Develop Reference

How to stop background worker in WPF?

I am using an MVVM model in my WPF application. I have an command binding to the cancel button. I have a start button which starts a few background workers. When i click on the cancel button, i want all the background workers to stop/quit.
With my current code when i click on cancel button, the background worker does not stop and the "StartEngineeringOperation" finishes. Can anyone please help me out with what i am doing wrong here?
Current code:
For EngineeringViewModel.cs:
public class EngineeringViewModel{
public EngineeringViewModel()
{
StartEngineering= new DelegateCommand(o =>
{
worker = new BackgroundWorker
{
WorkerReportsProgress = true,
WorkerSupportsCancellation = true
};
worker.ProgressChanged += Worker_ProgressChanged;
worker.RunWorkerCompleted += worker_RunWorkerCompleted;
if (worker.IsBusy != true) worker.RunWorkerAsync();
worker.DoWork += (s, e) =>
{
StartEngineeringOperation();
if (worker.CancellationPending)
{
e.Cancel = true;
return;
}
};
},
k => true);
Cancel = new DelegateCommand(CancelEngineeringOperation);
}
private void StartEngineeringOperation()
{
startAlarmService();
startTrendQualityCheck();
}
private void CancelEngineeringOperation(object param)
{
worker.DoWork += (s, e) =>
{
if (worker.IsBusy)
{
worker.CancelAsync();
e.Cancel = true;
return;
}
};
}
}
I tried this :
but doesn't seem to work:
private void StartEngineeringOperation()
{
startAlarmService();
if (worker.CancellationPending)
{
e.Cancel = true;
return;
}
startTrendQualityCheck();
}

As you may have learned from te comments, you need to poll the state of the BackgroundWorker in your operations that you want to support cancellation. Then take measures to cancel the ongoing operation gracefully.
The example shows how to cancel a background thread on button click. The first example uses the old BackgroundWorker and the second the modern and cleaner Task library.
BackgroundWorker
private BackgroundWorker Worker { get; set; }
private void StartWorker()
{
this.Worker = new BackgroundWorker
{
WorkerReportsProgress = true,
WorkerSupportsCancellation = true
};
this.Worker.DoWork += BackgroundWorker_DoWork;
}
private void BackgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
BackgroundWorker worker = sender as BackgroundWorker;
DoCancellableWork();
// Stop BackgroundWorker from executing
if (worker.CancellationPending)
{
e.Cancel = true;
}
}
private void DoCancellableWork()
{
// Check for cancellation before executing the cancellable operation and allocating resources etc..
if (this.Worker.CancellationPending)
{
return;
}
// Periodically/regularly check for the cancellation flag
for (int i = 0; i <= 10000000000; i++)
{
if (this.Worker.CancellationPending)
{
// Cancel operation gracefully e.g., do some cleanup, free resources etc.
return;
}
// Do some work
}
}
// Alternatively use a command e.g., in a view model class
private void CancelBackgroundWorker_Click(object sender, EventArgs e)
{
if (this.Worker.WorkerSupportsCancellation)
{
this.Worker.CancelAsync();
}
}
Task library
The example uses Progress<T> to report progress from the background thread to the UI thread.
private CancellationTokenSource CancellationTokenSource { get; set; }
private async Task StartWorker()
{
this.CancellationTokenSource = new CancellationTokenSource();
// Prepare callback to update UI from the background thread.
// The Progress<T> instance MUST be created on the UI thread
IProgress<int> progressReporter = new Progress<int>(progress => this.ProgressBar.Value = progress);
await Task.Run(
() => DoWork(progressReporter, this.CancellationTokenSource.Token),
this.CancellationTokenSource.Token);
this.CancellationTokenSource.Dispose();
}
private void DoWork(IProgress<int> progressReporter, CancellationToken cancellationToken)
{
DoCancellableWork(progressReporter, cancellationToken);
}
private void DoCancellableWork(IProgress<int> progressReporter, CancellationToken cancellationToken)
{
// Check for cancellation before executing the operation and allocating resources etc..
if (cancellationToken.IsCancellationRequested)
{
return;
}
// Periodically/regularly check for the cancellation flag
for (int i = 0; i <= 10000000000; i++)
{
if (cancellationToken.IsCancellationRequested)
{
// Cancel operation gracefully e.g., do some cleanup, free resources etc.
return;
}
// Do some work
// Report progress
progressReporter.Report(20);
}
}
// Alternatively use a command e.g., in a view model class
private void CancelBackgroundThread_Click(object sender, EventArgs e)
{
this.CancellationtokenSource?.Cancel();
}

