Avalonia API

Avalonia UI Framework - Complete API & Best Practices Guide

Target Framework: .NET 10.0+
File Extension: .axaml (Avalonia XAML)
Official Docs: https://docs.avaloniaui.net/

Table of Contents

1. AXAML Fundamentals
2. Controls & UI Elements
3. Layout System
4. Data Binding
5. MVVM Pattern with CommunityToolkit.Mvvm
6. Styling & Theming
7. Dependency & Attached Properties
8. Custom Controls
9. Control Templates
10. Resources & Converters
11. Events & Commands
12. Navigation
13. Cross-Platform Patterns
14. Performance & Best Practices
15. Developer Tools
16. Common Mistakes to Avoid
17. Common Patterns in XerahS

AXAML Fundamentals

File Structure

Every .axaml file follows this standard structure:

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:YourApp.ViewModels"
        x:Class="YourApp.Views.MainWindow"
        x:DataType="vm:MainViewModel"
        x:CompileBindings="True">
    
    <!-- Content here -->
    
</Window>

Required Namespace Declarations

Custom Namespace Syntax

<!-- Current assembly -->
<xmlns:myAlias1="using:AppNameSpace.MyNamespace">

<!-- Referenced assembly (library) -->
<xmlns:myAlias2="clr-namespace:OtherAssembly.MyNameSpace;assembly=OtherAssembly">

<!-- Alternative using: prefix (Avalonia style) -->
<xmlns:controls="using:XerahS.UI.Controls">

Control Content vs. Attributes

<!-- Using Content property (implicit) -->
<Button>Hello World!</Button>

<!-- Using Content attribute (explicit) -->
<Button Content="Hello World!" />

<!-- Using property element syntax -->
<Button>
    <Button.Content>
        <StackPanel>
            <TextBlock Text="Complex" />
            <TextBlock Text="Content" />
        </StackPanel>
    </Button.Content>
</Button>

Controls & UI Elements

Common Built-in Controls

Input Controls

• TextBox: Single/multi-line text input
• PasswordBox: Masked password input
• NumericUpDown: Numeric input with increment/decrement
• CheckBox: Boolean toggle
• RadioButton: Mutually exclusive selection
• Slider: Continuous range selection
• ComboBox: Dropdown selection
• AutoCompleteBox: Text input with suggestions
• DatePicker: Date selection
• TimePicker: Time selection
• ColorPicker: Color selection

Display Controls

• TextBlock: Read-only text display
• Label: Text with access key support
• Image: Display images
• Border: Visual border around content
• ContentControl: Single content container

Layout Panels

• Panel: Basic container (fills available space)
• StackPanel: Vertical/horizontal stack
• Grid: Row/column grid layout
• DockPanel: Edge-docked layout
• Canvas: Absolute positioning
• WrapPanel: Wrapping flow layout
• RelativePanel: Relative positioning
• UniformGrid: Equal-sized cells

Lists & Collections

• ListBox: Selectable list
• ListView: List with view customization
• TreeView: Hierarchical tree
• DataGrid: Tabular data with columns
• ItemsControl: Base collection display
• ItemsRepeater: Virtualizing collection

Containers

• Window: Top-level window
• UserControl: Reusable UI component
• ScrollViewer: Scrollable content
• Expander: Collapsible content
• TabControl: Tabbed interface
• SplitView: Hamburger menu pattern

Buttons

• Button: Standard button
• ToggleButton: Two-state button
• RepeatButton: Auto-repeating button
• RadioButton: Mutually exclusive button
• SplitButton: Button with dropdown
• DropDownButton: Dropdown menu button

Advanced

• Carousel: Cycling content display
• MenuFlyout: Modern flyout-based context menu (⚠️ Use this with FluentAvalonia)
• ContextFlyout: Right-click menu container (⚠️ Preferred over ContextMenu)
• ContextMenu: Legacy right-click menu (⚠️ Avoid with FluentAvalonia theme)
• Menu: Menu bar
• ToolTip: Hover information
• Flyout: Popup overlay
• Calendar: Calendar display

Layout System

ScrollViewer Activation Requirements

A ScrollViewer only activates (shows and enables the scrollbar) when it receives a finite (bounded) height constraint from its parent during the Measure pass. If any ancestor passes ∞ (infinity) down the chain, the ScrollViewer will never scroll.

