dotnet/wpf · error · NotSupportedException

Cannot convert to type.

Error message

Cannot convert to type.

What it means

ColorTypeConverter.ConvertTo checks the destinationType with IsSupportedType and throws NotSupportedException ('Cannot convert to type.') for unsupported target types. Only the destination types the XPS serializer needs are allowed.

Solutions

1. Use ColorConverter.ConvertToString for string conversion.
2. Convert manually: $"#{color.A:X2}{color.R:X2}{color.G:X2}{color.B:X2}".
3. Check the converter's supported destination types before calling ConvertTo.
4. Catch NotSupportedException and fall back to manual formatting.

Example fix

// before
string s = (string)new ColorTypeConverter().ConvertTo(null, null, color, typeof(string));
// after
string s = ColorConverter.ConvertToString(color);

if (destinationType != typeof(string) /* only whitelisted types */) throw new NotSupportedException();

static bool CanConvertTo(Type t) => t == typeof(string); // extend per supported set

try { return converter.ConvertTo(context, culture, color, destinationType); }
catch (NotSupportedException) { return ColorConverter.ConvertToString(color); }

When it happens

Trigger: Calling ConvertTo(color, typeof(string)) or any destination type not whitelisted by IsSupportedType on ColorTypeConverter.

Common situations: General string formatting of Color for display/logging, design-time property grids, or generic serialization utilities.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Cannot convert from type.
• Cannot convert to type.
• SR.Converter_ConvertToNotSupported
• SR.Converter_ConvertToNotSupported
• ArgumentNullException (nameof(targetObject))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/6d7a8be83b4b9669. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/ReachFramework/Serialization/ColorTypeConverter.cs:146

        /// <param name="destinationType">
        /// The Type to convert the value parameter to.
        /// </param>
        /// <returns>
        /// The Type to convert the value parameter to.
        /// </returns>
        public
        override
        object
        ConvertTo(
            ITypeDescriptorContext              context,
            System.Globalization.CultureInfo    culture,
            object                              value,
            Type                                destinationType
            )
        {
            if (!IsSupportedType(destinationType))
            {
                throw new NotSupportedException(SR.Converter_ConvertToNotSupported);
            }

            Color color = (Color)value;

            string colorString;

            if (color.ColorContext == null)
            {
                colorString = color.ToString(culture);

                if (colorString.StartsWith("sc#", StringComparison.Ordinal) && colorString.Contains('E'))
                {
                    //
                    // Fix bug 1588888: Serialization produces non-compliant scRGB color string.
                    //
                    // Avalon Color.ToString() will use "R"oundtrip formatting specifier for scRGB colors,
                    // which may produce numbers in scientific notation, which is invalid XPS.
                    //

View on GitHub (pinned to 81131a70a4)