amount of time between signals from the non-worker thread.
This thread contains a concrete example with Start, Pause, Resume and Stop methods. Note how Brannon's example never aborts the thread. It only fires an event, and then waits until the thread finishes gracefully.
Simply add MyThread = new Thread(MyFunction) before calling MyThread.Start() in doStart(). Do not create the thread outside of your methods, the space there is thought for declarations.
Please note that killing a thread with thread.Abort() can be very dangerous, as it might cause unexpected behavior or might not correctly dispose resources owned by the thread. You should try to accomplish clean multi threading, like Groo described in his answer.
The simple answer is, you can't. Once a thread has been aborted, you can't restart it. Just create a method or something, that returns a Thread object just how you need it. When you need a new Thread, just get it from that method.
No, there isn't, but why would you want to? Just start up a new thread, with the same ThreadStart, and the same parameter (if any).
If you really need to interrupt the thread function and resume, you should set a condition and then check it periodically during processing.
That would allow you to stop processing for some amount of time and then resume.
I've used events and Wait calls to accomplish a similar task.
The easiest way is to not abort the thread.
I really don't understand why people provide information if they do not know that is correct..
How can a real programmer suspend or stop processing a thread for sometime and then release it and thereby making the code vulnerable...
#Brad-- m sorry.. but your idea was not good..
#Rhythmic - You need to work on your way to approach things..
BFree was somewhat right if you people got him the same way he wanted to say..
You just need to re-declare that..
below is the example:
Public Shared Sub ResetAbort()
Dim ThreadPleaseWait As New Thread(New ThreadStart(AddressOf YourSubName))
YourThreadName.Start()
Thread.Sleep(2000)
YourThreadName.Abort()
End Sub
Now you can use this Sub anywhere you want to start the thread. It will automatically abort the thread.
If you want to start the thread on Button1_click() event and stop it on Button2_Click() event use this:
in Button1_click() event
Dim ThreadPleaseWait As New Thread(New ThreadStart(AddressOf YourSubName))
YourThreadName.Start()
in Button2_click() event
YourThreadName.Start()
doing this way you will abort you thread where ever you want and will initialize it again.
You can also use YourThreadName.ThreadState.Running property to check if the thread is running or not(Just to avoid multiple instances of the same thread.....