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Which is the better practice: Multiple BackgroundWorkers or Change the DoWork to a different method? [duplicate]

Suppose you have a search textbox and have a search algorithm attached to the TextChanged event, that runs with a BackgroundWorker. If there comes a new character in the textbox, i need to cancel the previous search and run it again.
I tried using events in between the main thread and the bgw, from this previous question, but I still get the error "currently busy and cannot run multiple tasks concurrently"
BackgroundWorker bgw_Search = new BackgroundWorker();
bgw_Search.DoWork += new DoWorkEventHandler(bgw_Search_DoWork);
private AutoResetEvent _resetEvent = new AutoResetEvent(false);
private void txtSearch_TextChanged(object sender, EventArgs e)
{
SearchWithBgw();
}
private void SearchWithBgw()
{
// cancel previous search
if (bgw_Search.IsBusy)
{
bgw_Search.CancelAsync();
// wait for the bgw to finish, so it can be reused.
_resetEvent.WaitOne(); // will block until _resetEvent.Set() call made
}
// start new search
bgw_Search.RunWorkerAsync(); // error "cannot run multiple tasks concurrently"
}
void bgw_Search_DoWork(object sender, DoWorkEventArgs e)
{
Search(txtSearch.Text, e);
}
private void Search(string aQuery, DoWorkEventArgs e)
{
int i = 1;
while (i < 3) // simulating search processing...
{
Thread.Sleep(1000);
i++;
if (bgw_Search.CancellationPending)
{
_resetEvent.Set(); // signal that worker is done
e.Cancel = true;
return;
}
}
}
EDIT To reflect answers. Don´t reuse the BackgroundWorker, create a new one:
private void SearchWithBgw()
{
if (bgw_Search.IsBusy)
{
bgw_Search.CancelAsync();
_resetEvent.WaitOne(); // will block until _resetEvent.Set() call made
bgw_Search = new BackgroundWorker();
bgw_Search.WorkerSupportsCancellation = true;
bgw_Search.DoWork += new DoWorkEventHandler(bgw_Search_DoWork);
}
bgw_Search.RunWorkerAsync();
}

When the _resetEvent.WaitOne() call completes, the worker thread isn't actually done. It is busy returning from DoWork() and waiting for an opportunity to run the RunWorkerCompleted event, if any. That takes time.
There is no reliable way to ensure the BGW is completed in a synchronous way. Blocking on IsBusy or waiting for the RunWorkerCompleted event to run is going to cause deadlock. If you really want to use only one bgw then you'll have to queue the requests. Or just don't sweat the small stuff and allocate another bgw. They cost very little.

Create a new background worker if the old one exists.
private void SearchWithBgw()
{
// cancel previous search
if (bgw_Search.IsBusy)
{
bgw_Search.CancelAsync();
// wait for the bgw to finish, so it can be reused.
_resetEvent.WaitOne(); // will block until _resetEvent.Set() call made
BackgroundWorker bgw_Search = new BackgroundWorker();
bgw_Search.DoWork += new DoWorkEventHandler(bgw_Search_DoWork);
}
// start new search
bgw_Search.RunWorkerAsync(); // error "cannot run multiple tasks concurrently"
}
Also I know you put fake code in, but you want to make sure you set _resetEvent when the code completes normally too.

Do not reuse a Backgroundworker. It is a cheap resource, it is not a Thread.
make sure your Bgw code stops, yours looks OK. The Bgw will release the Thread to the pool.
but in the mean time, create a new Task/Bgw for a new job.
You may want to unsubscribe your Completed event from the old Bgw.

I think you should consider not cancelling the background worker.
If you cancel requests and the user types faster than your server returns queries, he will not see suggestions until he is finished typing.
In interactive scenarios like this, It could be better to show responses that run behind with what the user's typing. Your user will know he can stop typing if the word he has in mind is your suggestions list.
This will be also better for your server when it is busy, because instead of many cancelled requests, who will cost something but that are ultimately not shown, there will be fewer requests whose response you actually use.
I ran into similar issues with (3d) rendering applications, where the beginner's mistake is to cancel and rerender on every mousemove. This lead to a lot of computation and little interactive feedback.

calling dialog and getting values from it in backgroundworker?

