I have attached a backgroundWorker component to my main form which runs a parallel task of capturing the screen for an animated gif. The worker's function has a while loop that runs until I use CancelAsync() on the worker, at which point it exits the loop, does some other things like saving the gif file and so on and returns some results to the UI thread.
private bool capturing = false;
public MainForm()
{
InitializeComponent();
backgroundWorker.DoWork += backgroundWorker_DoWork;
backgroundWorker.RunWorkerCompleted += backgroundWorker_RunWorkerCompleted;
backgroundWorker.WorkerSupportsCancellation = true;
}
private void captureBtn_Click(object sender, EventArgs e)
{
Debug.WriteLine("Button clicked");
if (capturing) { return; }
if (!backgroundWorker.IsBusy)
{
backgroundWorker.RunWorkerAsync();
}
}
private void stopCaptureBtn_Click(object sender, EventArgs e)
{
if (backgroundWorker.IsBusy)
{
backgroundWorker.CancelAsync();
}
}
private void backgroundWorker_DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
{
capturing = true;
Debug.WriteLine("DoWork running");
while (!backgroundWorker.CancellationPending)
{
Debug.WriteLine("Capturing frame {0}", frames);
//do the capturing to memory stream
}
Debug.WriteLine("DoWork cancelled");
//do some other things like saving the gif etc
e.Result = someResult;
}
private void backgroundWorker_RunWorkerCompleted(object sender, System.ComponentModel.RunWorkerCompletedEventArgs e)
{
Debug.WriteLine("RunWorkerCompleted running");
capturing = false;
//does something with the e.Result
}
My console output during a normal test is something like this:
Button clicked
DoWork running
Capturing frame 0
Capturing frame 1
Capturing frame 2
Capturing frame 3
Capturing frame 4
Capturing frame 5
Cancel button clicked
DoWork cancelled
The thread 0x2e4c has exited with code 0 (0x0).
DoWork running
DoWork cancelled
The thread 0x1010 has exited with code 0 (0x0).
RunWorkerCompleted running
The function seems to be running twice, I can see 2 separate threads exiting and also I don't seem to get any results from capturing. If I set a breakpoint inside the backgroundWorker_DoWork function and continue later, the first run does the capturing normally. What could be going on?
Its getting called twice because binding the event second time right after InitializeComponent().
Just comment those lines and it should work fine.
Here's the same example without the issue of running twice.
Sample Output
...
...
...
Capturing frame 2632
Capturing frame 2633
Capturing frame 2634
DoWork cancelled
RunWorkerCompleted running
public partial class Form1 : Form
{
private bool capturing = false;
public Form1()
{
InitializeComponent();
backgroundWorker1.WorkerSupportsCancellation = true;
// Don't need to re-bind
//backgroundWorker1.DoWork += backgroundWorker1_DoWork;
//backgroundWorker1.RunWorkerCompleted += backgroundWorker1_RunWorkerCompleted;
//backgroundWorker1.WorkerSupportsCancellation = true;
}
private void captureBtn_Click(object sender, EventArgs e)
{
Debug.WriteLine("Button clicked");
if (capturing) { return; }
if (!backgroundWorker1.IsBusy)
{
backgroundWorker1.RunWorkerAsync();
}
}
private void stopCaptureBtn_Click(object sender, EventArgs e)
{
if (backgroundWorker1.IsBusy)
{
backgroundWorker1.CancelAsync();
}
}
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
capturing = true;
Debug.WriteLine("DoWork running");
int frames = 1;
while (!backgroundWorker1.CancellationPending)
{
Debug.WriteLine("Capturing frame {0}", frames);
//do the capturing to memory stream
frames++;
}
Debug.WriteLine("DoWork cancelled");
//do some other things like saving the gif etc
//e.Result = someResult;
}
private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
Debug.WriteLine("RunWorkerCompleted running");
capturing = false;
//does something with the e.Result
}
The thread 0x3108 has exited with code 0 (0x0) means there was no error.
