How to wait for a BackgroundWorker to cancel?

https://stackoverflow.com/questions/123661

02-07-2019
|

Question

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.

Solution

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
}

OTHER TIPS

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.

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! :)

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).

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

}

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