I am working on a WPF project where I have to import data from a lot of single files.
The actual importing of those files and the data in them is being done in a backgroundworker doWork method.
It works like a charm, does the job and updating a progress bar also works perfectly.
Now though, depending on what I encounter in those files, I occasionally need to get a decision from the User before I can proceed processing the current file.
What is the best way to open a window/dialog, getting the values set in there back into the backgroundworker.doWork method and continue processing?
Is that even possible with a backgroundworker or do I need to keep that processing logic in the main/UI thread and update the progress bar from there somehow?
I hope some of you can give me some hints or point me to other resources since I have not found much useful information for my specific problem.

Background worker works in a different thread. You can not invoke UI directly from a background thread. One way of achiving what you are trying to do is by using a flag and Dispatcher to invoke UI for user input
bool WaitFor_Input = false; //flag to indicate user input status
private void ThreadRun()
{
//THIS IS IN Background worker thread
//do task
WaitFor_Input = true;
//ask for user input
Dispatcher.BeginInvoke(new Action(Show_Dialogue), null);
while (WaitFor_Input); //Background worker waits untill user input is complete
//continue further processing
}
private void Show_Dialogue()
{
//THIS IS IN UI THREAD
//show your dialogue box here, update data variables
//finally set
WaitFor_Input = false;
}
Keeping processing logic in a background thread is actually a good idea.

You can call the ShowDialog of an OpenFileDailog class on a new Thread or BackgroundWorker( this will also create a new thread)
But when you want to pass or update any property or control that is running on the main thread you will need to use the Disptacher like this
Dispatcher.BeginInvoke(new Action(() => { YourMethodThatUpdatesSomething(); }));

How to Pause backgroundWorker in Windows Form

I am a newbie in .net world so i am facing some problem,please help me out, here is one of them,I have a for loop and i have a backgroundWorker control and a progressBar Control and also have 1 button that id:"btnPause".So my requirement is when my form will load the peogressbar will show the progress how much it has completed ,whenever i click on the button(btnPause) the button text need to b change and set the text as pause, then whenever i clicked again on that button it need to be resume from the value it got paused.Please help me.
Thanks in advance.

My approach to this would be to use a boolean flag to let me know whether or not the operation is paused (and perhaps a second one letting me know whether or not the operation is cancelled). When the user presses the pause button, you set the IsPaused flag to true so that when the worker begins processing the next item, it can check this flag and know to go into a wait state.
There are a few ways to keep the BackgroundWorker in this paused state. My approach would be to use a ManualResetEvent. This allows the BackgroundWorker thread to enter a sleep state and come out when the event is set. I think this is better than using a while loop with a sleep body because the thread stays asleep until the event is set, rather than waking up to check if it should still be asleep. When the user wishes to continue, it can set this event and let the Background worker continue.
So the code would look a little like this:
private void backgroundWorker_doWork(object sender, DoWorkEventArgs args)
{
//Initialize any pre-work stuff here.
while(!finished)
{
if (Paused)
m_evtPause.WaitOne();
if (Cancelled)
break;
//lengthy thread procedure code.
}
}

Try to use this
class MyWorkerClass
{
volatile bool m_bPaused = false;
// Public property to control worker execution
public bool Paused
{
get { return m_bPaused; }
set { m_bPaused = value; }
}
long ThreadFunc (BackgroundWorker worker, DoWorkEventArgs e)
{
...
// While Paused property set to true
while (m_bPaused)
{
// Pause execution for some time
System.Threading.Thread.Sleep (100);
}
}
}

Need help to stop the BackgroundWorker thread

Need help to stop the BackgroundWorker thread.
I am trying to stop a background worker thread. This is what I am doing:
On stop button click (UI layer):
if (backgroundWorker.IsBusy == true &&
backgroundWorker.WorkerSupportsCancellation == true)
{
backgroundWorker.CancelAsync();
}
On DoWork event (UI layer):
if ((backgroundWorker.CancellationPending == true))
{
e.Cancel = true;
}
else
{
//Function which progresses the progress bar is called
//here with its definition in business layer
}
Once the DoWork event is fired and my program control is in the function defined in Business layer, how do I revert back to the DoWork event to set ‘e.Cancel = true’?

