dotnet/wpf · error · XamlParseException

type-parse error string from XamlTypeName.ParseInternal…

Error message

type-parse error string from XamlTypeName.ParseInternal (e.g. SR.PrefixNotFound/InvalidTypeString)

What it means

MeScanner.ResolveTypeName resolves the bare type name inside curly markup-extension form (e.g. '{StaticResource ...}' where 'StaticResource' must resolve to 'StaticResourceExtension'). It calls XamlTypeName.ParseInternal; if that returns null (PrefixNotFound, InvalidTypeString, etc.), the stored error string is thrown as XamlParseException.

Solutions

Verify the extension name is correct and the extension type exists
Register the extension's assembly/namespace via xmlns declarations or XmlnsDefinition attributes
Fix the type-string syntax inside the curly form
Catch XamlParseException around scanner/parser use and inspect the inner error text

Example fix

// before
<TextBlock Text="{CustomxExtension Foo}"/> // unregistered name
// after
xmlns:ext="clr-namespace:MyApp.Extensions;assembly=MyApp"
<TextBlock Text="{ext:CustomExtension Foo}"/>

Defensive patterns

Strategy: try-catch

Validate before calling

// verify the extension type resolves before curly-form scanning
bool ExtensionResolves(string name, XamlSchemaContext ctx)
{
    var tn = XamlTypeName.Parse(name, ctx.FindNamespaceByPrefix, out _);
    return tn != null && ctx.GetXamlType(tn, false) != null;
}

Try / catch

try { scanner/readResult = ...; }
catch (XamlParseException ex) { /* ex.Message carries the underlying PrefixNotFound/InvalidTypeString text from ParseInternal */ }

Prevention

Register custom extension types with XmlnsDefinition
Follow the 'NameExtension' naming convention for curly-form lookup
Test third-party namespaces resolve before deployment

When it happens

Trigger: Using a markup extension whose name cannot be resolved: unknown/misspelled extension name, missing xmlns prefix mapping, or a syntactically invalid type-name portion of the curly string.

Common situations: Custom markup extensions not registered via XmlnsDefinition, typos in extension names, missing namespace declarations when consuming third-party XAML libraries.

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/308add91f562829e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Parser/MeScanner.cs:293

                        builder.Append(value[idx + 1]);
                    }

                    // pick up again after that
                    start = idx + 2;
                }
            }
            while (start < value.Length);
            string result = builder.ToString();
            return result;
        }

        private void ResolveTypeName(string longName)
        {
            string error;
            XamlTypeName typeName = XamlTypeName.ParseInternal(longName, _context.FindNamespaceByPrefix, out error);
            if (typeName is null)
            {
                throw new XamlParseException(this, error);
            }

            // In curly form, we search for TypeName + 'Extension' before TypeName
            string bareTypeName = typeName.Name;
            typeName.Name += KnownStrings.Extension;
            XamlType xamlType = _context.GetXamlType(typeName, false);
            // This would be cleaner if we moved the Extension fallback logic out of XSC
            if (xamlType is null ||
                // Guard against Extension getting added twice
                (xamlType.UnderlyingType is not null &&
                 KS.Eq(xamlType.UnderlyingType.Name, typeName.Name + KnownStrings.Extension)))
            {
                typeName.Name = bareTypeName;
                xamlType = _context.GetXamlType(typeName, true);
            }

            _tokenXamlType = xamlType;
            _tokenNamespace = typeName.Namespace;

View on GitHub (pinned to 81131a70a4)