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C# backgroundworker RunworkerCompleted vs async await

Updated with answers:
The true way of wait until a number of different tasks to be finished would need async await instead of background worker.
#
I know there are numerous discussion about backgroundworker but I've being searched around and cannot find the answer.
Here is my code example(basic logic, the actual code is much longer), I wonder if there is a way to get around this:
BackgroundWorker MCIATS1Worker = new BackgroundWorker();
private AutoResetEvent _MCIATS1WorkerResetEvent = new AutoResetEvent(false);
public MainWindow()
{
InitializeComponent();
MCIATS1Worker = new BackgroundWorker();
MCIATS1Worker.DoWork += new DoWorkEventHandler(MCIATS1Worker_DoWork);
MCIATS1Worker.WorkerReportsProgress = true;
MCIATS1Worker.WorkerSupportsCancellation = true;
MCIATS1Worker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(MCIATS1_RunWorkerCompleted);
for (int i = 1; i <= 10; i++)
{
//some code
MCIATS1Worker.RunWorkerAsync();
_MCIATS1WorkerResetEvent.WaitOne();
}
}
DoWork and runworkercompleted
void MCIATS1Worker_DoWork(object sender, DoWorkEventArgs e)
{
//do something here
}
void MCIATS1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
MessageBox.Show("hello world");
_MCIATS1WorkerResetEvent.Set();
}
For some reasons, the MCIATS1_RunWorkerCompleted won't be triggered until the loop finished. And apparently the WaitOne is holding the loop.
Here is my question,
why RunWorkerCompleted won't be trigger the RunWorkerCompleted when the worker is actually finished the work?
Thank you.
###UPDATED SOLUTION
This is the right way of doing it.
private async void WhateverFunction()
{
await Task.WhenAll(MCIATS1WorkerDoWorkAsync(param),...other tasks);
}
private Task MCIATS1WorkerDoWorkAsync(bkgWorkParameter param)
{
return Task.Run(() =>
{
//Do whatever
});
}

It happens because when you use a BackgroundWorker it's RunWorkerCompleted event is posted to the SynchronizationContext of the thread that called RunWorkerAsync.
Because you call RunWorkerAsync on the UI thread the event can't run until the UI thread starts processing new messages in the message loop. However you prevented the UI thread from returning to the message loop by your _MCIATS1WorkerResetEvent.WaitOne(); call.
So what it boils down to is _MCIATS1WorkerResetEvent.Set(); is waiting for MCIATS1_RunWorkerCompleted to fire to stop blocking and MCIATS1_RunWorkerCompleted is waiting for _MCIATS1WorkerResetEvent.Set(); to stop blocking the UI thread so it's message to be processed.
Both things are waiting for the other to complete before itself completes and you have a classic deadlock.
There is no need for a for loop for this problem to happen, this same problem would happen with or without out the loop, in fact the loop never gets to run it's 2nd itteration because it will have deadlocked on the first time through so it does not matter that there is a loop at all.

Depend on what kind of work your MCIATS1Worker_DoWork method do, you can consider to use async-await approach, which makes code a little bid more cleaner.
private async Task MCIATS1WorkerDoWorkAsync()
{
await Task.Delay(1000) // do something asynchronously for 1 second
}
private async void MainWindow_Load(object sender, EventArgs e)
{
for (int i = 1; i <= 10; i++)
{
//some code
await MCIATS1WorkerDoWorkAsync();
MessageBox.Show("hello world");
}
}
Message box will be shown 10 times every 1 second. await keyword will continue loop only after MCIATS1WorkerDoWorkAsync method has successfully finished.
With async-await your form will remain responsive and if DoWork method do some IO operations, then you will not start another thread (as BackgroundWorker do) and whole execution will happens on one thread.

Is it possible to change views, once during the start of an event handler and once during end?

I get data from database on a click.
I have an event handler which when triggered should show "data retrieving..." in status bar and should change to "Ready" again just before the event handler ends.
But the text updates only once, the second Ready one. How is it generally done?
private void Next_Click(object sender, RoutedEventArgs e){
this.footerText = "Waiting for dataRetreival";
someRandomTimeTakingMethod(); //Gets Data from DB.
this.footerText = "Ready";
}
Even though code executes line 2, the view updates only when the function is over, ie only the second one actually works.

