Question

Here you have a simple WPF program:

<!-- Updater.xaml -->
<Window x:Class="Update.Updater"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525"
        DataContext="{Binding RelativeSource={RelativeSource Self}}">
    <Grid>
        <StackPanel>
            <Button Click="Button_Click" Height="50"></Button>
            <Label Content="{Binding Label1Text}" Height="50"></Label>
            <Label Content="{Binding Label2Text}" Height="50"></Label>
        </StackPanel>
    </Grid>
</Window>

// Updater.xaml.cs
using System.Threading;
using System.Windows;

namespace Update
{
    public partial class Updater : Window
    {
        public Updater()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Label1Text = "It is coming...";
            Thread.Sleep(3000);
            Label2Text = "It is here!";
        }

        public string Label1Text
        {
            get { return (string)GetValue(CategoryProperty); }
            set { SetValue(CategoryProperty, value); }
        }

        static readonly DependencyProperty CategoryProperty = DependencyProperty.Register("Label1Text", typeof(string), typeof(Updater));

        public string Label2Text
        {
            get { return (string)GetValue(Label2TextProperty); }
            set { SetValue(Label2TextProperty, value); }
        }

        static readonly DependencyProperty Label2TextProperty = DependencyProperty.Register("Label2Text", typeof(string), typeof(Updater));
    }
}

The intention is that when you click the button, the first label displays It is coming.... Then the program sleeps for 3 seconds, and finally the second label displays It is here!. However, the naïve implementation below doesn’t work. If you run it and click the button the following happens: The program sleeps for 3 seconds, and then the two label texts are displayed simultaneously. Do you know how to correct the program so that it does as intended?

Answer 1

Button_Click is invoked by the UI thread, and you should not do anything requiring more than a few miliseconds in it, let alone sleeping for 3 seconds. Messages are not processed during that time, your interface is unresponsive, and your application is considered "hung" by the system.

So, that long processing task should be handled by another thread.

Something like that (untested):

private void Button_Click(object sender, RoutedEventArgs e)
{
    Label1Text = "It is coming...";

    var backgroundWorker = new BackgroundWorker();

    backgroundWorker.DoWork += (s,e) => { Thread.Sleep(3000); }
    backgroundWorker.RunWorkerCompleted += (s,e) => { Label2Text = "It is here!"; }
    backgroundWorker.RunWorkerAsync();
}

Links:

BackgroundWorker

Build More Responsive Apps With The Dispatcher