Since the OP describes the task being done as "checking services", I would assume the work done looks something like this:
while(true){
// check service
// Post result back to UI thread
Thread.Sleep(...);
}
This is not the best way to write such such a check. As in most cases where Thread.Sleep is used, a timer would be a better alternative:
var myTimer = new System.Timers.Timer(...);
myTimer .Elapsed += OnTimedEvent;
myTimer .AutoReset = true;
myTimer .Enabled = true;
...
private void OnTimedEvent(Object source, ElapsedEventArgs e)
{
// check service
// Post result back to UI thread
}
This makes the problem of stopping/starting the task being done a simple matter of changing the Enabled-flag of the timer. It is also possible to use a timer, or a synchronization context to run the event directly on the UI thread, this is probably the best solution if "checking services" only takes a few ms.

Background task in UWP

I want to use a background task for my UWP app.
The below code, is my back button click event in windows mobile:
private async void MainPage_BackRequested(object sender, BackRequestedEventArgs e)
{
var access= await BackgroundExecutionManager.RequestAccessAsync();
var task = new BackgroundTaskBuilder
{
Name="My task",TaskEntryPoint=typeof(backGroundTask.Class1).ToString()
};
trigger = new ApplicationTrigger();
task.SetTrigger(trigger);
task.Register();
//var result = await trigger.RequestAsync();
if (Frame.CanGoBack)
{
Frame.GoBack();
e.Handled = true;
}
}
public void Run(IBackgroundTaskInstance taskInstance)
{
_deferral = taskInstance.GetDeferral();
clearData();
count1 = 0;
getDownloadedSongs();
dispatcherTimer1.Tick += DispatcherTimer1_Tick;
dispatcherTimer1.Interval = new TimeSpan(0, 0, 3);
dispatcherTimer1.Start();
_deferral.Complete();
}
DispatcherTimer dispatcherTimer1 = new DispatcherTimer();
private async void DispatcherTimer1_Tick(object sender, object e)
{
try
{
clearData();
}
catch (Exception ex)
{
}
}
But this code is not working, when I click back button. As per expectation background task code should work, but it is not working. What am I doing wrong?

Your background task is exiting before the DispatcherTimer gets a chance to ever execute, because you mark the Deferral as complete. You should hold on to the Deferral until all the work in your background task has been completed (or until you receive a TaskCanceled event from the system).

how to display/report back progress of Task(Async) actions?

I have the need to move some processes to async. I have 5 methods that I need to call individually and have run in the background so the user can continue on with their work.
The test code below seems to work... but I haven't been able to figure out how to return information (message) indicating that the a task has completed. The class will be called from a separate windows form so that the progress can be displayed....
from the form:
async void BtnGo_Click(object sender, System.EventArgs e)
{
label2.Text = #"Starting tasks...";
var progress = new Progress<string>(
p =>
{
label2.Text = p;
});
await TestTask.MyTestMain(progress);
}
the class:
public static class TestTask
{
public static Task MyTestMain(IProgress<string> pProgress)
{
return SomethingAsync(pProgress);
}
private static async Task SomethingAsync(IProgress<string> pProgress)
{
var t1 = SomeThing1(pProgress);
var t2 = SomeThing2(pProgress);
await Task.WhenAll(t1, t2);
if (pProgress != null) pProgress.Report(#"all tasks completed");
}
private static async Task SomeThing1()
{
await Task.Delay(9000);
var filename = #"c:\temp\tt1.txt";
if (File.Exists(filename))
File.Delete(filename);
using (TextWriter tw = new StreamWriter(filename))
{
await tw.WriteLineAsync(DateTime.Now.ToLongDateString());
}
if (pProgress != null) pProgress.Report(#"t1 completed");
}
private static async Task SomeThing2()
{
await Task.Delay(7000);
var filename = #"c:\temp\tt2.txt";
if (File.Exists(filename))
File.Delete(filename);
using (TextWriter tw = new StreamWriter(filename))
{
await tw.WriteLineAsync(DateTime.Now.ToLongDateString());
}
if (pProgress != null) pProgress.Report(#"t2 completed");
}
}
I would like know when each task has completed. Any help or direction would be appreciated.
EDIT
I have edited this post to reflect my changes... I still cannot get a progress report back to the UI... any thoughts?