Setting e.Cancel does nothing, if CancellationPending is true you need to basically break out of DoWork() using return or whatever (after you've stopped what you're doing).
Something like:
private void DoWork(...)
{
// An infinite loop of work!
while (true)
{
// If set to cancel, break the loop
if (worker.CancellationPending)
break;
// Sleep for a bit (do work)
Thread.Sleep(100);
}
}
DoWork() executes in a seperate thread to the UI thread, you can report back to the UI thread using BackgroundWorkr.ReportProgress().

DoWork will run in it's own thread and is not dependant of the GUI thread.
You do almost everything correct. From the GUI thread, set the CancellationPending to true.
In the DoWork method, you probably have a loop of some sort.
Here you check if CancellationPending == true, but in addition to setting e.Cancel to true, also include a return call to make the method return and effectively stopping the worker. This also causes the WorkerCompleted event to fire on the GUI thread if the method is hooked up.
If the DoWork method perform some long task that is not divided into parts (for example if your DoWork method looks like this:
void DoWork( (object sender, DoWorkEventArgs e)
{
myClass.SomeLongOperation();
}
Then you are out of luck, since you need to manually check the CancellationPending inside the DoWork method to be able to stop it. If the DoWork itself "hangs" and waits for a operation you can't control, you can't stop it (in any orderly fashion) by setting CancellationPending from the GUI thread.

Once the DoWork event is fired and my program control is in the function defined in Business layer, how do I revert back to the DoWork event to set ‘e.Cancel = true’?
You don't. If you want cancellation to be possible during execution of your business layer, then your business layer must support cancellation. So, you have two options:
In your DoWork method, call only short-time business layer methods and check for CancellationPending in between.
Make your business layer methods cancellation-aware, i.e., pass the BackgroundWorker to them and have them periodically check CancellationPending (and retun, once it turns true).

Keep checking the CancellationPending=True Flag in the else part of your logic, and return when true.

The code e.Cancel = true only sets a state on the BackgroundWorker, so that it knows it has been cancelled, it doesn't actually cancel the process.
You'll have to check the CancellationPending inside the loop of your method and break it or return.
void DoWork(object sender, DoWorkEventArgs e) {
for (int i = 0; i < length; i++) {
if(e.CancellationPending) {
return;
}
// Long running code
}
}

In C#, wait on the mainthread while continuing to process UI updates? (.NET 2.0 CF)

I want to otherwise block code execution on the main thread while still allowing UI changes to be displayed.
I tried to come up with a simplified example version of what I'm trying to do; and this is the best I could come up with. Obviously it doesn't demonstrate the behavior I'm wanting or I wouldn't be posting the question. I just hope it gives some code context to back my poor explanation of the problem I'm hoping to solve.
Within a button click handler on a form I have this:
private void button2_Click(object sender, EventArgs e)
{
AutoResetEvent autoResetEvent = new AutoResetEvent(false);
new Thread(delegate()
{
// do something that takes a while.
Thread.Sleep(1000);
// Update UI w/BeginInvoke
this.BeginInvoke(new ThreadStart(
delegate() {
this.Text = "Working... 1";
this.Refresh();
Thread.Sleep(1000); // gimme a chance to see the new text
}));
// do something else that takes a while.
Thread.Sleep(1000);
// Update UI w/Invoke
this.Invoke(new ThreadStart(
delegate() {
this.Text = "Working... 2";
this.Refresh();
Thread.Sleep(1000); // gimme a chance to see the new text
}));
// do something else that takes a while.
Thread.Sleep(1000);
autoResetEvent.Set();
}).Start();
// I want the UI to update during this 4 seconds, even though I'm
// blocking the mainthread
if (autoResetEvent.WaitOne(4000, false))
{
this.Text = "Event Signalled";
}
else
{
this.Text = "Event Wait Timeout";
}
Thread.Sleep(1000); // gimme a chance to see the new text
this.Refresh();
}
If I didn't set a timout on the WaitOne() the app would deadlock on the Invoke() call.
As to why I'd want to do this, I've been tasked with moving one subsystem of an app to do work in a background thread, but still have it block user's workflow (the main thread) only sometimes and for certain types of work related to that subsystem only.

