App Class

The App class is the app’s entry point
Defined in App.xaml and App.xaml.cs
Derives from Microsoft.UI.Xaml.Application
InitializeComponent method parses contents of App.xaml
OnLaunched method is invoked when app is launched creates and activates a new instance of MainWindow

App.xaml

<Application
    x:Class="LuckyNumbers.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:LuckyNumbers">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />
                <!-- Other merged dictionaries here -->
            </ResourceDictionary.MergedDictionaries>
            <!-- Other app resources here -->
        </ResourceDictionary>
    </Application.Resources>
</Application>

App.xaml.cs

public partial class App : Application
{
    public App()
    {
        this.InitializeComponent();
    }

    /// <param name="args">Details about the launch request and process.</param>
    protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
    {
        m_window = new MainWindow();
        m_window.Activate();
    }

    private Window m_window;
}

MainWindow Class

MainWindow class is the main window of the application
Defined in MainWindow.xaml and MainWindow.xaml.cs
Constructor calls its own InitializeComponent method to parse MainWindow.xaml into a graph of UI objects

MainWindow.xaml

<!-- x:Class is the fully-qualified class name. -->
<Window
    x:Class="LuckyNumbers.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:LuckyNumbers"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <!-- This Button element needs a name in order to be referenced in the code-behind file -->
    <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" VerticalAlignment="Center">
        <Button x:Name="myButton" Click="MyButton_Click">Click Me</Button>
    </StackPanel>
</Window>

MainWindow.xaml.cs

public sealed partial class MainWindow : Window
{
    public MainWindow()
    {
        this.InitializeComponent();
        // Set the title bar:
        this.Title = "Title text";
    }

    private void MyButton_Click(object sender, RoutedEventArgs e)
    {
        myButton.Content = "Clicked";
    }
}