Common sources of infinite height in XerahS layouts and their fixes:

Canonical scrollable settings page pattern:

<!-- ✅ Correct: padding lives inside the scroll extent -->
<TabControl VerticalContentAlignment="Stretch">
    <TabItem Header="General">
        <ScrollViewer>
            <StackPanel Spacing="24" Margin="24">
                <!-- content -->
            </StackPanel>
        </ScrollViewer>
    </TabItem>
</TabControl>

<!-- ❌ Wrong: padding cuts the viewport, last Npx of content unreachable -->
<TabControl>  <!-- default VerticalContentAlignment=Top → ∞ height -->
    <TabItem Header="General">
        <ScrollViewer Padding="24">  <!-- bottom 24px permanently cut off -->
            <StackPanel Spacing="24">
            </StackPanel>
        </ScrollViewer>
    </TabItem>
</TabControl>

Layout Process

Avalonia uses a two-pass layout system:

1. Measure Pass: Determines desired size of each control
2. Arrange Pass: Positions controls within available space

Control → Measure → MeasureOverride → DesiredSize
       → Arrange → ArrangeOverride → FinalSize

Panel Comparison

Recommendation: Use Panel instead of Grid with no rows/columns for better performance.

Common Layout Properties

<Control Width="100"                    <!-- Fixed width -->
         Height="50"                     <!-- Fixed height -->
         MinWidth="50"                   <!-- Minimum width -->
         MaxWidth="200"                  <!-- Maximum width -->
         Margin="10,5,10,5"              <!-- Left,Top,Right,Bottom -->
         Padding="5"                     <!-- Uniform padding -->
         HorizontalAlignment="Stretch"   <!-- Left|Center|Right|Stretch -->
         VerticalAlignment="Center"      <!-- Top|Center|Bottom|Stretch -->
         HorizontalContentAlignment="Center"  <!-- For content within -->
         VerticalContentAlignment="Center" />

Grid Layout

<Grid RowDefinitions="Auto,*,50"           <!-- Rows: auto-size, fill, fixed 50 -->
      ColumnDefinitions="200,*,Auto">      <!-- Cols: 200, fill, auto-size -->
    
    <TextBlock Grid.Row="0" Grid.Column="0" Text="Header" />
    <Border Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="3" />
    
    <!-- Star sizing for proportions -->
    <Grid ColumnDefinitions="*,2*,*">  <!-- 1:2:1 ratio -->
        <!-- ... -->
    </Grid>
</Grid>

DockPanel Layout

<DockPanel LastChildFill="True">
    <Menu DockPanel.Dock="Top" />
    <StatusBar DockPanel.Dock="Bottom" />
    <TreeView DockPanel.Dock="Left" Width="200" />
    
    <!-- Last child fills remaining space -->
    <ContentControl Content="{Binding CurrentView}" />
</DockPanel>

StackPanel Layout

<StackPanel Orientation="Vertical"    <!-- Vertical|Horizontal -->
            Spacing="10">              <!-- Space between items -->
    <TextBlock Text="Item 1" />
    <TextBlock Text="Item 2" />
    <TextBlock Text="Item 3" />
</StackPanel>

GridSplitter (Resizable Panes)

When a Grid has distinct content regions (sidebar + main, top/bottom split), add a GridSplitter so users can resize the panes. Dedicate a narrow column/row (3–6 px) for the splitter.

<!-- Two-pane resizable layout -->
<Grid ColumnDefinitions="250, 4, *">
    <TreeView Grid.Column="0" />
    <GridSplitter Grid.Column="1" ResizeDirection="Columns" />
    <ContentControl Grid.Column="2" Content="{Binding Detail}" />
</Grid>

Key rules:

• ResizeDirection must match the axis: Columns for a column splitter, Rows for a row splitter.
• Set MinWidth/MaxWidth (or MinHeight/MaxHeight) on adjacent cells to prevent collapsing to zero.
• Use GridSplitter whenever a Grid has two or more sizeable content regions.