While reading or writing the big data, you should divide it into parts. Otherwise, you cannot progress. Your current while loop is disabled when writing to memory stream.
so your backgroundWorker_DoWork method should be like this :
private void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
var worker = (BackgroundWorker)sender;
// this is an example file
string filePath = #"C:\file.gif";
// it determines how many bytes of data will be received from the stream each time.
byte[] buffer = new byte[4096];
int byteCount = 0;
// we are preparing to read stream.
using (FileStream fs = new FileStream(filePath, FileMode.Open, System.IO.FileAccess.Read))
using (MemoryStream ms = new MemoryStream())
{
// read the stream bytes and fill buffer
while ((byteCount = fs.Read(buffer, 0, buffer.Length)) > 0)
{
// if the task was canceled
if (worker.CancellationPending)
{
e.Cancel = true;
break;
}
// write buffer to memory stream
ms.Write(buffer, 0, byteCount);
}
}
}
I have a lengthily processing in my winform when I click a button; namely, i'm loading lots of files and processing them. For the duration of the processing, my GUI is frozen and unresponsive which is a problem since the processing can take an upward of 10 minutes. Is there a way of putting the code in some sort of bubble or something so I can use the GUI while processing the files? Maybe even add the "Cancel" button.
EDIT: René's solution works, also here's progressbar control I also wanted:
private async void button1_Click(object sender, EventArgs e)
{
progressBar1.Maximum = ValueWithTOtalNumberOfIterations.Length;
IProgress<int> progress = new Progress<int>(value => { progressBar1.Value = value;});
await Task.Run(() =>
{
var tempCount = 0;
//long processing here
//after each iteration:
if (progress != null)
{
progress.Report((tempCount));
}
tempCount++;
}
}
You could simply make your button's click handler async and start a Task for your long running operation:
public async void button1_Click(object sender, EventArgs e)
{
button1.Enabled = false; // disable button to not get called twice
await Task.Run(() =>
{
// process your files
}
button1.Enabled = true; // re-enable button
}
The compiler turns this into a state machine. The control flow is returned to the caller (the UI) at the await keyword. And execution of this method is resumed when your Task has completed.
To use a "Cancel"-Button you can use a TaskCancellationSource or simply define a flag that you check while you're processing your files, and return if the flag is set (by the click handler of your "Cancel" button):
private bool _stop = false;
private void cancelButton_Click(object sender, EventArgs e)
{
_stop = true;
}
private async void button1_Click(object sender, EventArgs e)
{
button1.Enabled = false; // disable button to not get called twice
_stop = false;
await Task.Run(() =>
{
// process your files
foreach(var file in files)
{
if (_stop) return;
// process file
}
}
button1.Enabled = true; // re-enable button
}
I'm working on an application that should read the data from Mifare smart card. I have to create a form will check the Mifare reader periodically and when the card is in range, read its serial number and send it to the parent form. I managed to get the background worker to read the serial number, but I can't close the form from it due to cross thread calling error it would cause. Is there a way to monitor the work that backGroundWorker does, and when it successfully reads the card ID, to stop it and close the child form?
This is the code I'm using in the DoWork method:
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
BackgroundWorker worker = sender as BackgroundWorker;
while (!worker.CancellationPending)
{
MifareReader.CommPort = 4;
MifareReader.PortOpen = true;
MifareReader.mfAutoMode(true);
MifareReader.mfRequest();
if (CardID == "0" || CardID == string.Empty)
{
MifareReader.mfRequest();
CardID = MifareReader.mfAnticollision().ToString();
MifareReader.mfHalt();
}
else if (CardID != "0" && CardID != string.Empty)
{
MessageBox.Show(ObrnutiID);
worker.CancelAsync();
}
MifareCitac.mfHalt();
}
}
This code does it's job, but I have to manually close the form. Is there a way to check if the CardID variable changes it's value in the main thread and if it does, close the form.
I tried to solve this problem by using a timer, but when I do that, the timer blocks the main form thread, and I can't close it manually (which of course I have to be able). Can you please suggest a way to solve this problem?
You can use the BackgroundWorker.RunWorkerCompleted event to monitor when the BackgroundWorker is done.
Occurs when the background operation has completed, has been canceled, or has raised an exception.
From there, you could close the form programmatically.
You can write your BackgroundWorker so that it returns when it has finished its work.
Then in your main form, subscribe to the BackgroundWorker.RunWorkerCompleted and respond appropriately, presumably by just closing the form.
Add RunWorkerCompleted event from your backgroundWorker:
private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
this.Close() // Closes the form.
}
You can avoid cross-thread calling error by checking InvokedRequired and try to BeginInvoke using delegates.
