Avalonia skill

Avalonia

Workflow

1. Inspect the SDK, target frameworks, Avalonia package versions, .axaml files, entrypoint, and supported OS/RIDs. Keep Avalonia packages on one compatible version. Use the Avalonia 12 docs for 12 and the Avalonia 11 docs for 11; do not copy 12-only APIs into 11.
2. Keep Program.BuildAvaloniaApp, App.axaml, views, view models, and domain/services separate. Desktop hosts use StartWithClassicDesktopLifetime and create MainWindow in OnFrameworkInitializationCompleted; single-view targets require their own lifetime. The previewer can have no application lifetime.
3. Use typed compiled bindings and explicit binding modes. x:DataType describes the binding type; it does not set the runtime DataContext. Avalonia 12 enables compiled bindings by default; explicitly enable them on 11. Read references/bindings-and-controls.md for templates, properties, resources, and command patterns.
4. Use CommunityToolkit.Mvvm for observable state and commands when appropriate. Prefer cancellable asynchronous service operations, surface errors, and marshal background updates to the UI dispatcher. Keep services independent of controls.
5. Choose StyledProperty for styleable/animatable values and DirectProperty for backed state without style precedence. Use DataTemplate to display data and ControlTemplate/ControlTheme to change a control's visual structure. Use UserControl for application composition, TemplatedControl for reusable skinnable controls, and custom rendering only when needed.
6. Validate native integrations, dialogs, resources, shutdown, DPI, accessibility, and packaging on each supported platform. Read references/platforms-and-publishing.md before trimming or Native AOT. Read the documentation map only for the selected topic.

flowchart LR
  Packages[Installed Avalonia major] --> Docs[Matching 11 or 12 documentation]
  Docs --> VM[View model and service]
  VM --> Bindings[Typed bindings and commands]
  Bindings --> UI[Views, templates, property system]
  UI --> Desktop[Desktop lifetime and UI dispatcher]
  Desktop --> Verify[Headless tests + native OS checks]
  Verify --> Publish[RID-specific publish and packaging]

Install

For a new application, use the official templates rather than inventing project scaffolding:

dotnet new install Avalonia.Templates
dotnet new avalonia.mvvm -o MyApp
dotnet add MyApp/MyApp.csproj package CommunityToolkit.Mvvm
dotnet run --project MyApp/MyApp.csproj

For an existing project, select versions matching its target framework and installed Avalonia major:

dotnet add MyApp.csproj package Avalonia.Desktop --version <compatible-version>
dotnet add MyApp.csproj package Avalonia.Themes.Fluent --version <compatible-version>

Read: typed view-model bindings

The view model implements notifications through the toolkit. Wire the runtime context when constructing the view:

using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

public partial class MainViewModel : ObservableObject
{
    [ObservableProperty]
    private string name = "Ada";

    [ObservableProperty]
    private string greeting = "Ready";

    [RelayCommand]
    private void Greet() => Greeting = $"Hello, {Name}";
}

// In App.OnFrameworkInitializationCompleted, after checking desktop lifetime:
// desktop.MainWindow = new MainWindow { DataContext = new MainViewModel() };

In the view, replace MyApp.ViewModels with the actual namespace:

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:MyApp.ViewModels"
        x:Class="MyApp.MainWindow"
        x:DataType="vm:MainViewModel" x:CompileBindings="True">
  <StackPanel Spacing="8" Margin="16">
    <TextBox Text="{Binding Name, Mode=TwoWay}" />
    <Button Content="Greet" Command="{Binding GreetCommand}" />
    <TextBlock Text="{Binding Greeting}" />
  </StackPanel>
</Window>

Write: a styleable custom-control property

using Avalonia;
using Avalonia.Controls.Primitives;

public class StatusBadge : TemplatedControl
{
    public static readonly StyledProperty<string> TextProperty =
        AvaloniaProperty.Register<StatusBadge, string>(nameof(Text), "Ready");

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

Define the visual tree in a control theme; keep CLR accessors limited to GetValue/SetValue, because styles and bindings can bypass them. Use property-change hooks for behavior and SetCurrentValue when changing a value without replacing its existing binding.

Options and constraints

• Set <AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault> explicitly for consistent 11/12 behavior. Use ReflectionBinding only for a justified dynamic path; review trimming risk.
• Use Dispatcher.UIThread.InvokeAsync when completion matters, Post for a queued notification, and CheckAccess when deciding whether to marshal. Update bound collections on the UI thread; avoid .Result/.Wait() there.
• Choose an intentional ShutdownMode and owner for ShowDialog<T>(owner). Inject a dialog service into view models instead of constructing windows there.
• Fluent themes, selectors, pseudoclasses, theme variants, and avares:// resources differ from WPF. Do not transplant WPF styles or assume platform behavior is identical.
• Avalonia 12 changes binding APIs, clipboard, window decorations, and diagnostics packages. Review the major-version migration guide before upgrading.
• A successful build or headless test does not prove native rendering, packaging, accessibility, or AOT compatibility.

Deliver and validate

• State the installed major, chosen lifetime, supported OS/RIDs, and matching docs.
• Run dotnet build <solution> -c Release and the repository's focused tests; verify that typed binding paths compile and commands update observable state.
• Use Avalonia.Headless.XUnit or Avalonia.Headless.NUnit for dispatcher-aware control tests. Keep unrelated tests parallel; constrain only a concrete destructive collision domain.
• Launch on each promised OS and check resizing, scaling, keyboard navigation, dialogs, resources, and shutdown.
• Publish for each RID and launch the actual output. Fix trim/AOT warnings and test native dependencies rather than suppressing them.
• Keep test output to warnings/errors plus one concise result, preserve native progress/ANSI and exit codes, and cap diagnostics at 80 lines / 8 KiB with bounded artifacts outside context.