Responsive Layouts

Avalonia provides four approaches. Prefer these over fixed-pixel layouts.

Container Queries (preferred for reusable components)

Respond to the size of an ancestor control — not the window. Works live as the control resizes.

<Border Container.Name="main" Container.Sizing="Width">
    <Panel.Styles>
        <ContainerQuery Name="main" Query="max-width:600">
            <Style Selector="StackPanel#sidebar">
                <Setter Property="IsVisible" Value="False" />
            </Style>
        </ContainerQuery>
    </Panel.Styles>
    <!-- content here -->
</Border>

• Combine conditions: Query="min-width:400 and max-width:800"
• Container.Sizing: Width, Height, or Width Height

OnFormFactor (static platform detection)

Resolves once at startup. Use for desktop-vs-mobile differences that don't respond to window resizing.

<Grid ColumnDefinitions="{OnFormFactor Desktop='250,*', Mobile='*'}">
    <Border IsVisible="{OnFormFactor Desktop=True, Mobile=False}" />
</Grid>

Reflowing Panels (self-adapting)

<ItemsRepeater ItemsSource="{Binding Items}">
    <ItemsRepeater.Layout>
        <UniformGridLayout MinItemWidth="200" MinItemHeight="150" />
    </ItemsRepeater.Layout>
</ItemsRepeater>

When to use what

Key rules:

• PREFER Container Queries over manual size-change event handling.
• ALWAYS use star sizing (*) and Auto in Grid definitions — avoid fixed pixel widths for content regions.
• Visibility enum replaced by bool IsVisible; for invisible-but-space-occupying use Opacity="0".
• NO VisualStateManager — use pseudo-class selectors or Container Queries instead.

Data Binding

Binding Syntax

<!-- Basic binding -->
<TextBlock Text="{Binding PropertyName}" />

<!-- Binding with path -->
<TextBlock Text="{Binding Person.Name}" />

<!-- Binding modes -->
<TextBox Text="{Binding Name, Mode=TwoWay}" />
<!-- Modes: OneWay (default), TwoWay, OneTime, OneWayToSource -->

<!-- Binding to named element -->
<TextBlock x:Name="MyText" Text="Hello" />
<TextBox Text="{Binding #MyText.Text}" />

<!-- Binding to parent DataContext -->
<TextBlock Text="{Binding $parent[Window].DataContext.Title}" />

<!-- Binding with fallback -->
<TextBlock Text="{Binding Name, FallbackValue='Unknown'}" />

<!-- Binding with string format -->
<TextBlock Text="{Binding Price, StringFormat='${0:F2}'}" />

<!-- Binding with converter -->
<TextBlock Text="{Binding IsEnabled, Converter={StaticResource BoolToStringConverter}}" />

Important:

• #ElementName.Property is an Avalonia binding-path extension and should be used with Avalonia Binding / compiled bindings.
• Do not write {ReflectionBinding #SomeElement.SomeCommand}. ReflectionBinding treats the #... token as a plain path segment, so command/property lookup can fail at runtime.

Compiled Bindings (Recommended)

Compiled bindings provide compile-time safety and better performance.

<!-- Enable compiled bindings globally in .csproj -->
<AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault>

<!-- Enable for specific view -->
<Window x:DataType="vm:MainViewModel"
        x:CompileBindings="True">
    
    <!-- Type-safe binding -->
    <TextBox Text="{Binding FirstName}" />
    <TextBox Text="{Binding LastName}" />
    
    <!-- Disable compile-time checking for a dynamic path only -->
    <Button Command="{ReflectionBinding DynamicCommandName}" />
</Window>

<!-- Or use CompiledBinding markup explicitly -->
<TextBox Text="{CompiledBinding FirstName}" />

Best Practice: Always use compiled bindings for type safety and performance. Use ReflectionBinding only for truly dynamic paths that cannot be typed, and never with #ElementName syntax.

