Is Application.DoEvents() just for forms?
I thought that this command was used to ensure that all of the commands before are processed but now after reading the documentation, I'm not sure anymore.
4 Answers
Yes, it's really aimed at Windows Forms. However, in my view it should be avoided where possible.
It's usually used as a hack by developers who don't want to be bothered with putting long-running operations on a different thread... but that means they're introducing re-entrancy issues which can be very hard to track down, as well as still blocking the UI thread for some of the time (and if that includes something like a file operation, you can't really predict whether the operation will complete quickly enough to not have a user visible effect).
8 Comments
Alastair Pitts
+1000, Not just in your view. There is really no reason to use it at all.
Vlad
DoEvents can be useful for implementing a blocking operation like MessageBox.
Jon Skeet
@Vlad: I believe it's better to just disable the rest of the UI input. Otherwise your DoEvents can still mean that the rest of the UI starts to respond when you don't want it to.
Vlad
@Jon: I basically meant an arbitrary potentially slow function, which has to be blocking (e.g., has to produce an output value). The need for such functions (and not the design with StartCalculation/OnFinish event/callback) is somehow justified by the existence of
MessageBox.Show.Vlad
@Jon: looking back from the 1 year's perspective: now I believe that
DoEvents is completely evil. :) |
Without WinForms, there is no standard event queue. (Well, there is an event queue in WPF, but this is just another framework).
4 Comments
Vlad
@Cody: It means that a C# program is not necessarily a WinForms or WPF program. WPF is a framework built on the top of C# (and so is WinForms). The event queues will be available only if you use some of the frameworks, e.g., WinForms or WPF, but not per default.
Cody Gray
Sure. But isn't ASP.NET (for example) also a framework?
DoEvents doesn't make much sense there.Vlad
@Cody: I have no experience with ASP.NET, but I would assume it is. Otherwise event queues would be available in console applications as well.
Vlad
@Cody: it's not necessary that a framework brings in the event queue. However, as the event queue is not something built-in into the .NET, the event queue must be introduced by some framework. (Note that the .NET itself is called a framework, making the meaning of this term somewhat ambiguous.)
If what you are trying to achieve is waiting for something to happen outside your application (eg. a file to be dropped in a certain directory), a possible workaround would be the Timer class of the System.Timers namespace.
An example (based on MSDN):
Private Sub SetTimer()
Dim aTimer As New System.Timers.Timer
AddHandler aTimer.Elapsed, AddressOf OnTimedEvent
aTimer.Interval = 5000
aTimer.Enabled = True
Console.WriteLine("Press q to exit")
While Console.Read <> Asc("q")
End While
End Sub
Private Sub OnTimedEvent(ByVal source As Object, ByVal e As ElapsedEventArgs)
'Do the job here
Console.WriteLine("HELLO WORLD!")
'Don't forget to disable the timer if you don't need it anymore
'Source.Enabled = False
End Sub
More info at MSDN: http://msdn.microsoft.com/en-us/library/system.timers.timer%28v=vs.71%29.aspx
Comments
Yes, it's only for Windows Forms. It wouldn't make sense in a console or ASP.NET application, because there is no message loop. It is possible to do it in WPF, using the dispatcher, as shown here. Anyway, I wouldn't recommend using DoEvents except perhaps in a quick and dirty application, for the reasons explained by Jon.
Application.DoEvents. What problem are you trying to solve? Perhaps someone can suggest a better way of doing so.DoEvents. It’s the entire class. That’s why it’sSystem.Windows.Forms.Application...