MaterialDesignInXAML/MaterialDesignInXamlToolkit · error · InvalidOperationException

Entry was not of type Color

Error message

Entry {foregroundKey} was not of type Color

What it means

The second guard inside `SwatchesProvider.GetHue`: after finding the hue colour, it looks up the matching `*Foreground` key via `.Single(...)` and requires its value to also be a `Color`. If the foreground entry exists but is not a `Color`, it throws `InvalidOperationException` naming the foreground key. Note the `.Single(...)` call also throws (`InvalidOperationException`/`ArgumentNullException`) if the foreground key is missing entirely.

Solutions

1. Provide a matching `*Foreground` key typed as `Color` for every hue key in the resource dictionary.
2. Use the bundled `MaterialDesignColors` assemblies whose keys are paired correctly, and keep NuGet versions aligned.
3. If authoring custom hues, define both `<Color x:Key="MyHue">` and `<Color x:Key="MyHueForeground">`.
4. Wrap swatch loading in try/catch around `InvalidOperationException` to skip malformed swatches.

Example fix

// before: foreground key is a brush -> throws
//   <SolidColorBrush x:Key="PrimaryHueMidForeground" Color="White"/>

// after: foreground key is a Color
//   <Color x:Key="PrimaryHueMidForeground">#FFFFFF</Color>

bool ForegroundKeyIsColor(ResourceDictionary d, string fgKey)
    => d.OfType<DictionaryEntry>()
         .Where(de => Equals(de.Key?.ToString(), fgKey))
         .All(de => de.Value is Color);

static bool EntryIsColor(DictionaryEntry entry) => entry.Value is Color;

try { hues.Add(SwatchesProvider.GetHue(dict, entry)); }
catch (InvalidOperationException) { /* skip malformed hue */ }

When it happens

Trigger: A hue resource has a valid `Color` but its companion `*Foreground` key is typed as a Brush/string, is missing, or is duplicated (which makes `.Single` throw).

Common situations: Resource dictionaries that define foregrounds as `SolidColorBrush` rather than `Color`; renamed foreground keys across MaterialDesign versions; partial resource packs lacking the foreground counterpart.

Related errors

• Entry was not of type Color
• ColorReference does not contain any color
• Could not locate required resource with key(s)
• Non primary hues provided.
• Cannot get theme outside of a WPF application. Use

AI-assisted analysis of MaterialDesignInXAML/MaterialDesignInXamlToolkit@98edec3a0b (2026-08-13). Data as JSON: /api/errors/7fc08e2a11c2e8e7. Report an issue: GitHub.

Appendix: source

Thrown at src/MaterialDesignColors.Wpf/SwatchesProvider.cs:85

                    hues.Add(GetHue(resourceDictionary, entry));
                }
            }
            return hues;
        }

        static Hue GetHue(ResourceDictionary dictionary, DictionaryEntry entry)
        {
            if (entry.Value is not Color colour)
            {
                throw new InvalidOperationException($"Entry {entry.Key} was not of type {nameof(Color)}");
            }
            string foregroundKey = entry.Key?.ToString() + "Foreground";
            if (dictionary.OfType<DictionaryEntry>()
                    .Single(de => string.Equals(de.Key.ToString(), foregroundKey, StringComparison.Ordinal))
                    .Value is not Color foregroundColour)
            {
                throw new InvalidOperationException($"Entry {foregroundKey} was not of type {nameof(Color)}");
            }

            return new Hue(entry.Key?.ToString() ?? "", colour, foregroundColour);
        }
    }

    private static ResourceDictionary? Read(string? assemblyName, string? path)
    {
        if (assemblyName is null || path is null)
            return null;

        lock (_syncLock)
        {
            //NB: Application.LoadComponent is not thread safe
            return (ResourceDictionary)Application.LoadComponent(new Uri(
                $"/{assemblyName};component/{path.Replace(".baml", ".xaml")}",
                UriKind.RelativeOrAbsolute));
        }

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