I have a simple UserControl for database paging, that uses a controller to perform the actual DAL calls. I use a BackgroundWorker to perform the heavy lifting, and on the OnWorkCompleted event I re-enable some buttons, change a TextBox.Text property and raise an event for the parent form.
Form A holds my UserControl. When I click on some button that opens form B, even if I don't do anything "there" and just close it, and try to bring in the next page from my database, the OnWorkCompleted gets called on the worker thread (and not my Main thread), and throws a cross-thread exception.
At the moment I added a check for InvokeRequired at the handler there, but isn't the whole point of OnWorkCompleted is to be called on the Main thread? Why wouldn't it work as expected?
EDIT:
I have managed to narrow down the problem to arcgis and BackgroundWorker. I have the following solution wich adds a Command to arcmap, that opens a simple Form1 with two buttons.
The first button runs a BackgroundWorker that sleeps for 500ms and updates a counter.
In the RunWorkerCompleted method it checks for InvokeRequired, and updates the title to show whethever the method was originaly running inside the main thread or the worker thread.
The second button just opens Form2, which contains nothing.
At first, all the calls to RunWorkerCompletedare are made inside the main thread (As expected - thats the whold point of the RunWorkerComplete method, At least by what I understand from the MSDN on BackgroundWorker)
After opening and closing Form2, the RunWorkerCompleted is always being called on the worker thread. I want to add that I can just leave this solution to the problem as is (check for InvokeRequired in the RunWorkerCompleted method), but I want to understand why it is happening against my expectations. In my "real" code I'd like to always know that the RunWorkerCompleted method is being called on the main thread.
I managed to pin point the problem at the form.Show(); command in my BackgroundTesterBtn - if I use ShowDialog() instead, I get no problem (RunWorkerCompleted always runs on the main thread). I do need to use Show() in my ArcMap project, so that the user will not be bound to the form.
I also tried to reproduce the bug on a normal WinForms project. I added a simple project that just opens the first form without ArcMap, but in that case I couldn't reproduce the bug - the RunWorkerCompleted ran on the main thread, whether I used Show() or ShowDialog(), before and after opening Form2. I tried adding a third form to act as a main form before my Form1, but it didn't change the outcome.
Here is my simple sln (VS2005sp1) - it requires
ESRI.ArcGIS.ADF(9.2.4.1420)
ESRI.ArcGIS.ArcMapUI(9.2.3.1380)
ESRI.ArcGIS.SystemUI (9.2.3.1380)
Isn't the whole point of OnWorkCompleted is to be called on the Main thread? Why wouldn't it work as expected?
No, it's not.
You can't just go running any old thing on any old thread. Threads are not polite objects that you can simply say "run this, please".
A better mental model of a thread is a freight train. Once it's going, it's off on it's own track. You can't change it's course or stop it. If you want to influence it, you either have to wait til it gets to the next train station (eg: have it manually check for some events), or derail it (Thread.Abort and CrossThread exceptions have much the same consequences as derailing a train... beware!).
Winforms controls sort of support this behaviour (They have Control.BeginInvoke which lets you run any function on the UI thread), but that only works because they have a special hook into the windows UI message pump and write some special handlers. To go with the above analogy, their train checks in at the station and looks for new directions periodically, and you can use that facility to post it your own directions.
The BackgroundWorker is designed to be general purpose (it can't be tied to the windows GUI) so it can't use the windows Control.BeginInvoke features. It has to assume that your main thread is an unstoppable 'train' doing it's own thing, so the completed event has to run in the worker thread or not at all.
However, as you're using winforms, in your OnWorkCompleted handler, you can get the Window to execute another callback using the BeginInvoke functionality I mentioned above. Like this:
// Assume we're running in a windows forms button click so we have access to the
// form object in the "this" variable.
void OnButton_Click(object sender, EventArgs e )
var b = new BackgroundWorker();
b.DoWork += ... blah blah
// attach an anonymous function to the completed event.
// when this function fires in the worker thread, it will ask the form (this)
// to execute the WorkCompleteCallback on the UI thread.
// when the form has some spare time, it will run your function, and
// you can do all the stuff that you want
b.RunWorkerCompleted += (s, e) { this.BeginInvoke(WorkCompleteCallback); }
b.RunWorkerAsync(); // GO!
}
void WorkCompleteCallback()
{
Button.Enabled = false;
//other stuff that only works in the UI thread
}
Also, don't forget this:
Your RunWorkerCompleted event handler should always check the Error and Cancelled properties before accessing the Result property. If an exception was raised or if the operation was canceled, accessing the Result property raises an exception.
