Could you please tell me how do I go about pausing my program for 500 milliseconds and then continue?
I read Thread.Sleep(500) is not good as it holds up the GUI thread.
Using a timer it fires a callback ...
I just want to wait 500ms and then continue to the next statement.
Please advise.
EDIT: I need to display a status bar message for 500ms and then update the message with a different one. Sorry, I meant 500 not 50.
EDIT: I do understand what all you have said. but: [I just want to wait 500ms and then continue to the next statement.] I think because it is such a short interval i am going do a Thread.Sleep(500) on the main GUI thread. Otherwise i would have to rewrite a lot of code to accomodate this brief interval of 500 milliseconds.
EDIT: i will try to reformat my status message so the pause is not needed.
Hmya, what you're trying to do is pretty fundamentally incompatible with the Windows programming model. A native Windows program is event driven. Your program is always idle, sitting inside a loop started by Application.Run(), waiting for Windows to tell it that something interesting happened that it should respond to. Paint requests, mouse clicks, timer expirations, stuff like that.
Your program should respond to this and filter what is interesting to you. When you drop a button on a form, you are always interested in the Click event, generated when Windows sends the MouseDown notification message. Your Click event handler runs some kind of custom code that you write. Like updating a status bar message in your case.
Updating the status bar message half a second later doesn't make a whole heckofalot of sense. What exactly happened during those 500 milliseconds that changed the way your program responds to events? You can call the Update() method of the StatusBar so the new message is visible, then call System.Threading.Thread.Sleep(500) to get what you want. You'll get away with it, the "Not Responding" ghost that Windows puts up takes your program going dead for several seconds.
But that doesn't make a lot of sense, nothing happened during that half second, the state of your program didn't change. It couldn't change, it was dead to Windows and not receiving any messages that would allow it to change state.
Well, that's about as far as I can take this. Please update your question and explain why you need to do this. Just in case: if you're contemplating this to fake doing something important for half a second, your user will not be impressed. She'll eventually notice your UI is dead for half a second without anything to show for it.
You have two choices:
Use a timer as you suggested. Split your method up into two methods, foo1 and foo2. Use the foo1 to start the timer and run foo2 in the callback.
Use a BackgroundWorker for running the entire function and use Thread.Sleep on the worker thread.
From your update it seems that the only thing you want to do is change a single field. I would definitely recommend the first method: using a timer. Starting a BackgroundWorker for this task is overkill and will just give you unnecessary extra work and complications.
Instead of pausing the UI directly for 500 ms, you can always use a BackgroundWorker. That will cause your callback to run in a separate thread, where you can use Thread.Sleep to pause it without blocking the UI. Then when you are done, just update the status bar with your new message.
More context to the question would be helpful.
Thread.Sleep(50) will pause the current thread for 50 milliseconds. If you're doing this in the UI thread, then yes, it will freeze the UI for 50 milliseconds. However, if you use a different thread to do this processing, then calling Sleep on that thread will pause it for 50 milliseconds without freezing your UI thread.
See Marc's answer to this question for an example on using a BackgroundWorker instance to do what you need.
In C# your best bet is to use the Timer and fire a callback.
In F# there is an awesome way to do what you want, see
F# async on the client side
which shows how to write straight-line code and have the language take care of the callbacks for you.
You need to allocate another thread. In that thread you Sleep(500) and change the needed data. Caution: you would need to use the original thread's dispatcher, because the data related to UI should be usually updated from the GUI thread.
Related
I'm new in C# and I'm using System.Threading.
I have this code:
UISystem.SetScene(Scene_Menu);
Thread.Sleep (9000);
p.Text="HELLO";
Thread.Sleep(9000);
p.Text="WORLD";
It delays 18 seconds, but the p.Text="HELLO" doesn't show between the sleep functions. What's the problem with my code?
Thanks.
Timers don't work since I can't edit p from a separate thread.
Application.DoEvents() is a Windows Forms function, I'm building an application in PS Vita.
You have discovered why you should never use Thread.Sleep. It is useful for only two things. (1) Writing test cases that need to simulate a thread being busy for a certain number of seconds, and (2) Sleeping for zero milliseconds tells the operating system "I cede the rest of my time slice to another process if there exists one that wants it"; it's a politeness thing.