As others mentioned, here is how to implement the BackgroundWorker.RunWorkerCompleted event:
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
bool found = false;
var worker = sender as BackgroundWorker;
while (!worker.CancellationPending && !found)
{
MifareReader.CommPort = 4;
MifareReader.PortOpen = true;
MifareReader.mfAutoMode(true);
MifareReader.mfRequest();
if (CardID == "0" || CardID == string.Empty)
{
MifareReader.mfRequest();
CardID = MifareReader.mfAnticollision().ToString();
MifareReader.mfHalt();
}
else
{
e.Result = ObrnutiID;
found = true;
MifareCitac.mfHalt();
}
}
if (worker.CancellationPending)
{
e.Cancel = true;
}
}
private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
if (e.Cancelled)
{
// The user canceled the operation.
MessageBox.Show("Operation was canceled");
}
else if (e.Error != null)
{
// There was an error during the operation.
string msg = String.Format("An error occurred: {0}", e.Error.Message);
MessageBox.Show(msg);
}
else
{
// The operation completed normally.
string msg = String.Format("Result = {0}", e.Result);
MessageBox.Show(msg);
}
this.Close() // Closes the form.
}
I have written code to save an image which is generated by the application. The size of the image is around 32-35 MB. While saving the image to a BMB file, it is taking a long time, around 3-5 secs. For this purpose, I have used a background worker but when running the background worker, it shows an error like..."can't access the object as it is created on different thread".
Following is the code:
private void btnSaveDesign_Click(object sender, RoutedEventArgs e)
{
Microsoft.Win32.SaveFileDialog sfd = new Microsoft.Win32.SaveFileDialog();
sfd.Title = "Save design as...";
sfd.Filter = "BMP|*.bmp";
if (sfd.ShowDialog() == true)
{
ww = new winWait();
ww.Show();
System.ComponentModel.BackgroundWorker bw = new System.ComponentModel.BackgroundWorker();
bw.DoWork += new System.ComponentModel.DoWorkEventHandler(bw_DoWork);
bw.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);
fName = sfd.FileName;
cache = new CachedBitmap((BitmapSource)imgOut.Source, BitmapCreateOptions.None, BitmapCacheOption.OnLoad);
bw.RunWorkerAsync();
}
}
void bw_RunWorkerCompleted(object sender, System.ComponentModel.RunWorkerCompletedEventArgs e)
{
ww.Close();
}
void bw_DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
{
BmpBitmapEncoder encoder = new BmpBitmapEncoder();
encoder.Frames.Add(BitmapFrame.Create(cache)); //here... it says cant access...
using (FileStream file = File.OpenWrite(fName))
{
encoder.Save(file);
}
}
I have declared "cache" as a global object. (A similar trick worked when I was programming in Windows Forms with VB.NET.)
ww is the wait window that I want to be displayed while the precess is being executed.
How to do this? Is there any other simple method for multi threading in WPF?
When WPF objects are created they are assigned to a Dispatcher object. This disallows any threads other than the creating thread to access the object. This can be circumvented by freezing the object by calling the freeze method. You would need to call Freeze on your bitmapsource object. Once you have frozen your object it becomes uneditable
Your problem comes about because you are accessing an object which is not created by the background worker thread. Normally this would happen if you access a UI control which is created in the main thread and accessed from different thread.
Use the code below.
Dispatcher.Invoke
(
new Action(
delegate()
{
BmpBitmapEncoder encoder = new BmpBitmapEncoder();
encoder.Frames.Add(BitmapFrame.Create(cache));
using (FileStream file = File.OpenWrite(fName))
{
encoder.Save(file);
}
}
)
);
I think you have to pass cache as a parameter to the new thread:
bw.RunWorkerAsync(cache);
and get it from the DoWork method:
var cache=(CacheType) e.Argument;
.NET framework provides a simple way to get started in threading with
the BackgroundWorker component. This wraps much of the complexity and
makes spawning a background thread relatively safe. In addition, it
allows you to communicate between your background thread and your UI
thread without doing any special coding. You can use this component
with WinForms and WPF applications. The BackgroundWorker offers
several features which include spawning a background thread, the
ability to cancel the background process before it has completed, and
the chance to report the progress back to your UI.