It looks like a bug:
http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=116930
http://thedatafarm.com/devlifeblog/archive/2005/12/21/39532.aspx
So I suggest using the bullet-proof (pseudocode):
if(control.InvokeRequired)
control.Invoke(Action);
else
Action()
The BackgroundWorker checks whether the delegate instance, points to a class which supports the interface ISynchronizeInvoke. Your DAL layer probably does not implement that interface. Normally, you would use the BackgroundWorker on a Form, which does support that interface.
In case you want to use the BackgroundWorker from the DAL layer and want to update the UI from there, you have three options:
you'd stay calling the Invoke method
implement the interface ISynchronizeInvoke on the DAL class, and redirect the calls manually (it's only three methods and a property)
before invoking the BackgroundWorker (so, on the UI thread), to call SynchronizationContext.Current and to save the content instance in an instance variable. The SynchronizationContext will then give you the Send method, which will exactly do what Invoke does.
The best approach to avoid issues with cross-threading in GUI is to use SynchronizationContext.
Related
This question already has answers here:
Async Progress Bar Update
(3 answers)
Closed 3 years ago.
I have a form (called Form1) and I have created a status strip (called toolStripProgressBar1), with a label and a progress bar. I have a function that is called when you press a button and gets some data and processes it, which works well.
I want to provide the user with some information as to its progress so I want to set the label and progress bar but I can't get it to set
private static async Task GetSurvey(string surid)
{
Form1.toolStripProgressBar1.Value = 10;
It will be a lot easier if you don't do it as static; doing the work in a method that has access to the form instance variables would make this quite trivial. Be aware though that your progress bar is (== should be) updated by the windowing thread, and most things like button click event handlers are actioned by that same thread. This means that you click your button and the thread that created the progressbar gets busy doing your long running code...
Now, you're running your task async and this is good, just make sure that you await any lengthy operations so that the windowing thread can hand off to a background process at that point and go back to its main job (updating the UI) otherwise you won't see your progress bar updating very effectively. Don't try to update form controls from code that is run on a different thread to the main windowing thread.
If you're not clear on how async/await works, visualise it that upon encountering await within some method block, the thread that is executing the code will create a background process to complete the await'd section, and it itself will travel back up the call stack til it reaches the first method not marked async and proceed from there. In practical terms in a windows forms app you want all your code that "does stuff" in response to a button click to be marked async so you can release the windowing thread out of your code context entirely and back to doing its job of keeping the UI responding, whenever you use await
An alternative way of doing this might be tonise a BackgroundWorker - you attach a DoWork event handler that does the work, and as part of this code it should regularly call the ReportProgress method, passing in an int percentage completion. The ProgressChanged event handler on the BGWorker is used to set the percentage bar/Uu elements -critically you need to know that DoWork runs on a thread that is NOT the windowing thread (and windows controls must not be accessed from a thread other than the thread that created them, usually the windowing thread) so we don't access the progress bar directly from DoWork- instead we call ReportProgress and that causes the ProgressChanged event to fire, and the BGWorker deliberately arranges things so the code in that event handler IS run on the windowing thread(actually the thread that creates the worker but this should be the same)
Closing a form from inside an Invoke like this:
Invoke(new Action(() => {
Close();
MessageBox.Show("closed in invoke from another thread");
new Form1();
}));
throws an exception as soon as the form is closed:
Invoke or BeginInvoke cannot be called on a control until the window
handle has been created.
But only on NET 4.0. On NET 4.5 no exceptions are thrown.
Is this expected behavior? How should I go about it?
That's because Close method closes the form and destroys it's handle and then the MessageBox is invoked in the Closed form with no handle, so the error message shows up.
I don't understand your purpose, but you should either move the code after Close out of invoke, or move the Close after them. For example:
Invoke(new Action(() => {
Hide();
MessageBox.Show("closed in invoke from another thread");
new Form1();
Close();
}));
Edit:
MSDN note about Control.Invoke:
The Invoke method searches up the control's parent chain until it finds a control or form that has a window handle if the current control's underlying window handle does not exist yet. If no appropriate handle can be found, the Invoke method will throw an exception. Exceptions that are raised during the call will be propagated back to the caller.
If you start a thread during initialisation, you do not know how far the initialisation has gone in another thread.
You notice differences in behavior on different .Net versions, but you cannot be sure about the order of things on different machines.
I have solved a lot of threading issues in Windows forms using my own messagepump, using a Queue and a normal Timer control:
Add a timer control to your form, with a small interval (250 ms)
Add a Queue to your form.
Let the timer event dequeue the actions, and execute it.
Add Actions to the queue during initialisation or even other background jobs.
Using this approach will issues with background jobs during initialisation, but also during closing/disposing of the form, since the timer will only trigger if the form is fully functional.