You should never use thread.Sleep to introduce a delay as you are doing for exactly the reason you have discovered. You are setting a property, but setting a property does not cause the operating system to repaint the screen. Consider if it did; you might have a thousand property sets in a method, and you would have to repaint the screen after all of them, which would look ugly and be very slow.
Instead what happens is the property is set and the object makes a note to the operating system that says when this thread is available to handle operating system messages again, please repaint me. Your program is, instead of telling the operating system "I'm done, go ahead and see if there are any message for me" that instead you want the thread to do nothing for nine seconds.
Now, you can tell the program to check for messages by calling DoEvents but using DoEvents is also a bad idea and you should not do it. Doing so essentially causes your program to exhibit symptoms of Attention Deficit Disorder; you have not finished the current job and you are looking to see if there are new jobs to do without removing the old jobs from the call stack! Suppose those new jobs in turn get interrupted, and so on, and so on. The stack grows without bound, which is very bad. DoEvents is a "worst practice", just like sleeping a thread. You can get away with it in small simple programs but it leads to big trouble when the program becomes complex.
Moreover: yes, DoEvents will paint your control, but that is all it will do. For the next nine seconds, the application will appear to the user to be completely hung. That is a very bad user experience.
The right thing to do if you want to introduce a delay is to asynchronously wait. In C# 4 and earlier the standard way to do that is to create a timer, and when the timer ticks, do the next thing.
Now, you say that you cannot use a timer because you need to access the control from the UI thread. That's fine. The timer's tick event handler will run on the UI thread, not on a separate thread. You can safely use a timer.
In C# 5, the right thing to do is to use the new await keyword to introduce an asynchronous wait. That is, a wait that does other stuff while it is waiting, instead of going to sleep while it is waiting. In C# 5 you would write your code as:
UISystem.SetScene(Scene_Menu);
await Task.Delay (9000);
p.Text="HELLO";
await Task.Delay(9000);
p.Text="WORLD";
C# 5 is at present in beta; for details on this new feature see:
http://msdn.microsoft.com/en-us/async
For a gentle introduction to async and an explanation of why DoEvents is bad news, see my MSDN magazine article:
http://msdn.microsoft.com/en-us/magazine/hh456401.aspx
I'm doing all this in C#, in Visual Studio 2008.
I want to slow down the work of my algorithm so that the user can watch it's work. There is a periodic change visible at the GUI so I added Thread.Sleep after every instance.
Problem is that Thread.Sleep, when set to at least a second, after a few instances of Thread.Sleep (after few loops) simply freezes entire GUI and keeps it that way till program completion. Not right away, but it always happens. How soon depends on the length of the sleep.
I have proof that entire program does not freeze, it's working it's thing, even the sleep is making pauses of correct length. But the GUI freezes at certain point until the algorithm ends, at which point it shows the correct final state.
How to solve this issue? Alternative to pausing algorithm at certain point?
First off, don't make the user wait for work that is done before they even think about when it will be finished. Its pointless. Please, just say no.
Second, you're "sleeping" the UI thread. That's why the UI thread is "locking up." The UI thread cannot be blocked; if it is, the UI thread cannot update controls on your forms and respond to system messages. Responding to system messages is an important task of the UI thread; failing to do so makes your application appear locked up to the System. Not a good thing.
If you want to accomplish this (please don't) just create a Timer when you start doing work that, when it Ticks, indicates its time to stop pretending to do work.
Again, please don't do this.
I'd guess everything is running out of a single thread. The user probably invokes this algorithm by clicking on a button, or some such. This is handled by your main thread's message queue. Until this event handler returns, your app's GUI cannot update. It needs the message queue to be pumped on regular basis in order to stay responsive.
Sleeping is almost never a good idea, and definitely not a good idea in the GUI thread. I'm not going to recommend that you continue to use sleep and make your GUI responsive by calling Application.DoEvents.
Instead, you should run this algorithm in a background thread and when it completes it should signal so to the main thread.