You're doing IO bound work, you don't need to use thread-pool threads.
Transform your methods to use the async APIs of StreamWriter:
private static async Task FirstThingAsync()
{
var filename = #"c:\temp\tt1.txt";
if (File.Exists(filename))
File.Delete(filename);
using (TextWriter tw = new StreamWriter(filename))
{
await tw.WriteLineAsync(DateTime.Now);
}
}
Same for your second method, and then you can asynchronously wait on them concurrently:
private static async Task SomethingAsync()
{
var firstThing = FirstThingAsync();
var secondThing = SecondThingAsync();
await Task.WhenAll(firstThing, secondThing);
}
Edit:
You're never reaching your first Progress.Report call because your code is throwing an InvalidOperationException when you call t.Start() on a promise-style task:
t1.Start();
await t1;
t2.Start();
await t2;
The task returned from both method calls is a "hot task", meaning it's operation is already started. The docs on Task.Start say:
InvalidOperationException: The Task is not in a valid state to be
started. It may have already been started, executed, or canceled, or
it may have been created in a manner that doesn't support direct
scheduling.
The reason you're not seeing that exception is because you're swallowing it:
var t = SomethingAsync(pProgress);
When you don't await on the async operation. Your method calls should look like this:
public static Task MyTestMain(IProgress<string> pProgress)
{
return SomethingAsync(pProgress);
}
async void BtnGo_Click(object sender, System.EventArgs e)
{
label2.Text = #"Starting tasks...";
var progress = new Progress<string>(
p =>
{
label2.Text = p;
});
await TestTask.MyTestMain(progress);
}

Using Task to run continuous Thread

I want run a thread continuously. This thread would poll and check for card status. Here is a sample implementation:
static void Main(string[] args)
{
var _cancelationTokenSource = new CancellationTokenSource();
new Task(() => chkRequestTask(_cancelationTokenSource), _cancelationTokenSource.Token, TaskCreationOptions.LongRunning).Start();
while (true)
{
}
}
static bool chkRequestTask(CancellationTokenSource _cancellationTokenSource)
{
bool noRequest = false;
while (!_cancellationTokenSource.Token.IsCancellationRequested)
{
var RequestTask = Task.Factory.StartNew(() => noRequest = chkRequestTask(_cancellationTokenSource), _cancellationTokenSource.Token);
if (noRequest)
{
_cancellationTokenSource.Token.WaitHandle.WaitOne(15000);
Console.WriteLine("Waiting for 15Seconds");
}
else
{
Console.WriteLine("Checking the card");
}
}
return noRequest;
}
What I want to achieve here is chkRequestTask should be run on a separate thread. This would continously poll the status of the card. For this sample I'm simply doing : Console.WriteLine("Checking the card");.
Once it checks the status of the card it should sleep for 15secs for this sample only (in general it should check every 50ms, but for testing purposes I have kept 15secs).
But in the above sample it's not sleeping it's simply giving me Checking the card continuously. It's not sleeping at all for 15secs. What is wrong with this code ?

You're calling chkRequestTask recursively using Task.Factory.StartNew which you don't need at all.
It's not clear why you need to poll the status, better idea is to check any event or callback or WaitHandle provided by the card API you're talking about. That should keep you away from the pain.
If at all you believe polling is the only option you've left with, you can do it as follows.
static async Task ChkRequestTask(CancellationToken token)
{
while (true)
{
token.ThrowIfCancellationRequested();
Console.WriteLine("Checking the card");
bool status = PollTheCardForStatus();
if(!status)
break;
await Task.Delay(15 * 1000, token);//Adjust the delay as you wish
}
}
Else where in code, if possible await the call, If not then attach a continuation or use blocking task.Wait.
await ChkRequestTask(token);
This method doesn't need to return bool as you're returning from the method only when it is false, it is safe to assume the status is false when the Task returned from ChkRequestTask completes, which means poll returned false or the CancellationToken is cancelled, in which case you'll get TaskCanceledException

