dotnet/wpf · error · ArgumentException

ArgumentException(SR.Format(SR.TypeHasInvalidXamlName…

Error message

ArgumentException(SR.Format(SR.TypeHasInvalidXamlName, type.Name), nameof(type))

What it means

WriteStartObject validates that the XamlType's Name is a valid XAML name before writing; if type.IsNameValid is false it throws ArgumentException naming the offending type. XAML names must be valid NCNames, so types with characters like dots-in-wrong-places or other illegal identifiers cannot be written to XML.

Solutions

Fix or sanitize the type's Name so it is a valid XAML (NCName) identifier before writing.
Map the type to a valid XAML representation (e.g. via a value serializer or different type mapping) instead of writing it directly.
Check type.IsNameValid yourself and handle invalid types before calling WriteStartObject.

Example fix

// before
writer.WriteStartObject(new XamlSchemaContext().GetXamlType(typeof(Foo)) with bad name); // ArgumentException
// after
if (type.IsNameValid) { writer.WriteStartObject(type); } else { /* sanitize or map type */ }

Defensive patterns

Strategy: validation

Validate before calling

if (type is null) throw new ArgumentNullException(nameof(type));
if (!type.IsNameValid) throw new ArgumentException($"Type '{type.Name}' has an invalid XAML name", nameof(type));
writer.WriteStartObject(type);

Type guard

bool IsWritableType(XamlType t) => t is not null && t.IsNameValid;

Try / catch

try { writer.WriteStartObject(type); }
catch (ArgumentException ex) { /* sanitize the type name or map to a valid XAML type */ }

Prevention

Check IsNameValid before every WriteStartObject when types come from external sources.
Avoid hand-constructing XamlType names; derive them from real CLR types.
Sanitize generated/obfuscated type names before serialization.

When it happens

Trigger: Calling xamlXmlWriter.WriteStartObject(type) with a XamlType whose Name contains invalid characters (spaces, leading digits, punctuation) or is otherwise not a valid XAML name.

Common situations: Constructing XamlType instances manually with fabricated names; reflection-based type names containing characters invalid in XML names; renamed/mangled types from generated or obfuscated assemblies.

Understand the failure class

Background: "invalid id" errors: invalid identifier format — why libraries reject IDs before lookup, and how to fix them — this error's family across 37 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/1f2908130cf36344. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlXmlWriter.cs:166

            Frame frame = namespaceScopes.Peek();

            if (frame.AllocatingNodeType == XamlNodeType.StartMember)
            {
                type = frame.Member.Type;
            }

            currentState.WriteObject(this, type, true);
        }

        public override void WriteStartObject(XamlType type)
        {
            CheckIsDisposed();

            ArgumentNullException.ThrowIfNull(type);

            if (!type.IsNameValid)
            {
                throw new ArgumentException(SR.Format(SR.TypeHasInvalidXamlName, type.Name), nameof(type));
            }

            currentState.WriteObject(this, type, false);

            if (type.TypeArguments is not null)
            {
                WriteTypeArguments(type);
            }
        }

        public override void WriteEndObject()
        {
            CheckIsDisposed();
            currentState.WriteEndObject(this);
        }

        public override void WriteStartMember(XamlMember property)
        {

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