MahApps/MahApps.Metro · error · InvalidOperationException

The Light Flyout theme only works if the window theme abides

Error message

The Light Flyout theme only works if the window theme abides the naming convention. See ThemeManager.GetInverseAppTheme for more infos

What it means

Thrown by Flyout.ChangeFlyoutTheme when Theme==FlyoutTheme.Light and the needed light theme cannot be resolved. If the window theme's BaseColorScheme is already Light, the window theme is used directly; otherwise GetInverseTheme is called and returns null when the window theme name doesn't follow the Light/Dark naming convention.

Source

Thrown at src/MahApps.Metro/Controls/Flyout.cs:714

                    ThemeManager.Current.ApplyThemeResourcesFromTheme(this.Resources, inverseTheme);
                    break;

                case FlyoutTheme.Dark:
                    var darkTheme = windowTheme.BaseColorScheme == ThemeManager.BaseColorDark ? windowTheme : ThemeManager.Current.GetInverseTheme(windowTheme);
                    if (darkTheme is null)
                    {
                        throw new InvalidOperationException("The Dark Flyout theme only works if the window theme abides the naming convention. " +
                                                            "See ThemeManager.GetInverseAppTheme for more infos");
                    }

                    ThemeManager.Current.ApplyThemeResourcesFromTheme(this.Resources, darkTheme);
                    break;

                case FlyoutTheme.Light:
                    var lightTheme = windowTheme.BaseColorScheme == ThemeManager.BaseColorLight ? windowTheme : ThemeManager.Current.GetInverseTheme(windowTheme);
                    if (lightTheme is null)
                    {
                        throw new InvalidOperationException("The Light Flyout theme only works if the window theme abides the naming convention. " +
                                                            "See ThemeManager.GetInverseAppTheme for more infos");
                    }

                    ThemeManager.Current.ApplyThemeResourcesFromTheme(this.Resources, lightTheme);
                    break;
            }
        }

        private void UpdateOpacityChange()
        {
            if (this.flyoutRoot is null || this.fadeOutFrame is null || System.ComponentModel.DesignerProperties.GetIsInDesignMode(this))
            {
                return;
            }

            if (!this.AnimateOpacity)
            {
                this.fadeOutFrame.Value = 1;

View on GitHub (pinned to 72099e310b)

Solutions

1. Register a light counterpart theme following the naming convention.
2. Use Flyout.Theme=Adapt to inherit the window theme without needing an inverse.
3. Use built-in MahApps themes which provide both variants.
4. Pre-check ThemeManager.Current.GetInverseTheme and fall back if null.

var windowTheme = ThemeManager.Current.DetectTheme(window);
FlyoutTheme desired = FlyoutTheme.Light;

if (windowTheme is not null
    && windowTheme.BaseColorScheme != ThemeManager.BaseColorLight
    && ThemeManager.Current.GetInverseTheme(windowTheme) is null)
{
    desired = FlyoutTheme.Adapt; // no light counterpart available
}
flyout.Theme = desired;

try { flyout.Theme = FlyoutTheme.Light; }
catch (InvalidOperationException ex) when (ex.Message.Contains("Light Flyout theme"))
{
    flyout.Theme = FlyoutTheme.Adapt;
}

When it happens

Trigger: Setting Flyout.Theme=Light on a dark-themed window whose custom theme name lacks the Light/Dark convention; the light inverse variant was never registered.

Common situations: Custom dark theme registered without a light counterpart; non-conventional theme naming; incomplete theme resource pack.

Related errors

• The inverse Flyout theme only works if the window theme abid
• The Dark Flyout theme only works if the window theme abides
• {nameof(AutoWatermarkProperty)} is not supported for {fe.Get
• The property 'IsSpellCheckContextMenuEnabled' may only be se
• The settings file '{filename ?? "<unknown>"}' seems to be co

AI-assisted analysis of MahApps/MahApps.Metro@72099e310b (2026-08-13). Data as JSON: /api/errors/c42daf4d04b13ab2. Report an issue: GitHub.