You want to use the "BackgroundWorker" class, which will take most of this pain out of this for you.. but as mentioned before, you'll also want to structure it so that the main thread is updating the UI and the worker is doing the heavy lifting.

It is easyer then you might think.
Suggestion: when you need a thread to perform some occasional work, get it from the threadpool, so you will not need strange/error prone recycling code.
When you want something on another thread to update your UI, you just need a reference to the form and to call Form.Invoke passing the UI code you want the main thread to execute; it's a best pactice, in an event, to release the UI thread as soon as possible.
Ie:
private void button1_Click(object sender, EventArgs e)
{
// this is the UI thread
ThreadPool.QueueUserWorkItem(delegate(object state)
{
// this is the background thread
// get the job done
Thread.Sleep(5000);
int result = 2 + 2;
// next call is to the Invoke method of the form
this.Invoke(new Action<int>(delegate(int res)
{
// this is the UI thread
// update it!
label1.Text = res.ToString();
}), result);
});
}
Hope this helps you:)
EDIT: I am sorry, I didn't read the "blocking user workflow" part.
WindowsForms is not designed to do that, blocking the main thread is BAD (it handles the messages from the OS).
You don't have to block the user workflow via freezing a form (which would then be considered "Not Responding" by windows), the way to block user workflow is by disabling any control you want (with the Invoke method above if from another thread), even the entire form!!

Common activities which 'block' the main thread are things like opening messages boxes or modal dialog. The main code appears to block at the MessageBox or ShowDialog call.
The way those items work (and MessageBox is just a specialized modal dialog) is that they contain their own message pump while they're blocking.
Although it's a nasty hack, you can do something like this in your app by looping calling Application.DoEvents() to keep the user messages pumping while you're waiting for your other task to complete. You need to be careful because all sorts of nasty things might lead from pumping messages like this - for example someone close the form or reenter your current message handler - the modal dialogs avoid this by effectively disabling input from the form which launches them.
I did mean to say that BackgroundWorker is a better solution, if you can make it fit. I sometimes combine it with a modal 'progress dialog' to give me the background thread / message pumping and the blocking of the UI thread.
Edit - to expand on the last bit:
One approach I've used is to have a 'progress form' class, which takes a BackgroundWorker object as a constructor parameter, and contains handlers for the progress and completion events of the background worker which gets passed to it.
The form which wants the work done creates the background worker and hooks up the 'work' event (can't remember what it's called right now), and then creates a progress dialog to which it passes the background worker. It then modally shows the progress dialog, which means it will wait (but pumping messages) until the progress dialog closes.
The progress form is responsible for starting the BackgroundWorker from its OnLoad override, and closes itself when it sees the BackgroundWorker complete. Obviously you can add message text, progress bars, cancel buttons, whatever to the progress form.

structure your app so that the main thread only performs UI updates, and all other work is done on secondary threads via a work queue; then add a waiting-for-godot flag to your main thread and use it to guard the method that adds items to the work queue
out of curiosity: why do you want to do this?

