Why does Thread.Sleep() behave this way

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Why does Thread.sleep() behave in this way

This is a simple code that i have written:

    private void button1_Click(object sender, EventArgs e)
    {
        label1.Text = "first";
        Thread.Sleep(1000);
        label1.Text = "second";
    }

When this code is executed and button is clicked, label1 displays text as only 'second' but not 'first'. I checked using break point, and statement label1.text="first" is executed but label does not display 'first'. Why is this so?

You've already asked a variant of this ([Why does Thread.sleep() behave in this way](http://stackoverflow.com/questions/11576637/why-does-thread-sleep-behave-in-this-way)) and gotten accurate answers. The same answer applies here again, it isn't processing the message queue so does not run the paint message. — Adam Houldsworth, Jul 20 '12 at 10:19
I have submitted the question twice by mistake.. Applogies for that. Thanks for the comments — Shiridish, Jul 20 '12 at 10:28
@John Well I know *you* did, but your solution assumes that the OP did too - it isn't specified. I'm not sure your solution would work in WinForms. — Adam Houldsworth, Jul 20 '12 at 10:36
Yes, I tried it in Windows Forms, and it doesn't work... :) But WPF is better than WinForms, in my opinion. — Ionică Bizău, Jul 20 '12 at 10:39

score 3 · Accepted Answer · answered Jul 20 '12 at 10:21

Because your code is running on the UI thread.

When your app is running and you're not clicking on things, the application's main thread is rushing around, waiting for events, processing Windows messages, and painting the screen. When you click your button, the app stops doing those things, and runs your function instead. While it's running your method, it can't paint the screen. Your function sets the label to "First", blocks the thread for a second, then sets the label to "second".

When you return, control passes back to the application, which is then free to repaint the form. It repaints it using the "second" label. The message loop never had chance to paint it with the "first" label.

score 3 · Answer 2 · answered Jul 20 '12 at 10:24

You're running a single threaded application. Although the statement setting the label1 object's Text property to "first" has been executed, because you are causing the main application thread to pause, the "Windows Message Pump" is not being executed.

score 1 · Answer 3 · answered Jul 20 '12 at 10:21

1

The problem is your a freezing the UI thread here.

The text "first" is set but then you put the main GUI thread to sleep - so the UI does not get painted.

After the sleep phase is over, the text "second" is set and the UI gets painted.

That's just the way Windows works, so you have to do that differently. e.g. using Threads, or Backgroundworker

answered Jul 20 '12 at 10:21

Wolfgang Ziegler

1,675
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1

This isn't actually what happens. The message pump is still working, but it is busy handling the button click. The paint message occurs after the method returns. – Adam Houldsworth Jul 20 '12 at 10:29
Of course this is what happens. As soon as you call Sleep on the UI thread (i.e.inside the message pump) your application basically freezes (concerning the UI thread) and will not be repainted. – Wolfgang Ziegler Jul 20 '12 at 10:47
1

The sleep call *slows down* the execution of the method, but it is not responsible for the lack of repaint, it is just responsible for the lack of repaint being *noticeable*. The message pump is actually alive, it is busy processing the event handler. You seem to imply that the sleep call stops the "first" from being painted, but even without a sleep call "first" would never be painted, because the paint call onto the label would occur after this method exits, when the label state is "second". – Adam Houldsworth Jul 20 '12 at 10:51

Ionică Bizău · Answer 4 · 2013-01-22T04:37:35.423

If you want delay use timers.

The reason why you don't see the result in UI is because Dipatcher's priorities. The Render task has lower priority than the execution.

This helps you: TIMERS!

DispatcherTimer dispatcherTimer = new DispatcherTimer();
dispatcherTimer.Tick += new EventHandler(dispatcherTimer_Tick);
dispatcherTimer.Interval = new TimeSpan(0, 0, 0, 1);
dispatcherTimer.Start();

private void dispatcherTimer_Tick(object sender, EventArgs e)
{
//your code goes here
}

If you work in WPF you can try this:

private void button1_Click(object sender, RoutedEventArgs e)
{
    label1.Text = "first";
    Dispatcher.Invoke(new Action(() => { }), DispatcherPriority.ContextIdle);
    Thread.Sleep(1000);                            
    label1.Text = "second";
}

Why does Thread.Sleep() behave this way

4 Answers4