dotnet/wpf · error · ArgumentException

SR.MalformedPropertyName

Error message

SR.MalformedPropertyName

What it means

MeScanner.ResolvePropertyName throws ArgumentException with SR.MalformedPropertyName when XamlPropertyName.Parse cannot decompose a markup-extension property name (the text between a markup extension's member references, e.g. `{x:Static Member=...}` long names) into a valid namespace/type/member form. System.Xaml uses this scanner when parsing values like `{StaticResource Key}`; a name that does not match the grammar `ns:Type.Member` or `Type.Member` is rejected. It is thrown during scanner Read, which the XAML pull parser drives, so it surfaces as an exception during XamlReader.Load/XamlXmlReader parse.

Solutions

1. Fix the markup-extension property name in the XAML so it matches ns:Type.Member or Type.Member grammar.
2. Ensure no leading/trailing '=' with an empty property name inside the braces.
3. If the value legitimately contains '=' or braces as literal text, escape the opening brace with {} (e.g. {}{literal}) or use x:Static/attribute syntax instead.
4. Re-run XamlReader.Load on a minimal snippet to isolate the offending element and line.

Example fix

<!-- before -->
<TextBlock Text="{StaticResource =HeaderKey}" />

<!-- after -->
<TextBlock Text="{StaticResource HeaderKey}" />

if (xaml.Contains("=") && System.Text.RegularExpressions.Regex.IsMatch(xaml, @"\{[^}]*=\s*[^\w:.]")) throw new FormatException("Suspicious markup-extension property assignment");

try { var obj = XamlReader.Parse(xaml); } catch (XamlParseException ex) { log(ex.Message, ex.LineNumber, ex.LinePosition); }

When it happens

Trigger: Calling XamlReader.Load / XamlXmlReader on XAML containing a markup extension whose property longName (the string before '=' inside the extension braces) does not parse — e.g. `{StaticResource =Value}` (empty name), `{StaticResource a..b=Value}`, or an unbalanced dotted name inside a markup extension property assignment.

Common situations: Hand-edited or tool-generated XAML with a typo in a markup-extension property name; string-interpolated XAML where an empty or malformed property name is emitted; WPF/.NET Core migration where previously silently-tolerated syntax is now strictly scanned.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

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

Related errors

• brokenRule (SR.UnexpectedToken-based parser rule error)
• Element ::= . EmptyElement | ( StartElement ElementBody ).
• EmptyPropertyElement ::= EMPTYPROPERTYELEMENT.
• IAmbientProvider
• IXamlSchemaContextProvider

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/a28a4a29c899aaf8. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Parser/MeScanner.cs:319

            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;
        }

        private void ResolvePropertyName(string longName)
        {
            XamlPropertyName propName = XamlPropertyName.Parse(longName);
            if (propName is null)
            {
                throw new ArgumentException(SR.MalformedPropertyName);
            }

            XamlMember prop = null;
            XamlType declaringType;
            XamlType tagType = _context.CurrentType;
            string tagNamespace = _context.CurrentTypeNamespace;

            if (propName.IsDotted)
            {
                prop = _context.GetDottedProperty(tagType, tagNamespace, propName, tagIsRoot: false);
            }

            // Regular property p
            else
            {
                string ns = _context.GetAttributeNamespace(propName, Namespace);
                declaringType = _context.CurrentType;
                prop = _context.GetNoDotAttributeProperty(declaringType, propName, Namespace, ns, tagIsRoot: false);

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