xaml-csharp-development-skill-for-avalonia

WinUI Theme Variants, High Contrast, and Live Theme Switching to Avalonia

Table of Contents

1. Scope and APIs
2. Concept Mapping
3. Conversion Example
4. Migration Notes

Scope and APIs

Primary WinUI APIs:

• Application.RequestedTheme, FrameworkElement.RequestedTheme
• AccessibilitySettings.HighContrast
• UISettings.ColorValuesChanged

Primary Avalonia APIs:

• Application.RequestedThemeVariant, TopLevel.RequestedThemeVariant, ThemeVariantScope
• Application.ActualThemeVariantChanged
• IPlatformSettings.GetColorValues, ColorValuesChanged
• PlatformColorValues.ContrastPreference

Concept Mapping

WinUI idiom	Avalonia idiom
app-level requested theme	Application.RequestedThemeVariant
subtree theme override	ThemeVariantScope RequestedThemeVariant
high-contrast preference checks	PlatformColorValues.ContrastPreference
live system color/theme updates	IPlatformSettings.ColorValuesChanged

Conversion Example

WinUI XAML:

<Grid RequestedTheme="Dark"
      Background="{ThemeResource ApplicationPageBackgroundThemeBrush}" />

WinUI C#:

Application.Current.RequestedTheme = ApplicationTheme.Dark;

var accessibility = new AccessibilitySettings();
bool isHighContrast = accessibility.HighContrast;

Avalonia XAML:

<ThemeVariantScope xmlns="https://github.com/avaloniaui"
                   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                   RequestedThemeVariant="Dark">
  <Border Background="{DynamicResource ThemeBackgroundBrush}" />
</ThemeVariantScope>

Avalonia C#:

Application.Current!.RequestedThemeVariant = ThemeVariant.Dark;

if (Application.Current.PlatformSettings is { } platform)
{
    ApplyContrast(platform.GetColorValues());
    platform.ColorValuesChanged += (_, values) => ApplyContrast(values);
}

void ApplyContrast(PlatformColorValues values)
{
    if (values.ContrastPreference == ColorContrastPreference.High)
        Application.Current!.RequestedThemeVariant = ThemeVariant.Dark;
}

Migration Notes

1. Keep theme switching reactive and resource-driven (DynamicResource) rather than repainting manually.
2. Handle system contrast/theme notifications as runtime events, not startup-only state.
3. Validate contrast and accent behavior on each target OS backend.

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