You are about to commit some fairly common user interface bloopers:
Don't spam the user with minutiae, she's only interested in the result
Don't force the user to work as fast as you demand
Don't forbid the user to interact with your program when you are busy.
Instead:
Display results in a gadget like a ListBox to allow the user to review results at her pace
Keep a user interface interactive by using threads
Slow down time for your own benefit with a debugger
This depends on a lot of things, so its hard to give a concrete answer from what you've said. Still, here are some matters that might be relevant:
Are you doing this on a UI thread (e.g. the thread the form-button or UI event that triggered the work started on)? If so, it may be better to create a new thread to perform the work.
Why do you sleep at all? If the state related to the ongoing work is available to all relevant threads, can the observer not just observe this without the working thread sleeping? Perhaps the working thread could write an indicator of the current progress to a volatile or locked variable (it must be locked if it's larger than pointer size - e.g. int or an object - but not otherwise. If not locked, then being volatile will prevent cache inconsistency between CPUs, though this may not be a big deal). In this case you could have a forms timer (there are different timers in .Net with different purposes) check the status of that variable and update the UI to reflect the work being done, without the working thread needing to do anything. At most it may be beneficial to Yield() in the working thread on occasion, but its not likely that even this will be needed.
Can Application.DoEvents() be used in C#?
Is this function a way to allow the GUI to catch up with the rest of the app, in much the same way that VB6's DoEvents does?
Hmya, the enduring mystique of DoEvents(). There's been an enormous amount of backlash against it, but nobody ever really explains why it is "bad". The same kind of wisdom as "don't mutate a struct". Erm, why does the runtime and the language supports mutating a struct if that's so bad? Same reason: you shoot yourself in the foot if you don't do it right. Easily. And doing it right requires knowing exactly what it does, which in the case of DoEvents() is definitely not easy to grok.
Right off the bat: almost any Windows Forms program actually contains a call to DoEvents(). It is cleverly disguised, however with a different name: ShowDialog(). It is DoEvents() that allows a dialog to be modal without it freezing the rest of the windows in the application.
Most programmers want to use DoEvents to stop their user interface from freezing when they write their own modal loop. It certainly does that; it dispatches Windows messages and gets any paint requests delivered. The problem however is that it isn't selective. It not only dispatches paint messages, it delivers everything else as well.
And there's a set of notifications that cause trouble. They come from about 3 feet in front of the monitor. The user could for example close the main window while the loop that calls DoEvents() is running. That works, user interface is gone. But your code didn't stop, it is still executing the loop. That's bad. Very, very bad.
There's more: The user could click the same menu item or button that causes the same loop to get started. Now you have two nested loops executing DoEvents(), the previous loop is suspended and the new loop is starting from scratch. That could work, but boy the odds are slim. Especially when the nested loop ends and the suspended one resumes, trying to finish a job that was already completed. If that doesn't bomb with an exception then surely the data is scrambled all to hell.
Back to ShowDialog(). It executes DoEvents(), but do note that it does something else. It disables all the windows in the application, other than the dialog. Now that 3-feet problem is solved, the user cannot do anything to mess up the logic. Both the close-the-window and start-the-job-again failure modes are solved. Or to put it another way, there is no way for the user to make your program run code in a different order. It will execute predictably, just like it did when you tested your code. It makes dialogs extremely annoying; who doesn't hate having a dialog active and not being able to copy and paste something from another window? But that's the price.
Which is what it takes to use DoEvents safely in your code. Setting the Enabled property of all your forms to false is a quick and efficient way to avoid problems. Of course, no programmer ever actually likes doing this. And doesn't. Which is why you shouldn't use DoEvents(). You should use threads. Even though they hand you a complete arsenal of ways to shoot your foot in colorful and inscrutable ways. But with the advantage that you only shoot your own foot; it won't (typically) let the user shoot hers.
The next versions of C# and VB.NET will provide a different gun with the new await and async keywords. Inspired in small part by the trouble caused by DoEvents and threads but in large part by WinRT's API design that requires you to keep your UI updated while an asynchronous operation is taking place. Like reading from a file.
It can be, but it's a hack.
See Is DoEvents Evil?.
Direct from the MSDN page that thedev referenced:
Calling this method causes the current
thread to be suspended while all
waiting window messages are processed.