You should put your data-intensive work on a background thread so the UI can update properly. This provides the best user experience.
To elaborate on FZysset's answer with some code...
private async void Next_Click(object sender, RoutedEventArgs e)
{
footerText.Text = "Waiting for dataRetreival";
IsEnabled = false;
await SomeRandomTimeTakingMethodAsync();
IsEnabled = true;
footerText.Text = "Ready";
}
private async Task SomeRandomTimeTakingMethodAsync()
{
await Task.Delay(TimeSpan.FromSeconds(new Random().Next(2, 5)));
// or await Task.Run(() => { ... });
}
The above example allows you to leverage await/async that was introduced in .NET 4.5. Notice how nicely it flows? No nonsense!
We're putting stuff onto the background thread so the UI can remain unblocked (thus it will show your updates to your status bar and allow user interaction.) Of course, you have to be careful not to update anything on the UI from your background thread.
If you are using an older version of .NET, you can just use TPL without async/await:
private void Next_Click(object sender, RoutedEventArgs e)
{
footerText.Text = "Waiting for dataRetreival";
IsEnabled = false;
Task.Factory.StartNew(() =>
{
SomeRandomTimeTakingMethod();
}).ContinueWith(t =>
{
IsEnabled = true;
footerText.Text = "Ready";
}, TaskScheduler.FromCurrentSynchronizationContext());
}
private void SomeRandomTimeTakingMethod()
{
Thread.Sleep(TimeSpan.FromSeconds(new Random().Next(2, 5)));
}
Two important things to note about the latter example:
You must provide TaskScheduler.FromCurrentSynchronizationContext() to the ContinueWith call, or you will encounter exceptions because the continuation is not on the UI thread. You must get the context in a method that isn't running on a background thread.
You will want to check for exceptions on the Task object in your ContinueWith.
This example is very rudimentary though. If you were to have a bunch of background operations kicked off with click handlers, you'd want to give yourself some helper classes/services to make life easier. (And investigate MVVM, which I cannot tell if you are using.)
A colleague of mine gave a presentation on using various asynchronous patterns in C# and .NET. You can check it out here: https://github.com/mtusk/TplLunchAndLearn

That's because you're "someRandomTimeTakingMethod" is launched on the UI Thread. Therefore it will not update the view until it is finished.
To go around this you have the following possibilities :
Make your method "someRandom..." asynchronous with a task, and use the await operator : http://msdn.microsoft.com/en-us/library/hh191443.aspx
Launch your randomTimeTaking method into a thread, and launch an event when your execution is finished, to update the footer text
I strongly recommend you the first option, for some sample : http://msdn.microsoft.com/en-us/library/hh873191.aspx

You need to run those lines asynchronously. You can do that using the Task class:
private void Next_Click(object sender, RoutedEventArgs e){
Task.Factory.StartNew(() => footerText = "Waiting for dataRetreival");
someRandomTimeTakingMethod(); //Gets Data from DB.
Task.Factory.StartNew(() => footerText = "Ready");
}

There is one way to do it using Dispatcher. The original post is here.
The code is:-
private void Next_Click(object sender, RoutedEventArgs e){
UpdateUI("Please wait for data retrieval", delegate() { someRandomTimeTakingMethod(); });
this.footerText = "Ready";
}
public delegate void NoArgsDelegate();
public void UpdateUI(string description, NoArgsDelegate operation)
{
this.FooterText= description;
DispatcherFrame frame = new DispatcherFrame();
DispatcherOperation dispatcherOperation = Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.ContextIdle, operation);
dispatcherOperation.Completed += delegate(object sender, EventArgs e)
{
frame.Continue = false;
};
Dispatcher.PushFrame(frame);
}
If my understanding is right, this uses Asynchronous programming, not different thread. The thread will update UI first and then call the someRandomTimeTakingMethod().

This BackgroundWorker is currently busy and cannot run multiple tasks concurrently

I'm trying to use a Background Worker in a WPF application. The heavy lifting task uses WebClient to download some HTML and parse some info out of it. Ideally I want to do that downloading and parsing without locking the UI and placing the results in the UI once it's done working.
And it works fine, however, if I quickly submit the "download and parse" command, I get the error:
This BackgroundWorker is currently busy and cannot run multiple tasks
concurrently
So I did some Googling and it seems that I can enable the .WorkerSupportsCancellation property of the background worker and just .CancelAsync(). However, this doesn't work as expected (canceling the current download and parse).
I still get the above error.
Here's my code:
//In window constructor.
_backgroundWorker.WorkerSupportsCancellation = true;
_backgroundWorker.DoWork += new DoWorkEventHandler(_backgroundWorker_DoWork);
_backgroundWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(_backgroundWorker_RunWorkerCompleted);
//Declared at class level variable.
BackgroundWorker _backgroundWorker = new BackgroundWorker();
//This is the method I call from my UI.
private void LoadHtmlAndParse(string foobar)
{
//Cancel whatever it is you're doing!
_backgroundWorker.CancelAsync();
//And start doing this immediately!
_backgroundWorker.RunWorkerAsync(foobar);
}
POCOClassFoo foo = new POCOClassFoo();
void _backgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
//This automagically sets the UI to the data.
Foo.DataContext = foo;
}
void _backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
//DOING THE HEAVY LIFTING HERE!
foo = parseanddownloadresult()!
}