This is how I have done this. It seems to be working properly. As it's a background thread it would exit when the application exits. Could someone advise If this is the right way to do it.
private void Form1_Load(object sender, EventArgs e)
{
m_dev = DASK.Register_Card(DASK.PCI_7250, 0);
if (m_dev < 0)
{
MessageBox.Show("Register_Card error!");
}
FunctionToCall();
}
private void FunctionToCall()
{
short ret;
uint int_value;
var thread = new Thread(() =>
{
while (true)
{
ret = DASK.DI_ReadPort((ushort)m_dev, 0, out int_value);
if (ret < 0)
{
MessageBox.Show("D2K_DI_ReadPort error!");
return;
}
if (int_value > 0)
{
textBox2.Invoke(new UpdateText(DisplayText), Convert.ToInt32(int_value));
}
Thread.Sleep(500);
}
});
thread.Start();
thread.IsBackground = true;
}
private void DisplayText(int i)
{
textBox2.Text = i.ToString();
}

Modal Progress Form showing IProgress and supporting Cancellation of async Task for WinForms

I have been attempting to have a re-usable modal progress window (I.e. progressForm.ShowDialog()) to show progress from a running async task, including enabling cancellation.
I have seen some implementations that launch start the async task by hooking the Activated event handler on the form, but I need to start the task first, then show the modal dialog that will show it's progress, and then have the modal dialog close when completed or cancellation is completed (note - I want the form closed when cancellation is completed - signalled to close from the task continuation).
I currently have the following - and although this working - are there issues with this - or could this be done in a better way?
I did read that I need to run this CTRL-F5, without debugging (to avoid the AggregateException stopping the debugger in the continuation - and let it be caught in the try catch as in production code)
ProgressForm.cs
- Form with ProgressBar (progressBar1) and Button (btnCancel)
public partial class ProgressForm : Form
{
public ProgressForm()
{
InitializeComponent();
}
public event Action Cancelled;
private void btnCancel_Click(object sender, EventArgs e)
{
if (Cancelled != null) Cancelled();
}
public void UpdateProgress(int progressInfo)
{
this.progressBar1.Value = progressInfo;
}
}
Services.cs
- Class file containing logic consumed by WinForms app (as well as console app)
public class MyService
{
public async Task<bool> DoSomethingWithResult(
int arg, CancellationToken token, IProgress<int> progress)
{
// Note: arg value would normally be an
// object with meaningful input args (Request)
// un-quote this to test exception occuring.
//throw new Exception("Something bad happened.");
// Procressing would normally be several Async calls, such as ...
// reading a file (e.g. await ReadAsync)
// Then processing it (CPU instensive, await Task.Run),
// and then updating a database (await UpdateAsync)
// Just using Delay here to provide sample,
// using arg as delay, doing that 100 times.
for (int i = 0; i < 100; i++)
{
token.ThrowIfCancellationRequested();
await Task.Delay(arg);
progress.Report(i + 1);
}
// return value would be an object with meaningful results (Response)
return true;
}
}
MainForm.cs
- Form with Button (btnDo).
public partial class MainForm : Form
{
public MainForm()
{
InitializeComponent();
}
private async void btnDo_Click(object sender, EventArgs e)
{
CancellationTokenSource cts = new CancellationTokenSource();
CancellationToken token = cts.Token;
// Create the ProgressForm, and hook up the cancellation to it.
ProgressForm progressForm = new ProgressForm();
progressForm.Cancelled += () => cts.Cancel();
// Create the progress reporter - and have it update
// the form directly (if form is valid (not disposed))
Action<int> progressHandlerAction = (progressInfo) =>
{
if (!progressForm.IsDisposed) // don't attempt to use disposed form
progressForm.UpdateProgress(progressInfo);
};
Progress<int> progress = new Progress<int>(progressHandlerAction);
// start the task, and continue back on UI thread to close ProgressForm
Task<bool> responseTask
= MyService.DoSomethingWithResultAsync(100, token, progress)
.ContinueWith(p =>
{
if (!progressForm.IsDisposed) // don't attempt to close disposed form
progressForm.Close();
return p.Result;
}, TaskScheduler.FromCurrentSynchronizationContext());
Debug.WriteLine("Before ShowDialog");
// only show progressForm if
if (!progressForm.IsDisposed) // don't attempt to use disposed form
progressForm.ShowDialog();
Debug.WriteLine("After ShowDialog");
bool response = false;
// await for the task to complete, get the response,
// and check for cancellation and exceptions
try
{
response = await responseTask;
MessageBox.Show("Result = " + response.ToString());
}
catch (AggregateException ae)
{
if (ae.InnerException is OperationCanceledException)
Debug.WriteLine("Cancelled");
else
{
StringBuilder sb = new StringBuilder();
foreach (var ie in ae.InnerExceptions)
{
sb.AppendLine(ie.Message);
}
MessageBox.Show(sb.ToString());
}
}
finally
{
// Do I need to double check the form is closed?
if (!progressForm.IsDisposed)
progressForm.Close();
}
}
}
Modified code - using TaskCompletionSource as recommended...
private async void btnDo_Click(object sender, EventArgs e)
{
bool? response = null;
string errorMessage = null;
using (CancellationTokenSource cts = new CancellationTokenSource())
{
using (ProgressForm2 progressForm = new ProgressForm2())
{
progressForm.Cancelled +=
() => cts.Cancel();
var dialogReadyTcs = new TaskCompletionSource<object>();
progressForm.Shown +=
(sX, eX) => dialogReadyTcs.TrySetResult(null);
var dialogTask = Task.Factory.StartNew(
() =>progressForm.ShowDialog(this),
cts.Token,
TaskCreationOptions.None,
TaskScheduler.FromCurrentSynchronizationContext());
await dialogReadyTcs.Task;
Progress<int> progress = new Progress<int>(
(progressInfo) => progressForm.UpdateProgress(progressInfo));
try
{
response = await MyService.DoSomethingWithResultAsync(50, cts.Token, progress);
}
catch (OperationCanceledException) { } // Cancelled
catch (Exception ex)
{
errorMessage = ex.Message;
}
finally
{
progressForm.Close();
}
await dialogTask;
}
}
if (response != null) // Success - have valid response
MessageBox.Show("MainForm: Result = " + response.ToString());
else // Faulted
if (errorMessage != null) MessageBox.Show(errorMessage);
}

