avalonia

internal static class Program
{
    [STAThread]
    public static void Main(string[] args) =>
        BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);

    // Must stay public static: the XAML previewer calls it
    public static AppBuilder BuildAvaloniaApp() =>
        AppBuilder.Configure<App>().UsePlatformDetect().LogToTrace();
}

public sealed partial class App : Application
{
    public override void Initialize() => AvaloniaXamlLoader.Load(this);

    public override void OnFrameworkInitializationCompleted()
    {
        var builder = Host.CreateApplicationBuilder(new HostApplicationBuilderSettings
        {
            ContentRootPath = AppContext.BaseDirectory
        });
        builder.Services.AddSingleton(TimeProvider.System);
        builder.Services.AddSingleton<MainWindowViewModel>();
        builder.Services.AddTransient<OrdersViewModel>();
        var host = builder.Build();
        host.Start();

        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
        {
            desktop.MainWindow = new MainWindow
            {
                DataContext = host.Services.GetRequiredService<MainWindowViewModel>()
            };
            desktop.Exit += (_, _) => host.Dispose();
        }

        base.OnFrameworkInitializationCompleted();
    }
}

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:vm="using:Contoso.Desktop.ViewModels"
             x:Class="Contoso.Desktop.Views.OrdersView"
             x:DataType="vm:OrdersViewModel">
    <DockPanel>
        <TextBox DockPanel.Dock="Top" Text="{Binding CustomerName}" PlaceholderText="Customer" />
        <Button DockPanel.Dock="Top" Content="Save" Command="{Binding SaveCommand}" />
        <ListBox ItemsSource="{Binding Orders}" SelectedItem="{Binding SelectedOrder}">
            <ListBox.ItemTemplate>
                <DataTemplate x:DataType="vm:OrderRow">
                    <TextBlock Text="{Binding Number}" />
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </DockPanel>
</UserControl>

<!-- App.axaml: typed templates, resolved at compile time -->
<Application.DataTemplates>
    <DataTemplate DataType="vm:OrdersViewModel"><views:OrdersView /></DataTemplate>
    <DataTemplate DataType="vm:SettingsViewModel"><views:SettingsView /></DataTemplate>
</Application.DataTemplates>

// If you prefer a ViewLocator: explicit factories, no reflection
public sealed class ViewLocator : IDataTemplate
{
    private static readonly Dictionary<Type, Func<Control>> Views = new()
    {
        [typeof(OrdersViewModel)] = () => new OrdersView(),
        [typeof(SettingsViewModel)] = () => new SettingsView(),
    };

    public Control? Build(object? param) =>
        param is not null && Views.TryGetValue(param.GetType(), out var factory)
            ? factory()
            : new TextBlock { Text = $"No view for {param?.GetType().Name}" };

    public bool Match(object? data) => data is ObservableObject;
}

public sealed class Badge : TemplatedControl
{
    // StyledProperty: settable from styles/themes, supports inheritance and animation
    public static readonly StyledProperty<string?> TextProperty =
        AvaloniaProperty.Register<Badge, string?>(nameof(Text));

    public static readonly StyledProperty<IBrush?> AccentProperty =
        AvaloniaProperty.Register<Badge, IBrush?>(nameof(Accent));

    // DirectProperty: plain backing field, cheap, NOT settable from styles — use for read-only state
    public static readonly DirectProperty<Badge, bool> IsEmptyProperty =
        AvaloniaProperty.RegisterDirect<Badge, bool>(nameof(IsEmpty), o => o.IsEmpty);

    private bool isEmpty = true;

    public string? Text
    {
        get => GetValue(TextProperty);
        set => SetValue(TextProperty, value);
    }

    public IBrush? Accent
    {
        get => GetValue(AccentProperty);
        set => SetValue(AccentProperty, value);
    }

    public bool IsEmpty
    {
        get => isEmpty;
        private set => SetAndRaise(IsEmptyProperty, ref isEmpty, value);
    }

    protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
    {
        base.OnPropertyChanged(change);
        if (change.Property == TextProperty)
        {
            IsEmpty = string.IsNullOrEmpty(Text);
            PseudoClasses.Set(":empty", IsEmpty);
        }
    }
}

<!-- Themes/Badge.axaml, merged into App.axaml resources -->
<ControlTheme x:Key="{x:Type local:Badge}" TargetType="local:Badge">
    <Setter Property="Accent" Value="{DynamicResource SystemAccentColor}" />
    <Setter Property="Template">
        <ControlTemplate>
            <Border Name="PART_Border" Background="{TemplateBinding Accent}" CornerRadius="8" Padding="6,2">
                <TextBlock Text="{TemplateBinding Text}" />
            </Border>
        </ControlTemplate>
    </Setter>
    <Style Selector="^:empty /template/ Border#PART_Border">
        <Setter Property="IsVisible" Value="False" />
    </Style>
</ControlTheme>

// Same Badge class as above, shown here for the template part only
[TemplatePart("PART_Border", typeof(Border))]
public sealed class Badge : TemplatedControl
{
    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
    {
        base.OnApplyTemplate(e);
        var border = e.NameScope.Find<Border>("PART_Border"); // may be null if a consumer re-templated
    }
}

public sealed class AvaloniaDialogService(Func<TopLevel> topLevel) : IFileDialogService
{
    public async Task<Stream?> OpenCsvAsync()
    {
        var files = await topLevel().StorageProvider.OpenFilePickerAsync(new FilePickerOpenOptions
        {
            Title = "Import orders",
            AllowMultiple = false,
            FileTypeFilter = [new FilePickerFileType("CSV") { Patterns = ["*.csv"] }]
        });
        return files is [var file] ? await file.OpenReadAsync() : null;
    }
}

<PropertyGroup>
  <PublishAot>true</PublishAot>          <!-- app project only -->
  <IsAotCompatible>true</IsAotCompatible> <!-- every project, enables the analyzers -->
</PropertyGroup>

dotnet publish src/Contoso.Desktop -c Release -r win-x64
dotnet publish src/Contoso.Desktop -c Release -r osx-arm64
dotnet publish src/Contoso.Desktop -c Release -r linux-x64

<!-- BAD — 12 compiles bindings by default: no x:DataType = build error; ReflectionBinding hides typos -->
<TextBlock Text="{ReflectionBinding CustmerName}" />

<!-- GOOD — typed, validated at build time -->
<UserControl x:DataType="vm:OrdersViewModel">
    <TextBlock Text="{Binding CustomerName}" />
</UserControl>

// BAD — in Avalonia 12, Binding always means ReflectionBinding
textBlock.Bind(TextBlock.TextProperty, new Binding("CustomerName"));

// GOOD — explicit compiled binding
textBlock.Bind(TextBlock.TextProperty,
    CompiledBinding.Create((OrdersViewModel vm) => vm.CustomerName));

// BAD — Windows-only, crashes on macOS/Linux
System.Diagnostics.Process.Start("explorer.exe", folder);

// GOOD — launcher from TopLevel (desktop OSes; sandboxed targets need IStorageFolder overloads)
await TopLevel.GetTopLevel(this)!.Launcher.LaunchDirectoryInfoAsync(new DirectoryInfo(folder));

// BAD — throws: call from invalid thread
_ = Task.Run(() => statusText.Text = "Done");

// GOOD — bind to a view model property and await the work; or dispatch explicitly
await Dispatcher.UIThread.InvokeAsync(() => Status = "Done");