Calling CancelAsync will still fire the RunWorkerCompleted event. In this event, you need to make sure that CancelAsync has not been called, by checking e.Cancelled. Until this event fires, you cannot call RunWorkerAsync.
Alternatively, I would recommend you do what Tigran suggested and create a new BackgroundWorker each time.
Further more, I would recommend storing the results of_backgroundWorker_DoWork in e.Result, then retrieve them from the same in _backgroundWorker_RunWorkerCompleted
Maybe something like this
BackgroundWorker _backgroundWorker;
private BackgroundWorker CreateBackgroundWorker()
{
var bw = new BackgroundWorker();
bw.WorkerSupportsCancellation = true;
bw.DoWork += _backgroundWorker_DoWork;
bw.RunWorkerCompleted += new _backgroundWorker_RunWorkerCompleted;
return bw.
}
private void LoadHtmlAndParse(string foobar)
{
//Cancel whatever it is you're doing!
if (_backgroundWorer != null)
{
_backgroundWorker.CancelAsync();
}
_backgroundWorker = CreateBackgroundWorker();
//And start doing this immediately!
_backgroundWorker.RunWorkerAsync(foobar);
}
//you no longer need this because the value is being stored in e.Result
//POCOClassFoo foo = new POCOClassFoo();
private void _backgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
if (e.Error != null)
{
//Error handling goes here.
}
else
{
if (e.Cancelled)
{
//handle cancels here.
}
{
//This automagically sets the UI to the data.
Foo.DataContext = (POCOClassFoo)e.Result;
}
}
private void _backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
//DOING THE HEAVY LIFTING HERE!
e.Result = parseanddownloadresult()!
}

The thing is that CancelAsync() does what it climes: cancel in async way. That means that it will not stop immediately, but after some time. That time can never be calculated or predicted, so you have a couple of options:
Wait until this backround worker stops really, by waiting in cycle until IsBusy property of it becomes false
Or, I think, better solution is to start another background worker, considering that request of cancelation was already sent to the first one, so it will be soon or later stop. In this case, you need to know from which background worker data comes, in order to process it or not, cause on start of second the first one will still run and pump the data from WebService.
Hope this helps.

CancelAsync returns before the worker cancels and stops its work. Hence, your RunWorkerAsync call is starting before the worker is ready, and you're getting that error. You'll need to wait for the worker to be ready first.

When I'm not interested in tracking progress of an async operation, I tend to prefer to just slap a lambda at ThreadPool.QueueUserWorkItem instead of instantiating and setting up a background worker that I have to check the state of to be able to reuse in a sane way.

You need to verify before you kicks in.
f( !bw.IsBusy )
bw.RunWorkerAsync();
else
MessageBox.Show("Can't run the bw twice!");

You are calling CancelAsync without waiting for the background worker to actually cancel the work. Also you must have your own logic for cancelling the work. There is a good example on MSDN which shows how to do it. Basically in your parseanddownloadresult() method you need to check the CancellationPending property.

Wait for a while without blocking main thread

I wish my method to wait about 500 ms and then check if some flag has changed. How to complete this without blocking the rest of my application?

You can use await Task.Delay(500); without blocking the thread like Sleep does, and with a lot less code than a Timer.

Thread.Sleep(500) will force the current thread to wait 500ms. It works, but it's not what you want if your entire application is running on one thread.
In that case, you'll want to use a Timer, like so:
using System.Timers;
void Main()
{
Timer t = new Timer();
t.Interval = 500; // In milliseconds
t.AutoReset = false; // Stops it from repeating
t.Elapsed += new ElapsedEventHandler(TimerElapsed);
t.Start();
}
void TimerElapsed(object sender, ElapsedEventArgs e)
{
Console.WriteLine("Hello, world!");
}
You can set AutoReset to true (or not set it at all) if you want the timer to repeat itself.

I don't really understand the question.
If you want to block before checking, use Thread.Sleep(500);
If you want to check asynchronously every x seconds, you can use a Timer to execute a handler every x milliseconds.
This will not block your current thread.

It the method in question is executing on a different thread than the rest of your application, then do the following:
Thread.Sleep(500);

System.Threading.Thread.Sleep(500);
Update
This won't block the rest of your application, just the thread that is running your method.