DataContext Type Inference (v11.3+)

<Window x:Name="MyWindow"
        x:DataType="vm:TestDataContext">
    
    <!-- Compiler infers DataContext type automatically -->
    <TextBlock Text="{Binding #MyWindow.DataContext.StringProperty}" />
    <TextBlock Text="{Binding $parent[Window].DataContext.StringProperty}" />
    
    <!-- No explicit type casting needed! -->
</Window>

Multi-Binding

<TextBlock>
    <TextBlock.Text>
        <MultiBinding StringFormat="{}{0} {1}">
            <Binding Path="FirstName" />
            <Binding Path="LastName" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

Element Binding

<!-- Bind to another element's property -->
<Slider x:Name="volumeSlider" Minimum="0" Maximum="100" Value="50" />
<TextBlock Text="{Binding #volumeSlider.Value}" />

<!-- Bind to parent control -->
<Border BorderThickness="{Binding $parent.IsMouseOver, 
                                  Converter={StaticResource BoolToThicknessConverter}}" />

MVVM Pattern with CommunityToolkit.Mvvm

⚠️ XerahS uses CommunityToolkit.Mvvm, NOT ReactiveUI. Do not add or reference ReactiveUI or Avalonia.ReactiveUI packages.

Install Package

dotnet add package CommunityToolkit.Mvvm

ViewModel Base Pattern

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

public partial class MainViewModel : ObservableObject
{
    [ObservableProperty]
    private string _firstName = string.Empty;

    [ObservableProperty]
    private bool _isLoading;

    [RelayCommand(CanExecute = nameof(CanSave))]
    private async Task SaveAsync()
    {
        IsLoading = true;
        try
        {
            await Task.Delay(1000); // Simulate save
        }
        finally
        {
            IsLoading = false;
        }
    }

    private bool CanSave() => !string.IsNullOrWhiteSpace(FirstName);
}

Source-generator notes:

• [ObservableProperty] on a private field generates a public PascalCase property + INotifyPropertyChanged notification (_firstName → FirstName).
• [RelayCommand] generates SaveAsyncCommand (an IAsyncRelayCommand) automatically.
• The class must be partial for source generators to work.
• [RelayCommand(CanExecute = nameof(...))] wires can-execute automatically; call SaveAsyncCommand.NotifyCanExecuteChanged() when the condition changes.

Architecture Layering

• Views (AXAML): Visual composition only. No business logic in code-behind beyond InitializeComponent().
• ViewModels: State, commands, and orchestration. UI-framework agnostic and unit-testable. Wire services via DI.
• Services / Domain: Business logic and data access — no references to Avalonia types.

View Setup (code-behind)

public partial class MainView : UserControl
{
    public MainView()
    {
        InitializeComponent();
        DataContext = new MainViewModel(); // Or resolve via ViewLocator / DI
    }
}

Command Binding in XAML

<!-- Generated command name: SaveAsyncCommand -->
<Button Content="Save" Command="{Binding SaveAsyncCommand}" />

<!-- With parameter -->
<Button Content="Delete"
        Command="{Binding DeleteCommand}"
        CommandParameter="{Binding SelectedItem}" />

Property Changed Callbacks

[ObservableProperty]
private string _name = string.Empty;

// Source-generated partial method — called automatically when Name changes
partial void OnNameChanged(string value)
{
    // React to change
}

Manual Property Notifications (when source generators are unavailable)

public partial class MyViewModel : ObservableObject
{
    private string _title = string.Empty;

    public string Title
    {
        get => _title;
        set => SetProperty(ref _title, value);
    }
}

Styling & Theming

Style Types

Avalonia has three styling mechanisms:

1. Styles: Similar to CSS, target controls by type or class
2. Control Themes: Complete visual templates (like WPF Styles)
3. Container Queries: Responsive styles based on container size

Basic Styles

<Window.Styles>
    <!-- Style by Type -->
    <Style Selector="TextBlock">
        <Setter Property="Foreground" Value="White" />
        <Setter Property="FontSize" Value="14" />
    </Style>

    <!-- Style by Class -->
    <Style Selector="TextBlock.header">
        <Setter Property="FontSize" Value="24" />
        <Setter Property="FontWeight" Value="Bold" />
    </Style>

    <!-- Style by property -->
    <Style Selector="Button:pointerover">
        <Setter Property="Background" Value="LightBlue" />
    </Style>

    <!-- Nested selectors -->
    <Style Selector="StackPanel > Button">
        <Setter Property="Margin" Value="5" />
    </Style>
</Window.Styles>

