dotnet/wpf · error · ArgumentException

SR.Format(SR.CannotConvertType, typeof(CornerRadius)…

Error message

SR.Format(SR.CannotConvertType, typeof(CornerRadius), destinationType.FullName)

What it means

CornerRadiusConverter.ConvertTo only supports converting to string and InstanceDescriptor; any other destinationType falls through to this ArgumentException built from SR.CannotConvertType, reporting typeof(CornerRadius) and the unsupported destination type's full name. This is the converter's 'unsupported target type' terminal case.

Solutions

Only request typeof(string) or typeof(InstanceDescriptor) from this converter.
Convert to string first, then parse/transform the string into your desired target type yourself.
Write a custom TypeConverter or mapper for unsupported destination types.

Example fix

// before
var n = (int)converter.ConvertTo(ctx, culture, cr, typeof(int));
// after
string s = (string)converter.ConvertTo(ctx, culture, cr, typeof(string));
// transform s into the target representation yourself

Defensive patterns

Strategy: validation

Validate before calling

static readonly Type[] Supported = { typeof(string), typeof(InstanceDescriptor) };
if (!Supported.Contains(destinationType))
    throw new NotSupportedException($"CornerRadiusConverter cannot convert to {destinationType?.FullName}");

Type guard

static bool IsSupportedDest(Type t) => t == typeof(string) || t == typeof(InstanceDescriptor);

Try / catch

try { return converter.ConvertTo(ctx, culture, cr, destType); } catch (ArgumentException) { return cr.ToString(CultureInfo.InvariantCulture); }

Prevention

Restrict generic conversion calls to string or InstanceDescriptor destinations.
Convert to string first, then transform for other formats.
Document converter-supported destinations wherever converters are invoked generically.

When it happens

Trigger: ConvertTo(ctx, culture, cornerRadius, typeof(int)) or any destinationType other than typeof(string) and typeof(InstanceDescriptor).

Common situations: Custom serializers exporting properties to CSV/JSON by asking converters for arbitrary types; tooling that requests byte[] or Xml representation of WPF values.

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/18d2d124f46d3dbd. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/CornerRadiusConverter.cs:134

        {
            ArgumentNullException.ThrowIfNull(value);

            ArgumentNullException.ThrowIfNull(destinationType);

            if (!(value is CornerRadius))
            {
                throw new ArgumentException(SR.Format(SR.UnexpectedParameterType, value.GetType(), typeof(CornerRadius)), nameof(value));
            }

            CornerRadius cr = (CornerRadius)value;
            if (destinationType == typeof(string)) { return ToString(cr, cultureInfo); }
            if (destinationType == typeof(InstanceDescriptor))
            {
                ConstructorInfo ci = typeof(CornerRadius).GetConstructor(new Type[] { typeof(double), typeof(double), typeof(double), typeof(double) });
                return new InstanceDescriptor(ci, new object[] { cr.TopLeft, cr.TopRight, cr.BottomRight, cr.BottomLeft });
            }

            throw new ArgumentException(SR.Format(SR.CannotConvertType, typeof(CornerRadius), destinationType.FullName));
        }

        #endregion Public Methods

        //-------------------------------------------------------------------
        //
        //  Internal Methods
        //
        //-------------------------------------------------------------------

        #region Internal Methods

        internal static string ToString(CornerRadius cr, CultureInfo cultureInfo)
        {
            char listSeparator = TokenizerHelper.GetNumericListSeparator(cultureInfo);

            return string.Create(cultureInfo, stackalloc char[64], $"{cr.TopLeft}{listSeparator}{cr.TopRight}{listSeparator}{cr.BottomRight}{listSeparator}{cr.BottomLeft}");
        }

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