SystemThemeHelper

UnoFeatures: Toolkit (add to <UnoFeatures> in your .csproj)

Provides utilities for managing and retrieving the current theme of the operating system and the application.

Theme Retrieval

Methods

Method	Return Type	Description
`GetCurrentOsTheme()`	`ApplicationTheme`	Retrieves the current theme of the operating system.
`GetApplicationTheme()`	`ApplicationTheme`	[Obsolete] Retrieves the current theme of the application. Will be removed in future versions.
`GetRootTheme(XamlRoot?)`	`ApplicationTheme`	Gets the `ApplicationTheme` of the provided `XamlRoot`, or falls back to the operating system theme.
`GetRootTheme(FrameworkElement?)`	`ApplicationTheme`	Gets the `ApplicationTheme` of the provided root element, or falls back to the operating system theme. Use when the app is hosted under a `XamlRoot` it doesn't own (e.g. ALC-hosted), where `XamlRoot.Content` is the host's root.
`GetRootTheme(Window?)`	`ApplicationTheme`	Gets the `ApplicationTheme` of the provided window's content, or falls back to the operating system theme.
`IsAppInDarkMode()`	`bool`	[Obsolete] Returns `true` if the application is currently in dark mode, otherwise `false`.
`IsRootInDarkMode(XamlRoot)`	`bool`	Determines if the provided `XamlRoot` is in dark mode.
`IsRootInDarkMode(FrameworkElement)`	`bool`	Determines if the provided root element is in dark mode.
`IsRootInDarkMode(Window)`	`bool`	Determines if the provided window's content is in dark mode.

Usage

Example of retrieving the current OS theme:

var osTheme = SystemThemeHelper.GetCurrentOsTheme();
Console.WriteLine($"Current OS Theme: {osTheme}");

Retrieve the application theme:

var xamlRoot = someElement.XamlRoot;
var appTheme = SystemThemeHelper.GetRootTheme(xamlRoot);
Console.WriteLine($"Current App Theme: {appTheme}");

Theme Management

Methods

Method	Description
`SetApplicationTheme(bool)`	[Obsolete] Sets the application theme to dark mode if `darkMode` is `true`, otherwise light mode. Will be removed in future versions.
`SetRootTheme(XamlRoot?, bool)`	Sets the theme for the provided `XamlRoot` based on the `darkMode` parameter.
`SetRootTheme(FrameworkElement?, bool)`	Sets the theme for the provided root element (and its subtree) based on the `darkMode` parameter. Use when the app is hosted under a `XamlRoot` it doesn't own (e.g. ALC-hosted), where `XamlRoot.Content` is the host's root.
`SetRootTheme(Window?, bool)`	Sets the theme for the provided window's content based on the `darkMode` parameter.
`SetApplicationTheme(XamlRoot?, ElementTheme)`	Sets the `ElementTheme` (`Light` or `Dark`) for the provided `XamlRoot`.
`SetApplicationTheme(FrameworkElement?, ElementTheme)`	Sets the `ElementTheme` (`Light` or `Dark`) on the provided root element, theming its whole subtree.
`SetApplicationTheme(Window?, ElementTheme)`	Sets the `ElementTheme` (`Light` or `Dark`) on the provided window's content.
`ToggleApplicationTheme()`	[Obsolete] Toggles the application theme between light and dark modes. Will be removed in future versions.

Usage

Set the theme for a specific XamlRoot:

var xamlRoot = someElement.XamlRoot;
SystemThemeHelper.SetApplicationTheme(xamlRoot, ElementTheme.Light);
Console.WriteLine("XamlRoot theme set to Light Mode.");

Set the theme when the app is hosted under a XamlRoot it doesn't own (e.g. ALC-hosted):

// Capture the app's root element once (e.g. in App.xaml.cs, right after setting window.Content):
var appRoot = window.Content as FrameworkElement;
SystemThemeHelper.SetApplicationTheme(appRoot, ElementTheme.Dark);

// Or, using the app's Window directly:
SystemThemeHelper.SetApplicationTheme(window, ElementTheme.Dark);

If the resolved root element is null (for example, the window content is not set yet, or is not a FrameworkElement), the Set* overloads are a no-op (a warning is logged when logging is enabled), and the Get* overloads fall back to the operating system theme.

Important

The XamlRoot-based overloads read and write the theme of XamlRoot.Content - the root visual of the XamlRoot - and are the API to use in normal situations. When the app's content is hosted under a XamlRoot it doesn't own (for example, an app loaded into another app via an AssemblyLoadContext, with its content re-parented into a shared XamlRoot), XamlRoot.Content is the host's root visual: theming it re-themes the host instead of the app. In that scenario, use the Window- or FrameworkElement-based overloads with the app's own window or root element (typically window.Content).