<!-- Apply class -->
<TextBlock Classes="header" Text="Title" />

Pseudo-classes

<Style Selector="Button:pointerover">         <!-- Mouse hover -->
<Style Selector="Button:pressed">             <!-- Mouse down -->
<Style Selector="Button:disabled">            <!-- Disabled state -->
<Style Selector="ListBoxItem:selected">       <!-- Selected item -->
<Style Selector="TextBox:focus">              <!-- Keyboard focus -->
<Style Selector="CheckBox:checked">           <!-- Checked state -->
<Style Selector="ToggleButton:unchecked">     <!-- Unchecked state -->

Style Selectors

<!-- Descendant (any depth) -->
<Style Selector="StackPanel TextBlock">

<!-- Direct child -->
<Style Selector="StackPanel > TextBlock">

<!-- Multiple conditions (AND) -->
<Style Selector="Button.primary:pointerover">

<!-- Multiple selectors (OR) -->
<Style Selector="Button, ToggleButton">

<!-- Negation -->
<Style Selector="Button:not(.primary)">

<!-- Template parts -->
<Style Selector="Button /template/ ContentPresenter">

Resources

<Window.Resources>
    <!-- Solid color brush -->
    <SolidColorBrush x:Key="PrimaryBrush" Color="#007ACC" />
    
    <!-- Static resource -->
    <x:Double x:Key="StandardSpacing">10</x:Double>
    
    <!-- Gradient brush -->
    <LinearGradientBrush x:Key="GradientBrush" StartPoint="0%,0%" EndPoint="0%,100%">
        <GradientStop Color="#FF0000" Offset="0" />
        <GradientStop Color="#00FF00" Offset="1" />
    </LinearGradientBrush>
</Window.Resources>

<!-- Use resources -->
<Button Background="{StaticResource PrimaryBrush}" 
        Margin="{StaticResource StandardSpacing}" />

<!-- DynamicResource (updates when changed) -->
<Button Background="{DynamicResource PrimaryBrush}" />

Themes

<!-- App.axaml -->
<Application.Styles>
    <!-- FluentTheme (Windows 11 style) -->
    <FluentTheme />
    
    <!-- Or Simple theme -->
    <SimpleTheme />
    
    <!-- Custom styles -->
    <StyleInclude Source="/Styles/CustomStyles.axaml" />
</Application.Styles>

Dependency & Attached Properties

StyledProperty (Dependency Property)

public class MyControl : ContentControl
{
    // Define the property
    public static readonly StyledProperty<string> TitleProperty =
        AvaloniaProperty.Register<MyControl, string>(
            nameof(Title), 
            defaultValue: string.Empty);

    // CLR wrapper
    public string Title
    {
        get => GetValue(TitleProperty);
        set => SetValue(TitleProperty, value);
    }

    // React to property changes
    protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
    {
        base.OnPropertyChanged(change);

        if (change.Property == TitleProperty)
        {
            // Handle change
            var oldValue = (string?)change.OldValue;
            var newValue = (string?)change.NewValue;
        }
    }
}

Attached Properties

public class MyPanel : Panel
{
    // Define attached property
    public static readonly AttachedProperty<int> ColumnProperty =
        AvaloniaProperty.RegisterAttached<MyPanel, Control, int>(
            "Column",
            defaultValue: 0);

    // Getters/Setters
    public static int GetColumn(Control element)
        => element.GetValue(ColumnProperty);

    public static void SetColumn(Control element, int value)
        => element.SetValue(ColumnProperty, value);
}

<!-- Use attached property -->
<local:MyPanel>
    <Button local:MyPanel.Column="0" Content="First" />
    <Button local:MyPanel.Column="1" Content="Second" />
</local:MyPanel>

Common Attached Properties

<!-- Grid -->
<Button Grid.Row="0" Grid.Column="1" Grid.RowSpan="2" Grid.ColumnSpan="3" />