I have a main GUI application, that does all of it's actual work in a referenced assembly. Right now, I DON'T do the work in a background worker, so it basically locks the main UI while it does it's processing. In my referenced assmbly, I added quite a few events to report back different progress back to the main UI form. On the main UI form, I update different text boxes with the values from those events. My question is, first of all, the processing appears to be much slower when throwing these events. So should I fire the events on a secondary thread (from the referenced assembly)? Should my original call to the referenced (static) assmebly be via a background worker? I'd like to report the different types of progress on a separate thread, just not sure which approach to take to have optimal performance.
Thanks
From your description it sounds like you would benefit from multithreading, as it would help keep the UI responsive.
And the easiest way to do this is to use a BackgroundWorker. Start by working through one of the many samples, then bite the bullet, and come back here if you have any problems.
In response to comment:
The best way to communicate from a BackgroundWorker worker thread to the main thread is to call the BackgroundWorker.ReportProgress method, which takes an optional object parameter userState which you can use to package up the data you want to communicate.
This causes the BackgroundWorker.ProgressChanged event to be raised on the main thread - and the data can be processed without the need for an explicit Invoke.
If you've already implemented events, you'll either have to do some rework to call ReportProgress instead of raising events, or implement some kind of adapter to handle the events and route them to ReportProgress method calls.
You could launch your process (the method on the other assembly) on a different thread, and handle the events raised by it on the main form.
Since the UI cannot be updated by another thread, you should wrap the code of those events on a this.Invoke().
Ex:
private void TheEventRaisedOnAnotherThread(object sender, EventArgs e)
{
_counter++;
this.Invoke(new MethodInvoker(delegate() { TextBox1.Text = _counter.ToString(); }));
}
I have a UI design question.
I would like to exit the application when it encounter an exception on a non-UI thread.
Basically, the event goes like this:
Main Form -> ShowDialog of sub WinForm (MainThread)-> Starts a background thread (WorkerThread) -> Exception occurs -> Show an ErrorForm (WorkerThread)
When the user click Exit button on the ErrorForm, i want to exit the entire application. However, doing the following call doesn't work.
Invoker.Invoke((Action)(() => { Application.Exit(); }), null);
The Invoker reference to the main form SynchronizedContext. However, since the MainThread is still waiting for the subWinForm to return its control, it probably can't handle the Application.Exit().
What would be a better design to handle exception that is thrown by a background worker thread?
Cancel the background worker and send an argument to the BackgroundWorker RunWorkerCompletedEvent to identify there is an exception. After that call the Application.Exit() from there would be fine.
I know that invoking like this works in Silverlight:
Dispatcher.BeginInvoke(() => Application.Exit());
Or if there's no Dispatcher for your WinForms classes:
Invoker.BeginInvoke(() => Application.Exit());
You had a lot of extra unnecessary code ((Action), null, unnecessary brackets and parentheses). I don't think it would have stopped it from working correctly, but in any case, it's easier to read like this.
I want to call ShowDialog() when a keyboard hook event is triggered, but I'm having some difficulties:
ShowDialog() blocks, so I can't call it from the hook triggered event, because it will block the OS.
I can start a new thread and call ShowDialog() from there, but I get some nasty exception. I guess I can't call ShowDialog() in any other thread.
I can start a timer: in the next 50 milliseconds call ShowDialog() (which is a nasty hack BTW, and I rather not do this). But then the timer fires in a new thread, and then I run into the same problem explained in the previous bullet.
Is there a way?
The problem may be that you are trying to put UI in a non-UI thread. Make your event fire from another thread and invoke the method that runs ShowDialog() from your UI thread.
Essentially, you want to keep your UI on the UI thread and move anything else to a back ground thread.
Check out Gekki Software for some details (there are zillions of others - this just happens to be the first one I found in research archives).
I'm not sure about ShowDialog, but whenever you get an exception when trying to do something with the UI in a background thread, it means you should use the UI dispatcher.
Try calling the BeginInvoke method (if you are on Windows Forms) of any UI object you control with a delegate that calls the showdialog.
Also, make sure to try (before this) passing a reference to a valid owner in the show dialog method.
Try this:
void MyKeyboardHookHandler(...)
{
WindowsFormsSynchronizationContext.Current.Post(state =>
{
Form f = new Form();
f.ShowDialog();
}, null);
}
You really should be able to show the dialog from a KeyPress type event.
Also, if you use ShowDialog() from another thread, it will not be modal (no parent). It would be the same as using Show().
Without the "nasty exception" it's hard to tell what's going on. I would assume it's because your thread isn't an STA thread, and the UI objects are throwing the exception when they get instantiated. Set your new thread's apartment model to be STA instead of MTA and see if that helps.
And if you don't know what the difference is, you should do some reading up, for instance Multithreaded Apartments (MSDN).
ShowDialog() will block your application's thread, but that's what it's supposed to do. If you don't want the form blocking your application, call Show() instead.
ShowDialog() will not "block the OS", so don't be reluctant to use it.