public BackgroudWorker()
{
InitializeComponent();
backgroundWorker = ((BackgroundWorker)this.FindResource("backgroundWorker"));
}
private int DoSlowProcess(int iterations, BackgroundWorker worker, DoWorkEventArgs e)
{
int result = 0;
for (int i = 0; i <= iterations; i++)
{
if (worker != null)
{
if (worker.CancellationPending)
{
e.Cancel = true;
return result;
}
if (worker.WorkerReportsProgress)
{
int percentComplete =
(int)((float)i / (float)iterations * 100);
worker.ReportProgress(percentComplete);
}
}
Thread.Sleep(100);
result = i;
}
return result;
}
private void startButton_Click(object sender, RoutedEventArgs e)
{
int iterations = 0;
if (int.TryParse(inputBox.Text, out iterations))
{
backgroundWorker.RunWorkerAsync(iterations);
startButton.IsEnabled = false;
cancelButton.IsEnabled = true;
outputBox.Text = "";
}
}
private void cancelButton_Click(object sender, RoutedEventArgs e)
{
// TODO: Implement Cancel process
this.backgroundWorker.CancelAsync();
}
private void BackgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
// e.Result = DoSlowProcess((int)e.Argument);
var bgw = sender as BackgroundWorker;
e.Result = DoSlowProcess((int)e.Argument, bgw, e);
}
private void BackgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
workerProgress.Value = e.ProgressPercentage;
}
private void BackgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
if (e.Error != null)
{
MessageBox.Show(e.Error.Message);
}
else if (e.Cancelled)
{
outputBox.Text = "Canceled";
workerProgress.Value = 0;
}
else
{
outputBox.Text = e.Result.ToString();
workerProgress.Value = 0;
}
startButton.IsEnabled = true;
cancelButton.IsEnabled = false;
}
Consider a hypothetical method of an object that does stuff for you:
public class DoesStuff
{
BackgroundWorker _worker = new BackgroundWorker();
...
public void CancelDoingStuff()
{
_worker.CancelAsync();
//todo: Figure out a way to wait for BackgroundWorker to be cancelled.
}
}
How can one wait for a BackgroundWorker to be done?
In the past people have tried:
while (_worker.IsBusy)
{
Sleep(100);
}
But this deadlocks, because IsBusy is not cleared until after the RunWorkerCompleted event is handled, and that event can't get handled until the application goes idle. The application won't go idle until the worker is done. (Plus, it's a busy loop - disgusting.)
Others have add suggested kludging it into:
while (_worker.IsBusy)
{
Application.DoEvents();
}
The problem with that is that is Application.DoEvents() causes messages currently in the queue to be processed, which cause re-entrancy problems (.NET isn't re-entrant).
I would hope to use some solution involving Event synchronization objects, where the code waits for an event - that the worker's RunWorkerCompleted event handlers sets. Something like:
Event _workerDoneEvent = new WaitHandle();
public void CancelDoingStuff()
{
_worker.CancelAsync();
_workerDoneEvent.WaitOne();
}
private void RunWorkerCompletedEventHandler(sender object, RunWorkerCompletedEventArgs e)
{
_workerDoneEvent.SetEvent();
}
But I'm back to the deadlock: the event handler can't run until the application goes idle, and the application won't go idle because it's waiting for an Event.
So how can you wait for an BackgroundWorker to finish?
Update
People seem to be confused by this question. They seem to think that I will be using the BackgroundWorker as:
BackgroundWorker worker = new BackgroundWorker();
worker.DoWork += MyWork;
worker.RunWorkerAsync();
WaitForWorkerToFinish(worker);
That is not it, that is not what I'm doing, and that is not what is being asked here. If that were the case, there would be no point in using a background worker.
If I understand your requirement right, you could do something like this (code not tested, but shows the general idea):
private BackgroundWorker worker = new BackgroundWorker();
private AutoResetEvent _resetEvent = new AutoResetEvent(false);
public Form1()
{
InitializeComponent();
worker.DoWork += worker_DoWork;
}
public void Cancel()
{
worker.CancelAsync();
_resetEvent.WaitOne(); // will block until _resetEvent.Set() call made
}
void worker_DoWork(object sender, DoWorkEventArgs e)
{
while(!e.Cancel)
{
// do something
}
_resetEvent.Set(); // signal that worker is done
}
There is a problem with this response. The UI needs to continue to process messages while you are waiting, otherwise it will not repaint, which will be a problem if your background worker takes a long time to respond to the cancel request.
