dotnet/wpf · error · NotSupportedException

SR.Converter_ConvertToNotSupported

Error message

SR.Converter_ConvertToNotSupported

What it means

BrushConverter.ConvertTo throws NotSupportedException when the target type is string but the Brush instance reports CanSerializeToString() == false. Only certain brush types (e.g. SolidColorBrush with a simple color) can be expressed as a string; others cannot round-trip and the converter refuses rather than emit a lossy value.

Solutions

  1. Check brush.CanSerializeToString() before calling ConvertTo and fall back to full XAML/BAML serialization for complex brushes.
  2. Convert the brush to a resource reference or serialized form instead of a plain string.
  3. If you only need simple brushes, restrict inputs to SolidColorBrush instances.
  4. Handle NotSupportedException and use XamlWriter.Save as fallback.

Example fix

// before
string s = (string)brushConverter.ConvertTo(ctx, culture, gradientBrush, typeof(string)); // throws
// after
string s = gradientBrush.CanSerializeToString()
    ? (string)brushConverter.ConvertTo(ctx, culture, gradientBrush, typeof(string))
    : XamlWriter.Save(gradientBrush);
Defensive patterns

Strategy: validation

Validate before calling

if (brush is SolidColorBrush scb) { /* safe to convert */ } else { bool convertible = ((Brush)brush).CanSerializeToString(); }

Type guard

bool canToString = brush is Brush b && b.CanSerializeToString();

Try / catch

try { s = (string)converter.ConvertTo(ctx, culture, brush, typeof(string)); } catch (NotSupportedException) { s = XamlWriter.Save(brush); }

Prevention

When it happens

Trigger: Calling ConvertTo(context, culture, value, typeof(string)) where value is a Brush whose CanSerializeToString() returns false (e.g. many gradient/complex brushes) while a context with a non-null Instance is supplied.

Common situations: Serializing a LinearGradientBrush/RadialGradientBrush to a string for settings storage or code generation; XAML designer/serialization engines attempting string conversion of complex brushes.

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/37bcbd8f9b0f00c5. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Generated/BrushConverter.cs:137

        /// </exception>
        /// <param name="context"> The ITypeDescriptorContext for this call. </param>
        /// <param name="culture"> The CultureInfo which is respected when converting. </param>
        /// <param name="value"> The object to convert to an instance of "destinationType". </param>
        /// <param name="destinationType"> The type to which this will convert the Brush instance. </param>
        public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
        {
            if (destinationType != null && value is Brush)
            {
                Brush instance = (Brush)value;

                if (destinationType == typeof(string))
                {
                    // When invoked by the serialization engine we can convert to string only for some instances
                    if (context != null && context.Instance != null)
                    {
                        if (!instance.CanSerializeToString())
                        {
                            throw new NotSupportedException(SR.Converter_ConvertToNotSupported);
                        }
                    }

                    // Delegate to the formatting/culture-aware ConvertToString method.
                    return instance.ConvertToString(null, culture);
                }
            }

            // Pass unhandled cases to base class (which will throw exceptions for null value or destinationType.)
            return base.ConvertTo(context, culture, value, destinationType);
        }
    }
}

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