<!-- DockPanel -->
<Menu DockPanel.Dock="Top" />

<!-- Canvas -->
<Rectangle Canvas.Left="50" Canvas.Top="100" />

<!-- ToolTip -->
<Button ToolTip.Tip="Click me!" />

<!-- ContextFlyout (Preferred with FluentAvalonia) -->
<Border>
    <Border.ContextFlyout>
        <MenuFlyout>
            <MenuItem Header="Copy" />
            <MenuItem Header="Paste" />
        </MenuFlyout>
    </Border.ContextFlyout>
</Border>

⚠️ XerahS-Specific: ContextMenu vs ContextFlyout

Critical Issue with FluentAvalonia Theme

Problem: Standard ContextMenu controls do not render correctly with FluentAvaloniaTheme. They use legacy Popup windows which are not fully styled and may appear unstyled or invisible.

Solution: ✅ Always use ContextFlyout with MenuFlyout instead of ContextMenu.

<!-- ❌ INCORRECT: May be invisible with FluentAvalonia -->
<Border.ContextMenu>
    <ContextMenu>
        <MenuItem Header="Action" Command="{Binding MyCommand}"/>
    </ContextMenu>
</Border.ContextMenu>

<!-- ✅ CORRECT: Use ContextFlyout with MenuFlyout -->
<Border.ContextFlyout>
    <MenuFlyout>
        <MenuItem Header="Action" Command="{Binding MyCommand}"/>
    </MenuFlyout>
</Border.ContextFlyout>

Binding in DataTemplates with Flyouts

Problem: When using ContextFlyout or ContextMenu inside a DataTemplate, bindings to the parent ViewModel fail because Popups/Flyouts exist in a separate visual tree, detached from the DataTemplate's hierarchy.

Solution: Use $parent[UserControl].DataContext to reach the main view's DataContext.

<DataTemplate x:DataType="local:MyItem">
    <Border>
        <Border.ContextFlyout>
            <MenuFlyout>
                <!-- ✅ Bind to parent UserControl's DataContext -->
                <MenuItem Header="Edit" 
                          Command="{Binding $parent[UserControl].DataContext.EditCommand}"
                          CommandParameter="{Binding}"/>
            </MenuFlyout>
        </Border.ContextFlyout>
        
        <TextBlock Text="{Binding Name}" />
    </Border>
</DataTemplate>

Key Points:

• Use $parent[UserControl].DataContext to access the View's ViewModel from within a flyout
• CommandParameter="{Binding}" passes the current data item (the DataTemplate's DataContext)
• For shared flyouts, define them in UserControl.Resources and reference via {StaticResource}

Custom Controls

Custom Control (Draws itself)

public class CircleControl : Control
{
    public static readonly StyledProperty<IBrush?> FillProperty =
        AvaloniaProperty.Register<CircleControl, IBrush?>(nameof(Fill));

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

    public override void Render(DrawingContext context)
    {
        var renderSize = Bounds.Size;
        var center = new Point(renderSize.Width / 2, renderSize.Height / 2);
        var radius = Math.Min(renderSize.Width, renderSize.Height) / 2;

        context.DrawEllipse(Fill, null, center, radius, radius);
    }
}

Templated Control (Look-less)

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

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

    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
    {
        base.OnApplyTemplate(e);
        
        // Find template parts
        var presenter = e.NameScope.Find<ContentPresenter>("PART_ContentPresenter");
    }
}

UserControl (Composite)

<!-- MyUserControl.axaml -->
<UserControl xmlns="https://github.com/avaloniaui"
             x:Class="MyApp.Controls.MyUserControl">
    <StackPanel>
        <TextBlock Text="{Binding Title}" />
        <Button Content="Click Me" />
    </StackPanel>
</UserControl>

// MyUserControl.axaml.cs
public partial class MyUserControl : UserControl
{
    public MyUserControl()
    {
        InitializeComponent();
    }
}

Control Templates

Define a ControlTheme

<ControlTheme x:Key="CustomButtonTheme" TargetType="Button">
    <Setter Property="Background" Value="Blue" />
    <Setter Property="Foreground" Value="White" />
    <Setter Property="Padding" Value="10,5" />
    <Setter Property="Template">
        <ControlTemplate>
            <Border Background="{TemplateBinding Background}"
                    BorderBrush="{TemplateBinding BorderBrush}"
                    BorderThickness="{TemplateBinding BorderThickness}"
                    CornerRadius="5">
                <ContentPresenter Name="PART_ContentPresenter"
                                  Content="{TemplateBinding Content}"
                                  Padding="{TemplateBinding Padding}"
                                  HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                                  VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" />
            </Border>
        </ControlTemplate>
    </Setter>
    
