name: avalonia
description: Expert guidance for developing cross-platform desktop applications with Avalonia UI framework. Use when building, debugging, or optimizing Avalonia apps including MVVM architecture, XAML design, data binding, styling, theming, custom controls, and cross-platform deployment for Windows, macOS, Linux, iOS, Android, and WebAssembly.
version: 2.0

Avalonia UI Framework - Orchestration Hub

Modular guidance for cross-platform desktop and mobile development using Avalonia, a WPF-inspired XAML-based framework for .NET.

Quick Reference: When to Load Which Resource

Framework Overview

Avalonia is a cross-platform XAML framework supporting:
- Platforms: Windows, macOS, Linux, iOS, Android, WebAssembly
- Architecture: MVVM with ReactiveUI support
- Styling: CSS-like selectors with Fluent/Simple themes
- Features: Data binding, reactive commands, observable collections, custom controls
- Modern .NET: .NET 6+ and .NET Standard 2.0

Standard Project Structure

MyAvaloniaApp/
├── MyAvaloniaApp/                  # Shared code
│   ├── App.axaml
│   ├── Views/                      # XAML views
│   ├── ViewModels/                 # Business logic + state
│   ├── Models/                     # Data models
│   ├── Services/                   # Application services
│   ├── Converters/                 # Value converters
│   ├── Assets/                     # Images, fonts
│   └── Styles/                     # Style resources
├── MyAvaloniaApp.Desktop/          # Desktop-specific (Win/Mac/Linux)
├── MyAvaloniaApp.Android/          # Android-specific (optional)
├── MyAvaloniaApp.iOS/              # iOS-specific (optional)
└── MyAvaloniaApp.Browser/          # WebAssembly (optional)

Getting Started

Minimal Setup

// Program.cs
public static void Main(string[] args)
{
    BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);
}

public static AppBuilder BuildAvaloniaApp() =>
    AppBuilder.Configure<App>()
        .UsePlatformDetect()
        .LogToTrace();

<!-- App.axaml -->
<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="MyApp.App">
    <Application.Styles>
        <FluentTheme />
    </Application.Styles>
</Application>

<!-- Views/MainWindow.axaml -->
<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        x:Class="MyApp.Views.MainWindow"
        Title="My Application"
        Width="800"
        Height="600">
    <StackPanel Padding="20" Spacing="10">
        <TextBlock Text="Hello, Avalonia!" FontSize="24" FontWeight="Bold" />
    </StackPanel>
</Window>

Core Patterns

MVVM Architecture Pattern

1. View (XAML): UI presentation with data bindings
2. ViewModel (C#): State management and commands
3. Model (C#): Business logic and data access
4. Service: Cross-cutting concerns (DI/IoC)

Load resources/mvvm-databinding.md for:
- ViewModel base classes
- Data binding modes and paths
- Multi-binding and converters
- Dependency injection setup
- Design-time data

Reactive Programming Pattern

Leverage ReactiveUI for event-driven UI updates:

this.WhenAnyValue(x => x.SearchText)
    .Debounce(TimeSpan.FromMilliseconds(300))
    .Subscribe(text => PerformSearch(text));

Load resources/reactive-animations.md for:
- Reactive properties and commands
- Observable sequences
- Animations and transitions
- Performance optimization

Platform-Adaptive Pattern

Design once, adapt per platform:

<OnPlatform Default="16">
    <On Options="Windows" Content="14" />
    <On Options="macOS" Content="15" />
</OnPlatform>

Load resources/platform-specific.md for:
- Runtime platform detection
- Platform-specific services
- Conditional UI rendering
- Native dialogs and features

Navigation by Task

"I need to build a form with validation"

1. Load resources/mvvm-databinding.md → Implement ViewModel with property validation
2. Load resources/controls-reference.md → Find TextBox, ComboBox, Button controls
3. Load resources/reactive-animations.md → Add debounced validation with observables

"I'm seeing poor performance with large lists"

1. Load resources/custom-controls-advanced.md → Enable virtualization
2. Load resources/mvvm-databinding.md → Use compiled bindings
3. Load resources/reactive-animations.md → Debounce/throttle updates

"I need platform-specific behavior"

1. Load resources/platform-specific.md → Implement service interfaces
2. Load resources/mvvm-databinding.md → Register platform implementations via DI
3. Platform-specific resources/ → Implement per-platform project

"I want custom styling and animations"

