dotnet/wpf · error · ArgumentException

SR.ParserAttributeArgsHigh with 'XmlnsDefinitionAttribute'…

Error message

SR.ParserAttributeArgsHigh with 'XmlnsDefinitionAttribute' (too many constructor arguments for XmlnsDefinitionAttribute)

What it means

In XmlnsCache, GetNamespacesFromDefinitionAttr reads the constructor arguments of an [XmlnsDefinitionAttribute]. If more than two positional arguments are present, the code throws ArgumentException with ParserAttributeArgsHigh — the attribute is only defined with (xmlNamespace, clrNamespace) and WPF's parser cannot interpret extra arguments.

Solutions

  1. Remove extra constructor arguments from the XmlnsDefinitionAttribute usage, keeping only xmlns and clrNamespace
  2. Ensure the attribute used is System.Windows.Markup.XmlnsDefinitionAttribute, not a custom type with the same name
  3. Check referenced assemblies for third-party extended attributes and split the metadata into separate attributes

Example fix

// before
[assembly: XmlnsDefinition("http://mycompany.com/schema", "MyCompany.Controls", AssemblyName = "MyCompany.Controls")] // 3rd ctor arg
// after
[assembly: XmlnsDefinition("http://mycompany.com/schema", "MyCompany.Controls")]
Defensive patterns

Strategy: validation

Validate before calling

foreach (var attr in asm.GetCustomAttributesData())
{
    if (attr.AttributeType.Name == "XmlnsDefinitionAttribute" && attr.ConstructorArguments.Count > 2)
        throw new InvalidOperationException($"{asm}: XmlnsDefinitionAttribute has too many ctor arguments");
}

Try / catch

try { xmlnsCache.LoadClrnsToAssemblyNameMappingCache(asm); }
catch (ArgumentException ex) when (ex.Message.Contains("XmlnsDefinitionAttribute"))
{ log.Error("Referenced assembly has malformed XmlnsDefinitionAttribute", ex); }

Prevention

When it happens

Trigger: An assembly referenced by the app declares [XmlnsDefinition] with a constructor having 3+ arguments (e.g. a custom or extended XmlnsDefinitionAttribute overload), and XmlnsCache scans the assembly via LoadClrnsToAssemblyNameMappingCache or GetClrnsToAssemblyNameMappingList.

Common situations: Using a redefined/extended XmlnsDefinitionAttribute with an extra parameter (e.g. assembly name or visibility hint); assembly referencing a different WPF version's attribute; hand-rolled attributes shadowing System.Windows.Markup.XmlnsDefinitionAttribute.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/5d81fe843f3cfb22. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XmlnsCache.cs:245

        }

        private void GetNamespacesFromDefinitionAttr(CustomAttributeData data, out string xmlns, out string clrns)
        {
            // typedConstructorArguments (the Attribute constructor arguments)
            // [MyAttribute("test", Name=Hello)]
            // "test" is the Constructor Argument
            xmlns = null;
            clrns = null;
            IList<CustomAttributeTypedArgument> constructorArguments = data.ConstructorArguments;
            for (int i = 0; i<constructorArguments.Count; i++)
            {
                CustomAttributeTypedArgument tca = constructorArguments[i];
                if (i == 0)
                    xmlns = tca.Value as String;
                else if (i == 1)
                    clrns = tca.Value as String;
                else
                    throw new ArgumentException(SR.Format(SR.ParserAttributeArgsHigh, "XmlnsDefinitionAttribute"));
            }
        }

        private void GetNamespacesFromCompatAttr(CustomAttributeData data, out string oldXmlns, out string newXmlns)
        {
            // typedConstructorArguments (the Attribute constructor arguments)
            // [MyAttribute("test", Name=Hello)]
            // "test" is the Constructor Argument
            oldXmlns = null;
            newXmlns = null;
            IList<CustomAttributeTypedArgument> constructorArguments = data.ConstructorArguments;
            for (int i=0; i<constructorArguments.Count; i++)
            {
                CustomAttributeTypedArgument tca = constructorArguments[i];
                if (i == 0)
                    oldXmlns = tca.Value as String;
                else if (i == 1)
                    newXmlns = tca.Value as String;

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