unoplatform/uno · error · XamlSchemaException

ConverterType '{0}' is not derived from '{1}' type

Error message

ConverterType '{0}' is not derived from '{1}' type

What it means

Thrown by XamlValueConverter<TConverterBase>.CreateInstance (accessed via the ConverterInstance property) when the ConverterType is not assignable to the generic TConverterBase constraint. CreateInstance instantiates the converter, and the assignability check guards against a type mismatch that would otherwise cause an InvalidCastException during Activator.CreateInstance.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml.Schema/XamlValueConverter.cs:96

		
		public bool Equals (XamlValueConverter<TConverterBase> other)
		{
			return !IsNull (other) && ConverterType == other.ConverterType && TargetType == other.TargetType && Name == other.Name;
		}

		public override bool Equals (object obj)
		{
			var a = obj as XamlValueConverter<TConverterBase>;
			return Equals (a);
		}

		protected virtual TConverterBase CreateInstance ()
		{
			if (ConverterType == null)
				return null;

			if (!typeof (TConverterBase).IsAssignableFrom (ConverterType))
				throw new XamlSchemaException (String.Format (CultureInfo.InvariantCulture, "ConverterType '{0}' is not derived from '{1}' type", ConverterType, typeof (TConverterBase)));

			if (TargetType != null && TargetType.UnderlyingType != null) {
				// special case: Enum
				if (TargetType.UnderlyingType.IsEnum)
					return (TConverterBase) (object) new EnumConverter (TargetType.UnderlyingType);
				// special case: Nullable<T>
				if (TargetType.IsNullable && TargetType.UnderlyingType.IsValueType)
					return (TConverterBase) (object) new NullableConverter (TargetType.UnderlyingType);
			}

			if (ConverterType.GetConstructor (Type.EmptyTypes) == null)
				return null;

			return (TConverterBase) Activator.CreateInstance (ConverterType, true);
		}

		public override int GetHashCode ()
		{

View on GitHub (pinned to 0418340488)

Solutions

1. Verify the converterType passed to XamlValueConverter<T> actually derives from (or implements) TConverterBase before construction.
2. In custom XamlSchemaContext overrides, ensure GetXamlValueConverter returns converters whose Type matches the requested generic kind (TypeConverter vs ValueSerializer).
3. Add a type guard: typeof(TConverterBase).IsAssignableFrom(converterType) before creating the XamlValueConverter.

Example fix

// before
var vc = new XamlValueConverter<ValueSerializer>(typeof(MyTypeConverter), targetType);
var inst = vc.ConverterInstance; // throws: MyTypeConverter is not a ValueSerializer

// after
if (typeof(ValueSerializer).IsAssignableFrom(typeof(MyValueSerializer)))
    var vc = new XamlValueConverter<ValueSerializer>(typeof(MyValueSerializer), targetType);

if (converterType != null && !typeof(TConverterBase).IsAssignableFrom(converterType))
    throw new ArgumentException("converterType mismatch");
var vc = new XamlValueConverter<TConverterBase>(converterType, targetType, name);

static bool IsCompatible<TBase>(Type converterType)
    => converterType == null || typeof(TBase).IsAssignableFrom(converterType);

try { return vc.ConverterInstance; }
catch (XamlSchemaException ex) { /* log converter type mismatch */ return null; }

When it happens

Trigger: Constructing an XamlValueConverter<ValueSerializer> with a converterType that derives from TypeConverter rather than ValueSerializer (or vice versa). Accessing ConverterInstance triggers the lazy CreateInstance.

Common situations: Schema-context code that pairs a converter type with the wrong TConverterBase generic parameter — e.g. assigning a TypeConverter-derived type to a ValueConverter-targeted XamlValueConverter. Incorrect custom XamlSchemaContext.GetXamlValueConverter overrides that return mismatched converter types.

Related errors

• Either of converterType, targetType or name must be non-null
• typeArguments array contains one or more null XamlTypeName
• Namespace must be set before calling ToString method.
• Name must be set before calling ToString method.
• Could not lookup prefix for namespace '{0}'

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/27f7bba08155b2a6. Report an issue: GitHub.