You should probably restructure your code as others have suggested, but depending on the behavior you're looking for, you might also want to have a look at using a Thread.Join on your background worker thread. Join actually allows the calling thread to process COM and SendMessage events while it waits for the other thread to finish. This seems like it could be dangerous in come cases, but I've actually had a couple scenarios where it was the only way to wait for another thread to finish cleanly.
Thread..::.Join Method
Blocks the calling thread until a
thread terminates, while continuing to
perform standard COM and SendMessage
pumping.
(from http://msdn.microsoft.com/en-us/library/95hbf2ta.aspx)

I agree with the others that are suggesting you use Background Worker. It does the heavy lifting and allows the UI to continue. You can use the Report Progress of Background Worker to initiate times where the Main Form can be set to be disabled while it performs the actions in the background and then re-enable once the 'certain instances' have completed processing.
Let me know if this helps!
JFV

If you could adjust your code so that you set a flag once a process has begun and then check that in the UI before you start an additional operation I think you'd have a much easier time coding this. I would create a delegate that could be called from the thread in the threadpool or user created thread to update on progress in the UI. Once the background process has been completed switch the flag and now normal UI operations can continue. The only caveat you need to be aware of is that when you update UI components you must do it on the thread they were created on, the main/UI thread. In order to accomplish this you can call the Invoke() method on any control that lives on that thread and pass it the delegate and parameters you need to call it.
Here's a link to a tutorial I wrote some time ago about how to use Control.Invoke():
http://xsdev.net/tutorials/pop3fetcher/

Just a code snippet: don't have much time sorry :)
private void StartMyDoSomethingThread() {
Thread d = new Thread(new ThreadStart(DoSomething));
d.Start();
}
private void DoSomething() {
Thread.Sleep(1000);
ReportBack("I'm still working");
Thread.Sleep(1000);
ReportBack("I'm done");
}
private void ReportBack(string p) {
if (this.InvokeRequired) {
this.Invoke(new Action<string>(ReportBack), new object[] { p });
return;
}
this.Text = p;
}

It is best to dispatch the work but if you must, maybe something like this. Just call this method to wait for the signal rather than calling the waitone.
private static TimeSpan InfiniteTimeout = TimeSpan.FromMilliseconds(-1);
private const Int32 MAX_WAIT = 100;
public static bool Wait(WaitHandle handle, TimeSpan timeout)
{
Int32 expireTicks;
bool signaled;
Int32 waitTime;
bool exitLoop;
// guard the inputs
if (handle == null) {
throw new ArgumentNullException("handle");
}
else if ((handle.SafeWaitHandle.IsClosed)) {
throw new ArgumentException("closed wait handle", "handle");
}
else if ((handle.SafeWaitHandle.IsInvalid)) {
throw new ArgumentException("invalid wait handle", "handle");
}
else if ((timeout < InfiniteTimeout)) {
throw new ArgumentException("invalid timeout <-1", "timeout");
}
// wait for the signal
expireTicks = (int)Environment.TickCount + timeout.TotalMilliseconds;
do {
if (timeout.Equals(InfiniteTimeout)) {
waitTime = MAX_WAIT;
}
else {
waitTime = (expireTicks - Environment.TickCount);
if (waitTime <= 0) {
exitLoop = true;
waitTime = 0;
}
else if (waitTime > MAX_WAIT) {
waitTime = MAX_WAIT;
}
}
if ((handle.SafeWaitHandle.IsClosed)) {
exitLoop = true;
}
else if (handle.WaitOne(waitTime, false)) {
exitLoop = true;
signaled = true;
}
else {
if (Application.MessageLoop) {
Application.DoEvents();
}
else {
Thread.Sleep(1);
}
}
}
while (!exitLoop);
return signaled;
}

I went with something I haven't seen posted yet which is to use MessageQueues.
The MainThread blocks while waiting for the next message on a queue.
The background thread posts different types of messages to the MessageQueue.
Some of the message types signal the MainThread to update UI elements.
Of course, there is a message to tell the MainThread to stop blocking and waiting for messages.
Seems over the top considering the windows message loop already exists somewhere, but it works.