A second flaw is that _resetEvent.Set() will never be called if the worker thread throws an exception - leaving the main thread waiting indefinitely - however this flaw could easily be fixed with a try/finally block.
One way to do this is to display a modal dialog which has a timer that repeatedly checks if the background worker has finished work (or finished cancelling in your case). Once the background worker has finished, the modal dialog returns control to your application. The user can't interact with the UI until this happens.
Another method (assuming you have a maximum of one modeless window open) is to set ActiveForm.Enabled = false, then loop on Application,DoEvents until the background worker has finished cancelling, after which you can set ActiveForm.Enabled = true again.
Almost all of you are confused by the question, and are not understanding how a worker is used.
Consider a RunWorkerComplete event handler:
private void OnRunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
if (!e.Cancelled)
{
rocketOnPad = false;
label1.Text = "Rocket launch complete.";
}
else
{
rocketOnPad = true;
label1.Text = "Rocket launch aborted.";
}
worker = null;
}
And all is good.
Now comes a situation where the caller needs to abort the countdown because they need to execute an emergency self-destruct of the rocket.
private void BlowUpRocket()
{
if (worker != null)
{
worker.CancelAsync();
WaitForWorkerToFinish(worker);
worker = null;
}
StartClaxon();
SelfDestruct();
}
And there is also a situation where we need to open the access gates to the rocket, but not while doing a countdown:
private void OpenAccessGates()
{
if (worker != null)
{
worker.CancelAsync();
WaitForWorkerToFinish(worker);
worker = null;
}
if (!rocketOnPad)
DisengageAllGateLatches();
}
And finally, we need to de-fuel the rocket, but that's not allowed during a countdown:
private void DrainRocket()
{
if (worker != null)
{
worker.CancelAsync();
WaitForWorkerToFinish(worker);
worker = null;
}
if (rocketOnPad)
OpenFuelValves();
}
Without the ability to wait for a worker to cancel, we must move all three methods to the RunWorkerCompletedEvent:
private void OnRunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
if (!e.Cancelled)
{
rocketOnPad = false;
label1.Text = "Rocket launch complete.";
}
else
{
rocketOnPad = true;
label1.Text = "Rocket launch aborted.";
}
worker = null;
if (delayedBlowUpRocket)
BlowUpRocket();
else if (delayedOpenAccessGates)
OpenAccessGates();
else if (delayedDrainRocket)
DrainRocket();
}
private void BlowUpRocket()
{
if (worker != null)
{
delayedBlowUpRocket = true;
worker.CancelAsync();
return;
}
StartClaxon();
SelfDestruct();
}
private void OpenAccessGates()
{
if (worker != null)
{
delayedOpenAccessGates = true;
worker.CancelAsync();
return;
}
if (!rocketOnPad)
DisengageAllGateLatches();
}
private void DrainRocket()
{
if (worker != null)
{
delayedDrainRocket = true;
worker.CancelAsync();
return;
}
if (rocketOnPad)
OpenFuelValves();
}
Now I could write my code like that, but I'm just not gonna. I don't care, I'm just not.
You can check into the RunWorkerCompletedEventArgs in the RunWorkerCompletedEventHandler to see what the status was. Success, canceled or an error.
private void RunWorkerCompletedEventHandler(sender object, RunWorkerCompletedEventArgs e)
{
if(e.Cancelled)
{
Console.WriteLine("The worker was cancelled.");
}
}
Update: To see if your worker has called .CancelAsync() by using this:
if (_worker.CancellationPending)
{
Console.WriteLine("Cancellation is pending, no need to call CancelAsync again");
}
You don't wait for the background worker to complete. That pretty much defeats the purpose of launching a separate thread. Instead, you should let your method finish, and move any code that depends on completion to a different place. You let the worker tell you when it's done and call any remaining code then.
If you want to wait for something to complete use a different threading construct that provides a WaitHandle.
Why can't you just tie into the BackgroundWorker.RunWorkerCompleted Event. It's a callback that will "Occur when the background operation has completed, has been canceled, or has raised an exception."
I don't understand why you'd want to wait for a BackgroundWorker to complete; it really seems like the exact opposite of the motivation for the class.
However, you could start every method with a call to worker.IsBusy and have them exit if it is running.
Hm maybe I am not getting your question right.
The backgroundworker calls the WorkerCompleted event once his 'workermethod' (the method/function/sub that handles the backgroundworker.doWork-event) is finished so there is no need for checking if the BW is still running.