    <!-- Pseudo-class styles -->
    <Style Selector="^:pointerover /template/ Border">
        <Setter Property="Background" Value="LightBlue" />
    </Style>
    
    <Style Selector="^:pressed /template/ Border">
        <Setter Property="Background" Value="DarkBlue" />
    </Style>
</ControlTheme>

<!-- Apply theme -->
<Button Theme="{StaticResource CustomButtonTheme}" Content="Custom" />

Template Parts

[TemplatePart("PART_ContentPresenter", typeof(ContentPresenter))]
public class MyTemplatedControl : TemplatedControl
{
    private ContentPresenter? _presenter;

    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
    {
        base.OnApplyTemplate(e);
        _presenter = e.NameScope.Find<ContentPresenter>("PART_ContentPresenter");
    }
}

Resources & Converters

Value Converters

public class BoolToVisibilityConverter : IValueConverter
{
    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
    {
        if (value is bool boolValue)
            return boolValue ? true : false; // Or specific logic
        return false;
    }

    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
    {
        if (value is bool visible)
            return visible;
        return false;
    }
}

<Window.Resources>
    <local:BoolToVisibilityConverter x:Key="BoolToVisConverter" />
</Window.Resources>

<Border IsVisible="{Binding IsActive, Converter={StaticResource BoolToVisConverter}}" />

Built-in Converters

<!-- Negation -->
<Button IsEnabled="{Binding !IsLoading}" />

<!-- Null check -->
<TextBlock IsVisible="{Binding MyObject, Converter={x:Static ObjectConverters.IsNotNull}}" />

<!-- String format -->
<TextBlock Text="{Binding Count, StringFormat='Items: {0}'}" />

Events & Commands

Event Handlers

<Button Click="OnButtonClick" Content="Click" />

private void OnButtonClick(object? sender, RoutedEventArgs e)
{
    // Handle event
}

Commands (MVVM)

<Button Command="{Binding SaveCommand}" 
        CommandParameter="{Binding CurrentItem}"
        Content="Save" />

// Using CommunityToolkit.Mvvm
[RelayCommand]
private void Save(object? parameter)
{
    // Execute command
}

// Async command
[RelayCommand]
private async Task SaveAsync()
{
    await Task.Delay(100);
}

// Command with can-execute
[RelayCommand(CanExecute = nameof(CanDelete))]
private void Delete() { }

private bool CanDelete() => SelectedItem is not null;

Routed Events

public static readonly RoutedEvent<RoutedEventArgs> MyEvent =
    RoutedEvent.Register<MyControl, RoutedEventArgs>(
        nameof(MyEvent), 
        RoutingStrategies.Bubble);

public event EventHandler<RoutedEventArgs> MyEvent
{
    add => AddHandler(MyEvent, value);
    remove => RemoveHandler(MyEvent, value);
}

// Raise event
RaiseEvent(new RoutedEventArgs(MyEvent));

Cross-Platform Patterns

Platform Detection

if (OperatingSystem.IsWindows())
{
    // Windows-specific code
}
else if (OperatingSystem.IsMacOS())
{
    // macOS-specific code
}
else if (OperatingSystem.IsLinux())
{
    // Linux-specific code
}

Platform-Specific Resources

<Application.Styles>
    <StyleInclude Source="/Styles/Common.axaml" />
    
    <!-- Conditionally include styles -->
    <OnPlatform>
        <On Options="Windows">
            <StyleInclude Source="/Styles/Windows.axaml" />
        </On>
        <On Options="macOS">
            <StyleInclude Source="/Styles/macOS.axaml" />
        </On>
    </OnPlatform>
</Application.Styles>

Design Principles

1. Use .NET Standard: Write business logic in .NET Standard libraries
2. MVVM Pattern: Separate UI from logic
3. Avalonia Drawing: Leverage Avalonia's drawn UI (not native controls)
4. Platform Abstractions: Use interfaces for platform-specific features
5. Responsive Design: Use container queries and adaptive layouts

