I have a problem that I have found many similar problems related to multi threading, but noone linked to me specific problem.
I have a method that does something, but when the method has been called, I don't want it to return untill a button is clicked.
So basically, at the end of the method, I would like it to stay put and wait for the Button_Click event to finish, and then the event should in some way "tell" the method to continue.
Currently I have done this by adding a loop at the end of the method like this:
while(someVariable){
Thread.Sleep(10);
Application.DoEvents();
}
Then the Button_Click event set someVariable to be false, and then the loop stops.
Of course, this looks very "iffy", and having this loop run every 10 milliseconds seems like a gigantic waste.
Are there any way of doing this properly? Since the method in question runs on the same thread as the rest of the application, it's also important that halting the method does not block the tread for other activities.
The reason I need this is because when the method is called from somewhere else, this method will draw up some components (including two buttons). Then the user clicks one of them, and then the method will return a different value depending on which button was clicked, and the program calling it can't continue untill it knows which button was clicked. So my program will look like this.
....
if( someMethod() == ButtonA ){
//do the proper action if button 1 is clicked
}else{
//do the proper action if button 2 is clicked
}
I hope this was not to confusing.
Assuming this is in the UI thread, you absolutely should not do this. You say "it's important that halting the method does not block the thread for other activities" - that basically means you need to find a different design. If the UI thread is "staying put and waiting for the Button_Click event to finish" then it can't be doing anything else without using the hack of Application.DoEvents().
Instead, you should make a call back to the original caller when the event occurs. Either expose an event that the caller can subscribe to, and raise that event when the button click finishes, or get the caller to pass in a delegate to call appropriately. (They're equivalent really, just different ways of propagating the callback.)
That's the way WinForms (and other rich client UIs) are designed to work - on an event-based model, basically.
Use AutoResetEvent, see sample and documentation at http://msdn.microsoft.com/en-us/library/58195swd.aspx
Your main thread does a WaitOne() on the event and in you Button.Click event, you Set() the event.
Related
I have an event to fire, named ValueGenerated. The code that generates values and fires ValueGenerated is running in a thread and the method which recieves this event is on a form.control (i.e. a form). As UI thread does not allow another thread to change the UI I wrote the following code on the event generation:
if (ValueGenerated.Target is System.windows.form.control)
{
Control targetForm = ValueGenerated.Target as control;
targetForm.Invoke(ValueChanged,new object[]{this,args});
}
But I think what happens if the event is registered by more than one methode. For example, by two or three destinations. Why on the event and delegate classes we have just the Target property which returns the instance object of the last method added? Do we always need just the last one?
You're doing it wrong.
As noted in the comments, you can get the full list of invocation targets by calling GetInvocationList() on the delegate instance. Then you can invoke each target individually.
But this is not the right way to do it. Your event should treat all handlers the same.
If the event is the kind of event that is always raised in a background thread, and is always handled by a UI object, then it should always use an appropriate mechanism to dispatch to the UI thread. See the BackgroundWorker class for an example of this sort of design, specifically its ProgressChanged and RunWorkerCompleted events.
If either of those conditions are not true, then your event should not attempt to deal with the cross-thread invocation in any way. Subscribers to the event that have thread affinity should be expected to deal with that themselves.
Unfortunately, there's not enough context in your question to provide any more specific advice than that. The only thing that is clear is that you've started down a dead-end road. Turn around, come back, and take the smoother path. :)
ok so I have a form with a button and a combobox,when the form loads I start a new thread. In that thread I want to listen for the click event from the form and get the selected item from the combobox. Whats the best way to go about doing this?
If you do some long running CPU expensive processing on button click, i would recommend to start a new thread every time the button is clicked. The best way would be to listen the click event in the main thread, and when the event fires - start a new thread and pass selected value there.
You can't strictly 'listen' to an event in C#. What you are doing when you attach an event handler to a control is providing a method (as a delegate) that you would like called when the event occurs. Your method will be invoked on whatever thread the invoking class decides to call it on, typically the same thread that the cause of the event happened on. For Windows Forms controls, this is always the UI thread.
In your event handler, you could (and often should) call another method to do the actual work, on another thread. There are many ways of doing this, from manually starting a thread, to the thread pool, to using the TPL.
If you want to have just one thread, that you control, processing all the events, the best idea is having a producer-consumer relationship. Have a queue into which your event handler places an instruction to do work, and have a thread which takes items out of the queue and processes them.
To signal your worker thread to know when there is new work, you can look into AutoResetEvent.
So I am doing a somewhat lengthy progress in another class, and I want to give some progress info to my GUI. I am aware of background worker, and may use it for this if I HAVE to, but this operation is so simple that I feel that background worker is a bit more than I need. Instead, I am using eventhandlers to handle the updates, but the operation is inconsistent.
When a point has been reached in the worker class, it puts up an event telling about its progress. In the GUI class, I have an eventhandler listening for that event. When it finds it, it makes a string about it, and puts that string as the text of a label.
I then call labelname.Update() in that eventhandler, but nothing happens. Here's the really confusing part, I put a textbox there instead, set its text, and then called textboxname.Update(), and it worked. Why would .Update() not work for one control, but not another. Is there a trick to get it to work for a label?
I am turning Roken's comment into an answer, but his remark is the answer. You must implement the background operation in a background worker, period.
See this excellent article about the why and how.
(1) Try to call labelname.Invalidate(); before calling labelname.Update();
Calling the Invalidate method does not force a synchronous paint; to
force a synchronous paint, call the Update method after calling the
Invalidate method. (MSDN)
(2) Another option is calling labelname.Refresh();
Forces the control to invalidate its client area and immediately
redraw itself and any child controls. (MSDN)
I have a Winforms App (.NET 3.X) That runs a method in a class to process some data. The method periodically raises a StatusUpdate event with a count of the number of items processed. I have a ToolStripStatuslabel on the Form that i would like to update with the count. The problem is that status label never updates with this count until the process is complete. Below is the code from the status update event handler
toolStripStatusLabel.Text = e.Count.ToString();
statusStrip.Refresh();
I think the problem is that the Refresh event is not firing because the processing method is being called from within a Button press event. I think there is a way to force the Refresh to process but I do not remember what it is.
My only other solution is to execute the processing in it's own thread.
Found the answer in another thread:
Call Application.DoEvents() after setting the label, but you should do all the work in a separate thread instead, so the user may close the window.
This is the command that I was thinking of...
Have you tried calling refresh on the label itself ?
toolStripStatusLabel.Refresh();
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.