Using a timer should do the trick
if you need to use a thread then here is an example
void Main()
{
System.Threading.Thread check= new System.Threading.Thread(CheckMethod);
check.Start();
}
private void CheckMethod()
{
//Code
Thread.Sleep(500);
}

Asynchron Task:
var task = new Task (() => function_test()); task.Start();
public void function_test() { `Wait for 5000 miliseconds` Task.Delay(5000);` }

I've recently been struggling with the same issue where I needed an action to be run on schedule without blocking the UI.
Here's my solution:
private void Button_Click(object sender, RoutedEventArgs e)
{
RunOnSchedule(interval, cancellationToken);
}
private void RunOnSchedule(int interval, CancellationToken cancellationToken)
{
// Start the task you want to run on schedule
TaskToRunOnSchedule(args);
Task.Run(async () =>
{
// This loop checks if the task was requested to be cancelled every 1000 ms
for (int x = 0; x < interval; x+=1000)
{
if (cancellationToken.IsCancellationRequested)
{
break;
}
await Task.Delay(1000);
}
}).GetAwaiter().OnCompleted(() =>
{
// Once the task for delaying is completed, check once more if cancellation is requested, as you will reach this point regardless of if it was cancelled or not.
if (!cancellationToken.IsCancellationRequested)
{
// Run this method again
RunOnSchedule(interval, cancellationToken);
}
});
}

In a WinForms application, when I want to wait on the main thread without blocking the app, I usually use
private void Wait (double milliseconds)
{
DateTime next = System.DateTime.Now.AddMilliseconds(milliseconds);
while (next > System.DateTime.Now)
Application.DoEvents();
}

Cancelling Background Tasks

When my C# application closes it sometimes gets caught in the cleanup routine. Specifically, a background worker is not closing. This is basically how I am attempting to close it:
private void App_FormClosing(object sender, FormClosingEventArgs e)
{
backgroundWorker1.CancelAsync();
while (backgroundWorker1.IsBusy) ; // Gets stuck here.
}
Is there a different way that I should be doing this? I am using Microsoft Visual C# 2008 Express Edition. Thanks.
ADDITIONAL INFORMATION:
The background worker does not appear to be exiting. This is what I have:
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
while (!backgroundWorker1.CancellationPending)
{
// Do something.
}
}
I've also modified the cleanup code:
private void App_FormClosing(object sender, FormClosingEventArgs e)
{
while (backgroundWorker1.IsBusy)
{
backgroundWorker1.CancelAsync();
System.Threading.Thread.Sleep(1000);
}
}
Is there something else that I should be doing?