Performance & Best Practices

Performance Tips

1. Use Panel over Grid when no rows/columns needed
2. Enable compiled bindings globally
3. Use virtualization for large lists (ItemsRepeater, VirtualizingStackPanel)
4. Avoid deep nesting of visual trees
5. Use RenderTransform instead of Margin for animations
6. Recycle DataTemplates with ItemsRepeater
7. Minimize layout passes by batching property changes

Memory Management

CommunityToolkit.Mvvm source-generated properties raise PropertyChanged automatically — no manual subscription disposal is needed for [ObservableProperty] fields. When subscribing to external observables or events, implement IDisposable:

public partial class MyViewModel : ObservableObject, IDisposable
{
    private readonly IDisposable _subscription;

    public MyViewModel(IMyService service)
    {
        _subscription = service.ValueStream.Subscribe(OnValue);
    }

    private void OnValue(string v) { }

    public void Dispose() => _subscription.Dispose();
}

Null Safety

XerahS uses strict nullable reference types. Always:

// Enable in .csproj
<Nullable>enable</Nullable>

// Handle nullability properly
public string? Title { get; set; }  // Nullable
public string Name { get; set; } = string.Empty;  // Non-nullable with default

AOT and Trimming Awareness

For apps targeting mobile (iOS/Android), WebAssembly, or NativeAOT:

• Prefer compiled bindings (already the default) — they avoid reflection.
• Avoid Activator.CreateInstance for ViewModel/service resolution; use a DI container with AOT support (e.g. Microsoft.Extensions.DependencyInjection with source-generated registrations).
• Avoid Type.GetType(), PropertyInfo.SetValue(), or runtime reflection in hot paths.
• Use [DynamicallyAccessedMembers] annotations when reflection is unavoidable.
• Use {ReflectionBinding} sparingly — it is not trimming-safe, and do not combine it with Avalonia #ElementName paths.

Developer Tools

⚠️ The legacy Avalonia.Diagnostics package is deprecated. NEVER use it.

Use the migrate_diagnostics MCP tool to set up or migrate Developer Tools in any project. It handles:

• Removing Avalonia.Diagnostics
• Installing AvaloniaUI.DiagnosticsSupport
• Configuring Program.cs and App.axaml.cs
• Replacing old API calls

# Install the global developer tools CLI
dotnet tool install --global AvaloniaUI.DeveloperTools

Rule: ALWAYS use AvaloniaUI.DiagnosticsSupport + the AvaloniaUI.DeveloperTools .NET global tool.

Common Mistakes to Avoid

Common Patterns in XerahS

StyledProperty Pattern

public static readonly StyledProperty<object?> SelectedObjectProperty =
    AvaloniaProperty.Register<PropertyGrid, object?>(nameof(SelectedObject));

public object? SelectedObject
{
    get => GetValue(SelectedObjectProperty);
    set => SetValue(SelectedObjectProperty, value);
}

Attached Property Pattern (Auditing)

public static readonly AttachedProperty<bool> IsUnwiredProperty =
    AvaloniaProperty.RegisterAttached<UiAudit, Control, bool>("IsUnwired");

public static bool GetIsUnwired(Control control)
    => control.GetValue(IsUnwiredProperty);

public static void SetIsUnwired(Control control, bool value)
    => control.SetValue(IsUnwiredProperty, value);

Window Structure

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:XerahS.ViewModels"
        x:Class="XerahS.UI.Views.MainWindow"
        x:DataType="vm:MainViewModel"
        x:CompileBindings="True"
        Title="{Binding Title}"
        Width="1000" Height="700">
    
    <Window.Styles>
        <!-- Local styles -->
    </Window.Styles>
    
    <DockPanel>
        <!-- Layout content -->
    </DockPanel>
</Window>

Quick Reference Tables

Alignment Values

Binding Modes

Grid Sizing

Additional Resources

• Official Docs: https://docs.avaloniaui.net/
• GitHub: https://github.com/AvaloniaUI/Avalonia
• Samples: https://gi

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