dotnet/wpf · error · ArgumentException

SR.Format(SR.MustBeOfType, "value"…

Error message

SR.Format(SR.MustBeOfType, "value", "DynamicResourceExtension")

What it means

DynamicResourceExtensionConverter.ConvertTo converts a DynamicResourceExtension to an InstanceDescriptor (for designer serialization). It throws ArgumentException with MustBeOfType when the supplied value is not a DynamicResourceExtension instance.

Solutions

Ensure the value passed to ConvertTo is a DynamicResourceExtension instance
Check the value type before converting
Use the converter only through the property system/designer pipeline, which supplies the correct types

Example fix

// before
converter.ConvertTo(ctx, culture, someString, typeof(InstanceDescriptor)); // throws
// after
if (value is DynamicResourceExtension)
    converter.ConvertTo(ctx, culture, value, typeof(InstanceDescriptor));

Defensive patterns

Strategy: type-guard

Validate before calling

if (!(value is DynamicResourceExtension)) throw new ArgumentException(nameof(value));

Type guard

bool IsDynamicResource(object v) => v is DynamicResourceExtension;

Try / catch

try { result = converter.ConvertTo(ctx, culture, value, typeof(InstanceDescriptor)); }
catch (ArgumentException ex) when (ex.ParamName == "value") { result = null; }

Prevention

Verify value types before invoking type converters
Route conversions through the WPF property system rather than calling converters directly

When it happens

Trigger: Calling TypeConverter.ConvertTo(context, culture, value, destinationType) passing an object that is not DynamicResourceExtension while destinationType is InstanceDescriptor.

Common situations: Custom designer/serialization code invoking the converter directly with a wrong object; type confusion when copying property values between resources and setters.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/312685b3e4b0786c. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/DynamicResourceExtensionConverter.cs:47

            {
                return true;
            }
            return base.CanConvertTo(context, destinationType);
        }

        /// <summary>
        /// Converts to an instance descriptor
        /// </summary>
        public override object ConvertTo(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value, Type destinationType)
        {
            if (destinationType == typeof(InstanceDescriptor))
            {
                ArgumentNullException.ThrowIfNull(value);

                DynamicResourceExtension dynamicResource = value as DynamicResourceExtension;

                if (dynamicResource == null)
                    throw new ArgumentException(SR.Format(SR.MustBeOfType, "value", "DynamicResourceExtension"), nameof(value)); 

                return new InstanceDescriptor(typeof(DynamicResourceExtension).GetConstructor(new Type[] { typeof(object) }), 
                    new object[] { dynamicResource.ResourceKey } );
            }
            return base.ConvertTo(context, culture, value, destinationType);
        }
    }
}

View on GitHub (pinned to 81131a70a4)