MaterialDesignInXAML/MaterialDesignInXamlToolkit · error · ArgumentNullException

theme

Error message

theme

What it means

SetTheme writes a Theme into the application resources but first validates that the Theme argument is non-null, throwing ArgumentNullException(nameof(theme)). This guard fires before the Application.Current check, so it triggers regardless of whether a WPF application is running.

Source

Thrown at src/MaterialDesignThemes.Wpf/PaletteHelper.cs:14

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;

View on GitHub (pinned to 98edec3a0b)

Solutions

1. Build the Theme with Theme.Create(...) or modify an existing GetTheme() result before passing it.
2. Null-check the theme at the call site before SetTheme.
3. If loading from settings, validate the deserialized theme and fall back to a default Theme rather than null.

Example fix

// before
helper.SetTheme(loadedTheme /* may be null */);
// after
helper.SetTheme(loadedTheme ?? Theme.Create(PrimaryColor.Blue, SecondaryColor.Amber));

if (theme is null) throw new ArgumentNullException(nameof(theme));
helper.SetTheme(theme);

theme is not null

When it happens

Trigger: Calling PaletteHelper.SetTheme(null), or passing a theme that a builder/factory returned null for.

Common situations: A Theme constructed from user input that failed to build; a refactor that dropped the Theme argument; a deserialization path that yielded null.

Related errors

• Cannot get theme outside of a WPF application. Use {nameof(R
• Cannot set theme outside of a WPF application. Use {nameof(R
• Cannot get ThemeManager outside of a WPF application. Use {n
• Non primary hues provided.
• Entry {entry.Key} was not of type Color

AI-assisted analysis of MaterialDesignInXAML/MaterialDesignInXamlToolkit@98edec3a0b (2026-08-13). Data as JSON: /api/errors/a6e334ad5cb0812f. Report an issue: GitHub.