Some pretty good suggestions, but I don't believe they address the underlying issue: canceling a background task.
Unfortunately, when using BackgroundWorker, termination of your task depends on the task itself. The only way your while loop will terminate, is if your background task checks its Cancel property and returns or breaks from its current process.
Example Base
For example, consider
private readonly BackgroundWorker worker = new BackgroundWorker ();
public void SomeFormEventForStartingBackgroundTask ()
{
worker.DoWork += BackgroundTask_HotelCalifornia;
worker.WorkerSupportsCancellation = true;
worker.RunWorkerAsync ();
}
// semantically, you want to perform this task for lifetime of
// application, you may even expect that calling CancelAsync
// will out and out abort this method - that is incorrect.
// CancelAsync will only set DoWorkEventArgs.Cancel property
// to true
private void BackgroundTask_HotelCalifornia (object sender, DoWorkEventArgs e)
{
for ( ; ;)
{
// because we never inspect e.Cancel, we can never leave!
}
}
private void App_FormClosing(object sender, FormClosingEventArgs e)
{
// [politely] request termination
worker.CancelAsync();
// [politely] wait until background task terminates
while (worker.IsBusy);
}
This is what is happening by default. Now, maybe your task isn't an infinite loop, perhaps it is just a long-running task. Either way, your main thread will block [actually it is spinning, but whatevs] until the task completes, or doesn't as the case may be.
If you have personally written and can modify the task, then you have a few options.
Example Improvement
For instance, this is a better implementation of the above example
private readonly BackgroundWorker worker = new BackgroundWorker ();
// this is used to signal our main Gui thread that background
// task has completed
private readonly AutoResetEvent isWorkerStopped =
new AutoResentEvent (false);
public void SomeFormEventForStartingBackgroundTask ()
{
worker.DoWork += BackgroundTask_HotelCalifornia;
worker.RunWorkerCompleted += BackgroundTask_Completed;
worker.WorkerSupportsCancellation = true;
worker.RunWorkerAsync ();
}
private void BackgroundTask_HotelCalifornia (object sender, DoWorkEventArgs e)
{
// execute until canceled
for ( ; !e.Cancel;)
{
// keep in mind, this task will *block* main
// thread until cancel flag is checked again,
// so if you are, say crunching SETI numbers
// here for instance, you could still be blocking
// a long time. but long time is better than
// forever ;)
}
}
private void BackgroundTask_Completed (
object sender,
RunWorkerCompletedEventArgs e)
{
// ok, our task has stopped, set signal to 'signaled' state
// we are complete!
isStopped.Set ();
}
private void App_FormClosing(object sender, FormClosingEventArgs e)
{
// [politely] request termination
worker.CancelAsync();
// [politely] wait until background task terminates
isStopped.WaitOne ();
}
While this is better, it's not as good as it could be. If you can be [reasonably] assured your background task will end, this may be "good enough".
However, what we [typically] want, is something like this
private void App_FormClosing(object sender, FormClosingEventArgs e)
{
// [politely] request termination
worker.CancelAsync();
// [politely] wait until background task terminates
TimeSpan gracePeriod = TimeSpan.FromMilliseconds(100);
bool isStoppedGracefully = isStopped.WaitOne (gracePeriod);
if (!isStoppedGracefully)
{
// KILL! KILL! KILL!
}
}
Alas, we cannot. BackgroundWorker does not expose any means of forceful termination. This is because it is an abstraction built on top of some hidden thread management system, one which could potentially destabalize other parts of your application if it were forcefully terminated.
The only means [that I have seen at least] to implement the above is to manage your own threading.
Example Ideal
So, for instance
private Thread worker = null;
// this time, 'Thread' provides all synchronization
// constructs required for main thread to synchronize
// with background task. however, in the interest of
// giving background task a chance to terminate gracefully
// we supply it with this cancel signal
private readonly AutoResetEvent isCanceled = new AutoResentEvent (false);
public void SomeFormEventForStartingBackgroundTask ()
{
worker = new Thread (BackgroundTask_HotelCalifornia);
worker.IsBackground = true;
worker.Name = "Some Background Task"; // always handy to name things!
worker.Start ();
}
private void BackgroundTask_HotelCalifornia ()
{
// inspect cancel signal, no wait period
//
// NOTE: so cheating here a bit, this is an instance variable
// but could as easily be supplied via parameterized thread
// start delegate
for ( ; !isCanceled.WaitOne (0);)
{
}
}
private void App_FormClosing(object sender, FormClosingEventArgs e)
{
// [politely] request termination
isCanceled.Set ();
// [politely] wait until background task terminates
TimeSpan gracePeriod = TimeSpan.FromMilliseconds(100);
bool isStoppedGracefully = worker.Join (gracePeriod);
if (!isStoppedGracefully)
{
// wipe them out, all of them.
worker.Abort ();
}
}
And that there, is a decent introduction on thread management.
Which is best suited for you? Depends on your application. It is probably best not to rock the boat, and modify your current implementation to ensure that
your background task inspects and respects the Cancel property
your main thread waits for completion, as opposed to polling
It is very important to compare and evaluate the pros and cons of each approach.
If you must control and guarantee termination of someone else's tasks, then writing a thread management system that incorporates the above may be the way to go. However you would lose out on out-of-box features like thread pooling, progress reporting, cross-thread data marshalling [worker does that, no?], and a bunch of other stuff. Not to mention, "rolling your own" is often error prone.
Anyway, hope this helps :)

Kevin Gale is correct in stating that your BackgroundWorker's DoWork handler needs to poll for CancellationPending and return if a cancellation is requested.
That being said, if this is happening when your application is shutting down, you can just ignore it safely, as well. BackgroundWorker uses a ThreadPool thread, which is, by definition, a background thread. Leaving this running will not prevent your application from terminating, and the thread will automatically be torn down when your application shuts down.

In the background worker thread you need to check the BackgroundWorker.CancellationPending flag and exit if it is true.
The CancelAsync() just sets this flag.
Or to put it another way. CancelAsync() doesn't actually cancel anything. It won't abort the thread or cause it to exit. If the worker thread is in a loop and checks the CancellationPending flag periodically it can catch the cancel request and exit.
MSDN has an example here although it doesn't use a loop in the worker routine.

This code is guaranteed to deadlock when the BGW is still running. BGW cannot complete until its RunWorkerCompleted event finished running. RunWorkerCompleted cannot run until the UI thread goes idle and runs the message loop. But the UI thread isn't idle, it is stuck in the while loop.
If you want the BGW thread to complete cleanly, you have to keep your form alive. Check this thread to see how to do that.

Try:
if (this.backgroundWorker1.IsBusy) this.backgroundWorker1.CancelAsync();