If you want to stop your worker check the cancellation pending property inside your 'worker method'.
The workflow of a BackgroundWorker object basically requires you to handle the RunWorkerCompleted event for both normal execution and user cancellation use cases. This is why the property RunWorkerCompletedEventArgs.Cancelled exists. Basically, doing this properly requires that you consider your Cancel method to be an asynchronous method in itself.
Here's an example:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.ComponentModel;
namespace WindowsFormsApplication1
{
public class AsyncForm : Form
{
private Button _startButton;
private Label _statusLabel;
private Button _stopButton;
private MyWorker _worker;
public AsyncForm()
{
var layoutPanel = new TableLayoutPanel();
layoutPanel.Dock = DockStyle.Fill;
layoutPanel.ColumnStyles.Add(new ColumnStyle());
layoutPanel.ColumnStyles.Add(new ColumnStyle());
layoutPanel.RowStyles.Add(new RowStyle(SizeType.AutoSize));
layoutPanel.RowStyles.Add(new RowStyle(SizeType.Percent, 100));
_statusLabel = new Label();
_statusLabel.Text = "Idle.";
layoutPanel.Controls.Add(_statusLabel, 0, 0);
_startButton = new Button();
_startButton.Text = "Start";
_startButton.Click += HandleStartButton;
layoutPanel.Controls.Add(_startButton, 0, 1);
_stopButton = new Button();
_stopButton.Enabled = false;
_stopButton.Text = "Stop";
_stopButton.Click += HandleStopButton;
layoutPanel.Controls.Add(_stopButton, 1, 1);
this.Controls.Add(layoutPanel);
}
private void HandleStartButton(object sender, EventArgs e)
{
_stopButton.Enabled = true;
_startButton.Enabled = false;
_worker = new MyWorker() { WorkerSupportsCancellation = true };
_worker.RunWorkerCompleted += HandleWorkerCompleted;
_worker.RunWorkerAsync();
_statusLabel.Text = "Running...";
}
private void HandleStopButton(object sender, EventArgs e)
{
_worker.CancelAsync();
_statusLabel.Text = "Cancelling...";
}
private void HandleWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
if (e.Cancelled)
{
_statusLabel.Text = "Cancelled!";
}
else
{
_statusLabel.Text = "Completed.";
}
_stopButton.Enabled = false;
_startButton.Enabled = true;
}
}
public class MyWorker : BackgroundWorker
{
protected override void OnDoWork(DoWorkEventArgs e)
{
base.OnDoWork(e);
for (int i = 0; i < 10; i++)
{
System.Threading.Thread.Sleep(500);
if (this.CancellationPending)
{
e.Cancel = true;
e.Result = false;
return;
}
}
e.Result = true;
}
}
}
If you really really don't want your method to exit, I'd suggest putting a flag like an AutoResetEvent on a derived BackgroundWorker, then override OnRunWorkerCompleted to set the flag. It's still kind of kludgy though; I'd recommend treating the cancel event like an asynchronous method and do whatever it's currently doing in the RunWorkerCompleted handler.
I'm a little late to the party here (about 4 years) but what about setting up an asynchronous thread that can handle a busy loop without locking the UI, then have the callback from that thread be the confirmation that the BackgroundWorker has finished cancelling?
Something like this:
class Test : Form
{
private BackgroundWorker MyWorker = new BackgroundWorker();
public Test() {
MyWorker.DoWork += new DoWorkEventHandler(MyWorker_DoWork);
}
void MyWorker_DoWork(object sender, DoWorkEventArgs e) {
for (int i = 0; i < 100; i++) {
//Do stuff here
System.Threading.Thread.Sleep((new Random()).Next(0, 1000)); //WARN: Artificial latency here
if (MyWorker.CancellationPending) { return; } //Bail out if MyWorker is cancelled
}
}
public void CancelWorker() {
if (MyWorker != null && MyWorker.IsBusy) {
MyWorker.CancelAsync();
System.Threading.ThreadStart WaitThread = new System.Threading.ThreadStart(delegate() {
while (MyWorker.IsBusy) {
System.Threading.Thread.Sleep(100);
}
});
WaitThread.BeginInvoke(a => {
Invoke((MethodInvoker)delegate() { //Invoke your StuffAfterCancellation call back onto the UI thread
StuffAfterCancellation();
});
}, null);
} else {
StuffAfterCancellation();
}
}
private void StuffAfterCancellation() {
//Things to do after MyWorker is cancelled
}
}
In essence what this does is fire off another thread to run in the background that just waits in it's busy loop to see if the MyWorker has completed. Once MyWorker has finished cancelling the thread will exit and we can use it's AsyncCallback to execute whatever method we need to follow the successful cancellation - it'll work like a psuedo-event. Since this is separate from the UI thread it will not lock the UI while we wait for MyWorker to finish cancelling. If your intention really is to lock and wait for the cancel then this is useless to you, but if you just want to wait so you can start another process then this works nicely.
