In any .NET 2.0 Winforms application, if you run the code in Debug Mode, and you either hit a breakpoint or otherwise Step Into the code line by line, and you want to see the GUI of the application being debugged, then .NET does not draw the application screen.
For example, I have an application which writes some messages to a TextBox on a Form. When I debug the code step by step, or when a breakpoint is hit, I want to see what all messages are logged in the TextBox, but if I press Alt-Tab to switch from VS2005 window to the WinForms application window, all I see is white color. The form is not redrawn, until you press F5 in the debug mode in VS2005.
What is the reason for this, and is there a way to overcome this, without introducing any threads in the code?
What is the reason for this
While you're debugging, you're effectively blocking the UI thread - you're manually stepping through its execution flow. The UI thread can't draw the UI while you're stopping it from executing.
and is there a way to overcome this
You could try calling Application.DoEvents() manually, but I'd generally recommend against it.
It would be better to just wait until you got to the end of the method and let the UI redraw itself normally. If your method is very long (in terms of time) then bear in mind that when not debugging, the UI still wouldn't be able to update itself while that method is executing. This may cause you to change your design (it's hard to tell just from the information we've got at the moment).
The reason for this is because you can only have one UI thread, and when you enter your method that updates that code that code begins blocking the UI thread. It will not update until your method exits.
Here is a good SO on message pumps, which are what drive the UI updates
You should be able to use Add Watch/Quick Watch to look at any values at the time of debugging. This sounds like what you are really looking for, anyway.
Like everyone else has said in answers and comments, the UI thread is blocked so it cannot be redrawn.
However, if all you want to do is see the GUI, and not interact with it, and you are running Windows 7/8 (which sounds unlikely since you're using VS2005) and haven't disabled aero peek, you can mouse over your application in the task bar and Windows will show the preview thumbnail. When you mouse over the thumbnail, you can "peek" at the application even when the breakpoint is blocking the UI thread.
Related
My question is similar to this:
Wait for gui to finish - matlab
But, the uiwait(f) doesn't work - when the user hits Ctrl-C - it closes the app rather than doing a copy.
Here's the situation:
We're compiling MATLAB, and running WPF on top to provide the UI. However, we need to block the MATLAB execution in order to display our WPF UI. In the past we've used the input() command - but this causes severe CPU load.
Using waitfor or uiwait is better, but again, prevents our users from using Ctrl-C in the WPF UI. No Good!
Anyone have any thoughts?
You have to stop the processing, not the UI.
Often stopping the UI will stop the processing as a side effect (the Process is waiting for the UI to accept a write order or something). But it will also stop any input processing, like Ctrl+C.
Google says there should be a "pause" command of some sort to do this:
https://de.mathworks.com/matlabcentral/answers/68698-can-i-pause-matlab-excution-while-it-is-already-running
Stop and continue execution from debugger possible?
I made a program based on Aforge (it's a video library). This library creates its own refresh events for the next video frame from camera. So far so good, earlier people have helped me with multi threading so these Aforge threads could report back to the main program thread, again so far so good, it works great.
The code can be seen here: how to do multithreading when using outside referenced code.
But now I regularly notice a new program error. The problem starts when I want to exit the program. At the moment the Aforge thread might still be active. And it wants to write back to main form that is closed.
Somehow I need to stop the other thread before I close the program. There is an option in Aforge to do something like camera stop, but it's not enough. But then still my code wants to write on the main form that's already closing.
Is there a way to stop all threads, or some safe way to close from the originating thread? I even placed the back reporting to the main program in a try catch construction but it didn't work well, the only way of stopping it in these situations is to press the stop execution button within Visual Studio itself.
If I understand your problem correctly, you may have an Invoke call about to happen on your form just as you've closed the form. I've had this happen as well.
What I've done is to set a "shutdown" variable when I start to close the form, and then use AppDomain.CurrentDomain.UnhandledException to watch for InvalidOperationException (or whatever you're getting), and ignore if shutdown is set.
I'm writing a c# application that requires user authentication.
When the user hits the log in button, quite a bit of stuff is done in the background, but I'm having trouble informing the user that something is happening, and that the program isn't just frozen.
I have some hidden text fields that I would like to have show up after they log in, while this stuff is running, but I can't seem to get it to show up.
Basically, when the user hits the log in button, it checks to see if their credentials are correct, then the messages should show up, and then either some other functions might run, followed by a different form being shown.
After the credentials are checked, and I know that the user is valid, I tried running this:
please_wait.Visible = true;
But it doesn't change when it gets to that point in the code.
I've tried threading it, to see if that would help. Instead of calling the above line, I just start a thread that does it.
That doesn't work either. The field still doesn't show up.
If I return out of the function right after I either start the thread or change the Visible property, it works just fine.
How do I get it to work fine and have more code run after the change?
The problem isn't that you need to update the UI from a background thread. Rather, it's that you should be performing your long-running task in a background thread and marshalling updates to the foreground. This is frequently done via a BackgroundWorker with progress notification (on a progressbar, for example).
Basically, your UI foreground thread is either loaded or blocked doing work, so it isn't handling messages in its message queue to update your user interface.
Along with what Greg recommends (which is certainly the first step if you're not already doing it) you may also need to give up some quantum.
If you are taxing the scheduler, it's possible that the UI updates (which are pretty low priority) may be getting preempted until your worker is complete. Adding an Application.DoEvents (or maybe Thread.Sleep(1) in the background thread) could give the UI a little scheduler time to paint.
Have you tried adding a call to Application.DoEvents()? It's a hack, but it's all you sometimes need.
K I am looking at a primarily single thread windows forms application in 3.0. Recently my boss had a progress dialogue added on a separate thread so the user would see some activity when the main thread went away and did some heavy duty work and locked out the GUI.
