MaterialDesignInXAML/MaterialDesignInXamlToolkit · error · InvalidOperationException

Cannot get theme outside of a WPF application. Use {nameof(R

Error message

Cannot get theme outside of a WPF application. Use {nameof(ResourceDictionaryExtensions)}.{nameof(ResourceDictionaryExtensions.GetTheme)} on the appropriate resource dictionary instead.

What it means

PaletteHelper.GetTheme() reads the theme from Application.Current.Resources, so it only works inside a running WPF Application. When Application.Current is null it throws InvalidOperationException and directs you to ResourceDictionaryExtensions.GetTheme, which operates on an explicit ResourceDictionary instead.

Source

Thrown at src/MaterialDesignThemes.Wpf/PaletteHelper.cs:8

namespace MaterialDesignThemes.Wpf;

public class PaletteHelper
{
    public virtual Theme GetTheme()
    {
        if (Application.Current is null)
            throw new InvalidOperationException($"Cannot get theme outside of a WPF application. Use {nameof(ResourceDictionaryExtensions)}.{nameof(ResourceDictionaryExtensions.GetTheme)} on the appropriate resource dictionary instead.");
        return GetResourceDictionary().GetTheme();
    }

    public virtual void SetTheme(Theme theme)
    {
        if (theme is null) throw new ArgumentNullException(nameof(theme));
        if (Application.Current is null)
            throw new InvalidOperationException($"Cannot set theme outside of a WPF application. Use {nameof(ResourceDictionaryExtensions)}.{nameof(ResourceDictionaryExtensions.SetTheme)} on the appropriate resource dictionary instead.");

        GetResourceDictionary().SetTheme(theme);
        RecreateThemeDictionaries();
    }

    public virtual IThemeManager? GetThemeManager()
    {
        if (Application.Current is null)
            throw new InvalidOperationException($"Cannot get ThemeManager outside of a WPF application. Use {nameof(ResourceDictionaryExtensions)}.{nameof(ResourceDictionaryExtensions.GetThemeManager)} on the appropriate resource dictionary instead.");
        return GetResourceDictionary().GetThemeManager();

View on GitHub (pinned to 98edec3a0b)

Solutions

1. Call ResourceDictionaryExtensions.GetTheme on your ResourceDictionary directly, e.g. myResourceDictionary.GetTheme().
2. Guard the call: `if (Application.Current is not null) helper.GetTheme();`.
3. In tests/design-time, construct a ResourceDictionary, merge the MaterialDesign theme, and read from it instead of PaletteHelper.

Example fix

// before
var theme = new PaletteHelper().GetTheme();
// after (works in tests/designer)
var theme = myResourceDictionary.GetTheme();

if (Application.Current is null)
    return myResourceDictionary.GetTheme();
return new PaletteHelper().GetTheme();

Application.Current is not null

When it happens

Trigger: Calling new PaletteHelper().GetTheme() (directly or transitively) when Application.Current is null: design-time data contexts, unit tests, console hosts, or code that runs before the WPF Application object is constructed.

Common situations: Reading the theme in a designer or xUnit/nunit test; a shared service calling PaletteHelper from a non-WPF process; calling GetTheme during static initialization or before app startup completes.

Related errors

• Cannot set theme outside of a WPF application. Use {nameof(R
• Cannot get ThemeManager outside of a WPF application. Use {n
• theme
• Could not locate required resource with key(s) '{string.Join
• Non primary hues provided.

AI-assisted analysis of MaterialDesignInXAML/MaterialDesignInXamlToolkit@98edec3a0b (2026-08-13). Data as JSON: /api/errors/d74f7c97e7f85d7d. Report an issue: GitHub.