1. Load resources/styling-guide.md → Define styles and themes
2. Load resources/reactive-animations.md → Add animations to styles
3. Load resources/custom-controls-advanced.md → Custom control templates

"I'm building a complex control"

1. Load resources/custom-controls-advanced.md → TemplatedControl or UserControl pattern
2. Load resources/mvvm-databinding.md → Attached properties and data binding
3. Load resources/styling-guide.md → Control templates and styling

Resource Organization

mvvm-databinding.md (Primary)

• Architecture overview
• ViewModel patterns with ReactiveUI
• Binding modes and syntax
• Value converters
• Collections and list binding
• Design-time data
• Master-detail and tab patterns

controls-reference.md (Primary)

• Layout controls (Grid, StackPanel, DockPanel, etc.)
• Input controls (TextBox, Button, CheckBox, ComboBox, etc.)
• Display controls (TextBlock, Image, ProgressBar, etc.)
• Collection controls (ListBox, DataGrid, TreeView, etc.)
• Navigation (Menu, TabControl, SplitView, etc.)
• Shapes and drawing

styling-guide.md (Primary)

• CSS-like selectors (type, class, pseudo-classes)
• Resource dictionaries and themes
• Control templates
• Data templates
• Animations and transitions
• Easing functions
• Theme variants (light/dark)

reactive-animations.md (Advanced)

• ReactiveUI integration
• Reactive properties
• Reactive commands (sync and async)
• Observable sequences
• Filtering, transformation, combining
• Programmatic animations
• Common patterns (search, validation, auto-complete)

custom-controls-advanced.md (Advanced)

• Custom TemplatedControl creation
• User control composition
• Advanced layouts
• Virtualization
• Performance optimization
• Render transforms
• Graphics and drawing

platform-specific.md (Advanced)

• Runtime platform detection
• Multi-project structure
• Service abstractions
• Platform-specific implementations
• Window management per platform
• File system access
• Native features (Windows DLL, macOS Cocoa, etc.)

Common Workflows

Build a Desktop App (Windows/macOS/Linux)

1. → Setup: Standard project structure + FluentTheme
2. → Create Views and ViewModels following MVVM
3. → Use controls-reference for UI layouts
4. → Add styles with styling-guide
5. → Implement services with DI (mvvm-databinding)
6. → Add animations with reactive-animations
7. → Test on each platform with platform-specific guidance

Build a Cross-Platform Mobile+Desktop App

1. → Create shared project + platform-specific projects
2. → Define service interfaces in shared code (mvvm-databinding)
3. → Implement services per platform (platform-specific)
4. → Use OnPlatform for adaptive UI
5. → Register platform implementations via DI
6. → Test thoroughly on each target (iOS/Android/Windows/Mac)

Add Real-Time Search

1. → Create SearchViewModel (mvvm-databinding)
2. → Use ObservableCollection for results (mvvm-databinding)
3. → Implement with reactive search pattern (reactive-animations)
4. → Debounce input to reduce API calls
5. → Display with ListBox (controls-reference)
6. → Style with appropriate CSS selectors (styling-guide)

Build Complex Data-Driven UI

1. → Design ViewModel hierarchy (mvvm-databinding)
2. → Create master-detail view (mvvm-databinding)
3. → Use DataGrid for tabular data (controls-reference)
4. → Add sorting/filtering with observables (reactive-animations)
5. → Optimize with virtualization (custom-controls-advanced)
6. → Add custom controls if needed (custom-controls-advanced)

Best Practices Summary

Architecture
- Maintain strict MVVM separation of concerns
- Use dependency injection for testability
- Keep business logic in ViewModels, not Views

Performance
- Enable compiled bindings with x:DataType
- Virtualize large collections
- Debounce rapid updates

Styling
- Use resource dictionaries for consistency
- Support light and dark themes
- Test styles on all target platforms

Reactive Patterns
- Use observables for event-driven updates
- Debounce/throttle input-triggered operations
- Always handle ThrownExceptions on commands

Testing
- Unit test ViewModels in isolation
- Use Avalonia.Headless for UI testing
- Provide design-time DataContext in XAML

Cross-Platform Deployment

• Windows: ClickOnce, MSI, portable exe
• macOS: DMG, homebrew
• Linux: AppImage, snap, flatpak
• Mobile: Apple App Store, Google Play Store
• Web: Static hosting (WASM runtime required)

Refer to resources/platform-specific.md for platform-specific build and deployment guidance.

Navigation: Choose a resource above based on your task. Each resource is self-contained with comprehensive examples and best practices.