Use of Application DoEvents

Question

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Check out the MSDN Documentation for the Application DoEvents method

[c#] Use of Application.DoEvents()

Examples related to c#

Examples related to winforms

Examples related to doevents