I know this is really late (5 years) but what you are looking for is to use a Thread and a SynchronizationContext. You are going to have to marshal UI calls back to the UI thread "by hand" rather than let the Framework do it auto-magically.
This allows you to use a Thread that you can Wait for if needs be.
Imports System.Net
Imports System.IO
Imports System.Text
Public Class Form1
Dim f As New Windows.Forms.Form
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
BackgroundWorker1.WorkerReportsProgress = True
BackgroundWorker1.RunWorkerAsync()
Dim l As New Label
l.Text = "Please Wait"
f.Controls.Add(l)
l.Dock = DockStyle.Fill
f.StartPosition = FormStartPosition.CenterScreen
f.FormBorderStyle = Windows.Forms.FormBorderStyle.None
While BackgroundWorker1.IsBusy
f.ShowDialog()
End While
End Sub
Private Sub BackgroundWorker1_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
Dim i As Integer
For i = 1 To 5
Threading.Thread.Sleep(5000)
BackgroundWorker1.ReportProgress((i / 5) * 100)
Next
End Sub
Private Sub BackgroundWorker1_ProgressChanged(ByVal sender As Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged
Me.Text = e.ProgressPercentage
End Sub
Private Sub BackgroundWorker1_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
f.Close()
End Sub
End Class
Fredrik Kalseth's solution to this problem is the best I've found so far. Other solutions use Application.DoEvent() that can cause problems or simply don't work. Let me cast his solution into a reusable class. Since BackgroundWorker is not sealed, we can derive our class from it:
public class BackgroundWorkerEx : BackgroundWorker
{
private AutoResetEvent _resetEvent = new AutoResetEvent(false);
private bool _resetting, _started;
private object _lockObject = new object();
public void CancelSync()
{
bool doReset = false;
lock (_lockObject) {
if (_started && !_resetting) {
_resetting = true;
doReset = true;
}
}
if (doReset) {
CancelAsync();
_resetEvent.WaitOne();
lock (_lockObject) {
_started = false;
_resetting = false;
}
}
}
protected override void OnDoWork(DoWorkEventArgs e)
{
lock (_lockObject) {
_resetting = false;
_started = true;
_resetEvent.Reset();
}
try {
base.OnDoWork(e);
} finally {
_resetEvent.Set();
}
}
}
With flags and proper locking, we make sure that _resetEvent.WaitOne() really gets only called if some work has been started, otherwise _resetEvent.Set(); might never been called!
The try-finally ensures that _resetEvent.Set(); will be called, even if an exception should occur in our DoWork-handler. Otherwise the application could freeze forever when calling CancelSync!
We would use it like this:
BackgroundWorkerEx _worker;
void StartWork()
{
StopWork();
_worker = new BackgroundWorkerEx {
WorkerSupportsCancellation = true,
WorkerReportsProgress = true
};
_worker.DoWork += Worker_DoWork;
_worker.ProgressChanged += Worker_ProgressChanged;
}
void StopWork()
{
if (_worker != null) {
_worker.CancelSync(); // Use our new method.
}
}
private void Worker_DoWork(object sender, DoWorkEventArgs e)
{
for (int i = 1; i <= 20; i++) {
if (worker.CancellationPending) {
e.Cancel = true;
break;
} else {
// Simulate a time consuming operation.
System.Threading.Thread.Sleep(500);
worker.ReportProgress(5 * i);
}
}
}
private void Worker_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
progressLabel.Text = e.ProgressPercentage.ToString() + "%";
}
You can also add a handler to the RunWorkerCompleted event as shown here:
BackgroundWorker Class (Microsoft documentation).