The above works fine unless the user switches applications or minimizes as the progress form sits top most and will not disappear with the main application. This is not so bad if there are lots of little operations as the event structure of the main form catches up with its events when it gets time so minimized and active flags can be checked and thus the dialog thread can hide or show itself accordingly.
But if a long running sql operation kicks off then no events fire. I have tried intercepting the WndProc command but this also appears queued when a long running sql operation is executing. I have also tried picking up the processes, finding the current app and checking various memory values isiconic and the like inside the progress thread but until the sql operation finishes none of these get updated. Removing the topmost causes the dialog to disappear when another app activates but if the main app is then brought back it does not appear again.
So I need a way to find out if the other thread is minimized or no longer active that does not involve querying the actual thread as that locks until the sql operation finishes.
Now I know that this is not the best way to write this and it would be better to have all the heavy processing on separate threads leaving the GUI free but as this is a huge ancient legacy app the time to re-write in that fashion will not be provided so I have to work with what I have got.
Any help is appreciated
It sounds as if the long running operation is bound to the progress dialog? That's usually a bad idea and I wonder whether the progress can be showed at all.
However you should consider using a BackgroundWorker for your long running operations. So your GUI (the main form as well as the progress dialog stays alive).
This way you should be able to send the minimize event of the main form to the progress dialog which can react to it instantly.
Btw. the BackgroundWorker supports progress reports on its own.
I'm building a UI for a program, and I can't figure out why my progress bar won't become visible after the convert button is clicked.
private void convertButton_Click(object sender, EventArgs e)
{
toolStripProgressBar.Visible = true;
...
toolStripProgressBar.Visible = false;
}
I ran into a similar problem with tkinter in Python, and I had to call a function to update the idle tasks. Is there a way to do this with windows forms without using threads?
Edit: On a side note, this is a progress bar in a toolStrip that also contains a label that gets updated with status bar text. Is there any way to get the label on the left side and the progress bar on the other instead of right next to each other on the left?
Well, there is a way to do this without using threads (Application.DoEvents) but I strongly recommend against you using it. Re-entrancy is nasty, and you really don't want the UI thread tied up at all.
Use BackgroundWorker instead - it's easy, and it's pretty much designed for progress bars. It takes the hassle out of using a separate thread and reporting progress back to the UI thread. No need for Control.Invoke etc - it takes care of that for you.
There are lots of tutorials for BackgroundWorker - it shouldn't take you too long to get going with it.
Per the question you asked for the way to do this WITHOUT threads, that is to do it with Application.DoEvents();. (Just add that call right after setting the progress bar as visible.)
Now I do agree with Jon Skeet though that BackgroundWorker is a better way of doing this, but it does use a separate thread.
You need to execute your process in a thread separate from the UI thread, and then have it periodically report back to the UI thread with it's progress. If your convert operation is working inside the UI thread, it will simply go unresponsive until the operation is complete.
The progress bar can only become visible when it is allowed to paint which occurs during the processing of messages. Message processing cannot normally happen while you are in the middle of an event handler. If you want the progress bar to show up you will have to set the visiblitity to true, start a background thread to complete the work and return from the handler.
I'm guessing the problem is that the "..." in your code is a long-running process. UI updates are not instantaneous, but must run through the message queue in windows and then be painted to the screen. The queue is pumped and painting takes place in the same thread as your events.
As a result, any long-running tasks need to be moved to a different thread. More than that, your line line of code needs to called after that thread terminates. Otherwise you set the progress bar and then immediately turn it off again.
One way to do that is with a BackgroundWorker control.
Here go two links trying to explain you how things work:
(1) (2)
Now, I will try to explain it as shortly as I can. Most of what happens inside a windows forms application happens in a single thread, usually the same thread Main() runs in. If you open Program.cs, you will see that Main() has a line that looks like the following:
Application.Run(new Form1());
If you debug the application at any moment and examine the call stack, you will see it will trace back to that Run method. This means that a Windows Forms application is in fact a continuous run of the Run method. So, what is Run doing? Run is eating a message queue through which Windows sends messages to it. Run then dispatches those messages to the correct controls, which themselves do things like add text which corresponds to the key being pressed, redraw themselves, etc. Notice that all this happens during and endless loop running alongside a single thread, so weather you are typing or simply moving the window around, loads of those messages are being passed onto the application, which in turn is processing them and reacting accordingly, all in that single thread. Controls can also send messages to themselves through the queue and even you can place messages in the pump via Control.BeginInvoke. One of the things those controls do is to raise events according to what happens. So, if you click a button, the code you've written to handle that click will ultimately and indirectly be run by the Application.Run method.
Now, what is happening with your code is that even though you are changing the visible status of your progress bar to visible and then updating its Value, you are then changing its visibility to false, all in the same method. This means that only after you leave the method, will Application.Run() be able to continue iterating and consuming the message queue, effectively asking the progress bar to update its display. When that happens, you've already left the progress bar's visibility to false, the last thing you did before exiting the method. DoEvents() is a quick and dirty workaround to your problem as it reads the messages in the queue and processes them. I don't really feel comfortable using it as it can bring reentrancy problems.
Using threads is a good solution, but I would recommend using a ThreadPool thread instead of a custom thread in this kind of situation, as I tend to use custom threads only in cases where I have a limited number of long lived threads and I need to control their life cycles. The easiest and most practical way to use threads is to use the BackgroundWorker component, even though I would recommend going through the pains of understanding how to do Windows Forms multithreading with delegates if you want to really understand what is going on.
My solution is to call refresh on the status strip.
I believe this causes the UI thread to repaint the status strip.
toolStripStatusBar1.PerformStep();
statusStrip1.Refresh();
This is for .NET 4.0. Even though this question is old it was the first I found on googling this issue.