MaterialDesignInXAML/MaterialDesignInXamlToolkit · error · InvalidOperationException

Cannot set theme outside of a WPF application. Use

Error message

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

What it means

The SetTheme counterpart of the GetTheme guard: SetTheme writes into Application.Current.Resources and cannot run when Application.Current is null. It throws InvalidOperationException pointing to ResourceDictionaryExtensions.SetTheme, which applies the theme to a specific ResourceDictionary.

Solutions

1. Use ResourceDictionaryExtensions.SetTheme on the target ResourceDictionary directly.
2. Defer PaletteHelper.SetTheme until after Application.Current is set (e.g. in the Startup handler).
3. In tests, build a ResourceDictionary, merge MaterialDesign resources, and call SetTheme on it.

Example fix

// before
new PaletteHelper().SetTheme(theme);
// after
myResourceDictionary.SetTheme(theme);

if (Application.Current is null)
    myResourceDictionary.SetTheme(theme);
else
    new PaletteHelper().SetTheme(theme);

Application.Current is not null

When it happens

Trigger: Calling PaletteHelper.SetTheme from a non-WPF host, before the Application object is constructed, or in a unit test.

Common situations: Applying a persisted theme during bootstrap before Application exists; applying a theme in a designer or test harness.

Related errors

• Cannot get theme outside of a WPF application. Use
• Cannot get ThemeManager outside of a WPF application. Use
• Could not locate required resource with key(s)
• theme
• ColorReference does not contain any color

AI-assisted analysis of MaterialDesignInXAML/MaterialDesignInXamlToolkit@98edec3a0b (2026-08-13). Data as JSON: /api/errors/9ccec79b285dd1d5. Report an issue: GitHub.

Appendix: source

Thrown at src/MaterialDesignThemes.Wpf/PaletteHelper.cs:16

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();
    }

    private static ResourceDictionary GetResourceDictionary()
        => Application.Current.Resources.MergedDictionaries.FirstOrDefault(x => x is IMaterialDesignThemeDictionary) ??
            Application.Current.Resources;

    /// <summary>
    /// Removes and readds resource dictionaries with static resource that will be re-evaluated with new theme brushes.

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