If a message causes an event to be
triggered, then other areas of your
application code may execute. This can
cause your application to exhibit
unexpected behaviors that are
difficult to debug. If you perform
operations or computations that take a
long time, it is often preferable to
perform those operations on a new
thread. For more information about
asynchronous programming, see
Asynchronous Programming Overview.
So Microsoft cautions against its use.
Also, I consider it a hack because its behavior is unpredictable and side effect prone (this comes from experience trying to use DoEvents instead of spinning up a new thread or using background worker).
There is no machismo here - if it worked as a robust solution I would be all over it. However, trying to use DoEvents in .NET has caused me nothing but pain.
Yes, there is a static DoEvents method in the Application class in the System.Windows.Forms namespace. System.Windows.Forms.Application.DoEvents() can be used to process the messages waiting in the queue on the UI thread when performing a long-running task in the UI thread. This has the benefit of making the UI seem more responsive and not "locked up" while a long task is running. However, this is almost always NOT the best way to do things.
According to Microsoft calling DoEvents "...causes the current thread to be suspended while all waiting window messages are processed." If an event is triggered there is a potential for unexpected and intermittent bugs that are difficult to track down. If you have an extensive task it is far better to do it in a separate thread. Running long tasks in a separate thread allows them to be processed without interfering with the UI continuing to run smoothly. Look here for more details.
Here is an example of how to use DoEvents; note that Microsoft also provides a caution against using it.
From my experience I would advise great caution with using DoEvents in .NET. I experienced some very strange results when using DoEvents in a TabControl containing DataGridViews. On the other hand, if all you're dealing with is a small form with a progress bar then it might be OK.
The bottom line is: if you are going to use DoEvents, then you need to test it thoroughly before deploying your application.
Yes.
However, if you need to use Application.DoEvents, this is mostly an indication of a bad application design. Perhaps you'd like to do some work in a separate thread instead?
I saw jheriko's comment above and was initially agreeing that I couldn't find a way to avoid using DoEvents if you end up spinning your main UI thread waiting for a long running asynchronous piece of code on another thread to complete. But from Matthias's answer a simple Refresh of a small panel on my UI can replace the DoEvents (and avoid a nasty side effect).
More detail on my case ...
I was doing the following (as suggested here) to ensure that a progress bar type splash screen (How to display a "loading" overlay...) updated during a long running SQL command:
IAsyncResult asyncResult = sqlCmd.BeginExecuteNonQuery();
while (!asyncResult.IsCompleted) //UI thread needs to Wait for Async SQL command to return
{
System.Threading.Thread.Sleep(10);
Application.DoEvents(); //to make the UI responsive
}
The bad: For me calling DoEvents meant that mouse clicks were sometimes firing on forms behind my splash screen, even if I made it TopMost.
The good/answer: Replace the DoEvents line with a simple Refresh call to a small panel in the centre of my splash screen, FormSplash.Panel1.Refresh(). The UI updates nicely and the DoEvents weirdness others have warned of was gone.
I've seen many commercial applications, using the "DoEvents-Hack". Especially when rendering comes into play, I often see this:
while(running)
{
Render();
Application.DoEvents();
}
They all know about the evil of that method. However, they use the hack, because they don't know any other solution. Here are some approaches taken from a blog post by Tom Miller:
Set your form to have all drawing occur in WmPaint, and do your rendering there. Before the end of the OnPaint method, make sure you do a this.Invalidate(); This will cause the OnPaint method to be fired again immediately.
P/Invoke into the Win32 API and call PeekMessage/TranslateMessage/DispatchMessage. (Doevents actually does something similar, but you can do this without the extra allocations).
Write your own forms class that is a small wrapper around CreateWindowEx, and give yourself complete control over the message loop.
-Decide that the DoEvents method works fine for you and stick with it.
Check out the MSDN Documentation for the Application.DoEvents method.
The DoEvents does allow the user to click around or type and trigger other events, and background threads are a better approach.
However, there are still cases where you may run into issues that require flushing event messages. I ran into a problem where the RichTextBox control was ignoring the ScrollToCaret() method when the control had messages in queue to process.
