Exemplos de WPF 3.5 (Windows Presentation Foundation) - 006 - Application - Current

<!--App.xaml-->
    <Application x:Class="_006_ApplicationCurrent.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    StartupUri="Window1.xaml" Startup="Application_Startup">
</Application>

//App.xaml.cs
using System;
using System.Windows;


namespace _006_ApplicationCurrent
{
    public partial class App : Application
    {
        private void Application_Startup(object sender, StartupEventArgs e)
        {
            foreach (var item in Application.Current.Windows)
            {
                MessageBox.Show(String.Format(
                    "Application_Startup: a janela {0} está aberta", item));
            }
        }
    }
}

<!--Window1.xaml-->
<Window x:Class="_006_ApplicationCurrent.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300">
    <Grid>
        <Button Padding="10" HorizontalAlignment="Center" 
                VerticalAlignment="Center" Name="button1" 
                Click="button1_Click">esse é um Button</Button>
    </Grid>
</Window>

//Window1.xaml.cs
using System;
using System.Windows;


namespace _006_ApplicationCurrent
{
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
        }


        private void button1_Click(object sender, RoutedEventArgs e)
        {
            foreach (var item in Application.Current.Windows)
            {
                MessageBox.Show(String.Format(
                    "button1_Click: a janela {0} está aberta", item));
            }


            var janelaPrincipal = Application.Current.MainWindow;

            MessageBox.Show(String.Format(
                "A janela principal do aplicativo é: {0}", janelaPrincipal));
        }
    }
}