I think the biggest issue I have, is that using await (instead of
ContinueWith) means I can't use ShowDialog because both are blocking
calls. If I call ShowDialog first the code is blocked at that point,
and the progress form needs to actually start the async method (which
is what I want to avoid). If I call await
MyService.DoSomethingWithResultAsync first, then this blocks and I
can't then show my progress form.
The ShowDialog is indeed a blocking API in the sense it doesn't return until the dialog has been closed. But it is non-blocking in the sense it continues to pump messages, albeit on a new nested message loop. We can utilize this behavior with async/await and TaskCompletionSource:
private async void btnDo_Click(object sender, EventArgs e)
{
CancellationTokenSource cts = new CancellationTokenSource();
CancellationToken token = cts.Token;
// Create the ProgressForm, and hook up the cancellation to it.
ProgressForm progressForm = new ProgressForm();
progressForm.Cancelled += () => cts.Cancel();
var dialogReadyTcs = new TaskCompletionSource<object>();
progressForm.Load += (sX, eX) => dialogReadyTcs.TrySetResult(true);
// show the dialog asynchronousy
var dialogTask = Task.Factory.StartNew(
() => progressForm.ShowDialog(),
token,
TaskCreationOptions.None,
TaskScheduler.FromCurrentSynchronizationContext());
// await to make sure the dialog is ready
await dialogReadyTcs.Task;
// continue on a new nested message loop,
// which has been started by progressForm.ShowDialog()
// Create the progress reporter - and have it update
// the form directly (if form is valid (not disposed))
Action<int> progressHandlerAction = (progressInfo) =>
{
if (!progressForm.IsDisposed) // don't attempt to use disposed form
progressForm.UpdateProgress(progressInfo);
};
Progress<int> progress = new Progress<int>(progressHandlerAction);
try
{
// await the worker task
var taskResult = await MyService.DoSomethingWithResultAsync(100, token, progress);
}
catch (Exception ex)
{
while (ex is AggregateException)
ex = ex.InnerException;
if (!(ex is OperationCanceledException))
MessageBox.Show(ex.Message); // report the error
}
if (!progressForm.IsDisposed && progressForm.Visible)
progressForm.Close();
// this make sure showDialog returns and the nested message loop is over
await dialogTask;
}