Just wanna say I came here because I need a background worker to wait while I was running an async process while in a loop, my fix was way easier than all this other stuff^^
foreach(DataRow rw in dt.Rows)
{
//loop code
while(!backgroundWorker1.IsBusy)
{
backgroundWorker1.RunWorkerAsync();
}
}
Just figured I'd share because this is where I ended up while searching for a solution. Also, this is my first post on stack overflow so if its bad or anything I'd love critics! :)
Closing the form closes my open logfile. My background worker writes that logfile, so I can't let MainWin_FormClosing() finish until my background worker terminates. If I don't wait for my background worker to terminate, exceptions happen.
Why is this so hard?
A simple Thread.Sleep(1500) works, but it delays shutdown (if too long), or causes exceptions (if too short).
To shut down right after the background worker terminates, just use a variable. This is working for me:
private volatile bool bwRunning = false;
...
private void MainWin_FormClosing(Object sender, FormClosingEventArgs e)
{
... // Clean house as-needed.
bwInstance.CancelAsync(); // Flag background worker to stop.
while (bwRunning)
Thread.Sleep(100); // Wait for background worker to stop.
} // (The form really gets closed now.)
...
private void bwBody(object sender, DoWorkEventArgs e)
{
bwRunning = true;
BackgroundWorker bw = sender as BackgroundWorker;
... // Set up (open logfile, etc.)
for (; ; ) // infinite loop
{
...
if (bw.CancellationPending) break;
...
}
... // Tear down (close logfile, etc.)
bwRunning = false;
} // (bwInstance dies now.)
You can piggy back off of the RunWorkerCompleted event. Even if you've already added an event handler for _worker, you can add another an they will execute in the order in which they were added.
public class DoesStuff
{
BackgroundWorker _worker = new BackgroundWorker();
...
public void CancelDoingStuff()
{
_worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler((sender, e) =>
{
// do whatever you want to do when the cancel completes in here!
});
_worker.CancelAsync();
}
}
this could be useful if you have multiple reasons why a cancel may occur, making the logic of a single RunWorkerCompleted handler more complicated than you want. For instance, cancelling when a user tries to close the form:
void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
if (_worker != null)
{
_worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler((sender, e) => this.Close());
_worker.CancelAsync();
e.Cancel = true;
}
}
I use async method and await to wait for the worker finishing its job:
public async Task StopAsync()
{
_worker.CancelAsync();
while (_isBusy)
await Task.Delay(1);
}
and in DoWork method:
public async Task DoWork()
{
_isBusy = true;
while (!_worker.CancellationPending)
{
// Do something.
}
_isBusy = false;
}
You may also encapsulate the while loop in DoWork with try ... catch to set _isBusy is false on exception. Or, simply check _worker.IsBusy in the StopAsync while loop.
Here is an example of full implementation:
class MyBackgroundWorker
{
private BackgroundWorker _worker;
private bool _isBusy;
public void Start()
{
if (_isBusy)
throw new InvalidOperationException("Cannot start as a background worker is already running.");
InitialiseWorker();
_worker.RunWorkerAsync();
}
public async Task StopAsync()
{
if (!_isBusy)
throw new InvalidOperationException("Cannot stop as there is no running background worker.");
_worker.CancelAsync();
while (_isBusy)
await Task.Delay(1);
_worker.Dispose();
}
private void InitialiseWorker()
{
_worker = new BackgroundWorker
{
WorkerSupportsCancellation = true
};
_worker.DoWork += WorkerDoWork;
}
private void WorkerDoWork(object sender, DoWorkEventArgs e)
{
_isBusy = true;
try
{
while (!_worker.CancellationPending)
{
// Do something.
}
}
catch
{
_isBusy = false;
throw;
}
_isBusy = false;
}
}
To stop the worker and wait for it runs to the end:
await myBackgroundWorker.StopAsync();
The problems with this method are:
You have to use async methods all the way.
await Task.Delay is inaccurate. On my PC, Task.Delay(1) actually waits ~20ms.
oh man, some of these have gotten ridiculously complex. all you need to do is check the BackgroundWorker.CancellationPending property inside the DoWork handler. you can check it at any time. once it's pending, set e.Cancel = True and bail from the method.
// method here
private void Worker_DoWork(object sender, DoWorkEventArgs e)
{
BackgroundWorker bw = (sender as BackgroundWorker);
// do stuff
if(bw.CancellationPending)
{
e.Cancel = True;
return;
}
// do other stuff
}