The following code blocks all user input while executing DoEvents:
using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;
namespace Integrative.Desktop.Common
{
static class NativeMethods
{
#region Block input
[DllImport("user32.dll", EntryPoint = "BlockInput")]
[return: MarshalAs(UnmanagedType.Bool)]
private static extern bool BlockInput([MarshalAs(UnmanagedType.Bool)] bool fBlockIt);
public static void HoldUser()
{
BlockInput(true);
}
public static void ReleaseUser()
{
BlockInput(false);
}
public static void DoEventsBlockingInput()
{
HoldUser();
Application.DoEvents();
ReleaseUser();
}
#endregion
}
}
Application.DoEvents can create problems, if something other than graphics processing is put in the message queue.
It can be useful for updating progress bars and notifying the user of progress in something like MainForm construction and loading, if that takes a while.
In a recent application I've made, I used DoEvents to update some labels on a Loading Screen every time a block of code is executed in the constructor of my MainForm. The UI thread was, in this case, occupied with sending an email on a SMTP server that didn't support SendAsync() calls. I could probably have created a different thread with Begin() and End() methods and called a Send() from their, but that method is error-prone and I would prefer the Main Form of my application not throwing exceptions during construction.
I'm doing a windows form and would like an audit task to happen every 30 seconds. This audit is essentially checking a series of services on remote computers and reporting back into a richtextbox the status.
Current I have this running in an endless background thread and using an invoker to update the richtextbox in the main form.
Is this best practice? If I made an endless loop in my main form that would prevent any of my buttons from working, correct?
I'm just curious if every time I want to create a periodic audit check I have to create a new thread which checks the status or file or what have you?
Edit: I looked further into the Timer class and decided to go with System Timer as it proved to be better with a longer function. Thanks for pointing me in the right direction.
You should look into the Windows Forms Timer class. You don't want a loop in your main form. It's better to use the timer to fire events which can be processed asynchronously on another thread.
(I assume this is a winform application)
Invoking on the main thread is the way to go. But what about using a timer instead of an endless loop? It gives you more control. And a the timer function would execute on it's own thread.
It's good practice to let long going work execute on a background thread, so that the main thread can work with the UI.
I have a problem with interface lag in C#.
Since I'm still learning please be patient whilst I explain.
I have narrowed the problem to my timer object.
Basically my program queries a device through TCP/IP socket and outputs it to a textbox on screen.
Now I am polling the device for data every second which requires some logic to be buried within timer object and the following is what happens between ticks:
Increment a value.
Construct the 2 strings that represents the command to be sent to
the box (encapsulated in a function
Encode the command
Send command
Clear the byte array
Receive reply.
Could this be too much processing being done in the event handler? Every time I try to move the window during the polling session i.e. when the timer is running I get a very bad input lag.
The timer you are using is executing on the windows message thread. Therefore, while the polling is running the windows message queue is blocked. This isn't a problem with doing too much processing, most of the time the thread will be waiting for the TCP/IP response.
To fix this, you just have to do the do the work on a background thread and then update the UI on the UI thread.
There are a heap of different timers in the .NET framework that work in different ways, the one you are using works processed the timer event on the same thread, others work on background threads. Check this article out about the different timers.
You could also just use your current timer to invoke a BackgroundWorker component to do the work on the background thread. The main benefit of this is the the BackgroundWorker will do the work on a background thread, but will raise the work complete event on the UI thread so that it is simple to update the UI without having to worry about which thread you are on.
I think this is because you're trying to do work in your UI thread. Have your timer run in a background work thread.
It seems like there are a few things going on. First, you may be doing too much in your timer tick handler. How are you constructing the string and encoding the command? Can any of this be done once outside the tick handler or simplified in any way (using String.Format calls, for instance)? There are actually three different timers available in .NET, with different resolutions. Which timer are you using?
The biggest issue is the fact that your interval is 1 second. No matter what, that is a lot of processing overhead. Keep in mind that, for the most part, every time the interval is hit and the tick handler is invoked you are causing a context switch between threads. There is a bit of overhead involved in this (nothing which you can do anything about) and the more often you context switch the slower your performance appears.