dotnet/wpf · error · XamlSchemaException

SR.BadXmlnsDefinition

Error message

SR.BadXmlnsDefinition

What it means

LoadNsDefHelper throws this XamlSchemaException when an XmlnsDefinitionAttribute read from an assembly carries invalid data — e.g. a null/empty XmlNamespace or ClrNamespace — so the xmlns definition cannot be registered while building the XAML namespace table.

Solutions

1. Ensure the assembly contains a valid XmlnsDefinitionAttribute with non-empty XmlNamespace and ClrNamespace
2. Fix the [assembly: XmlnsDefinition(...)] declaration in the referenced assembly
3. Verify the assembly being scanned is a legitimate WPF/Silverlight assembly

When it happens

Trigger: Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/MS/Impl/XmlNsInfo.cs:198 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c1d969719dd8a33f. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/MS/Impl/XmlNsInfo.cs:198

                attributes = Attribute.GetCustomAttributes(assembly, typeof(XmlnsDefinitionAttribute));
                foreach (Attribute attr in attributes)
                {
                    XmlnsDefinitionAttribute xmlnsDefAttr = (XmlnsDefinitionAttribute)attr;

                    string xmlns = xmlnsDefAttr.XmlNamespace;
                    string clrns = xmlnsDefAttr.ClrNamespace;
                    LoadNsDefHelper(result, xmlns, clrns, assembly);
                }
            }

            return result;
        }

        private void LoadNsDefHelper(IList<XmlNsDefinition> result, string xmlns, string clrns, Assembly assembly)
        {
            if (string.IsNullOrEmpty(xmlns) || clrns is null)
            {
                throw new XamlSchemaException(SR.Format(SR.BadXmlnsDefinition, assembly.FullName));
            }

            result.Add(new XmlNsDefinition { ClrNamespace = clrns, XmlNamespace = xmlns });
        }

        private ConcurrentDictionary<string, IList<string>> LoadClrToXmlNs()
        {
            ConcurrentDictionary<string, IList<string>> result =
                XamlSchemaContext.CreateDictionary<string, IList<string>>();

            Assembly assembly = Assembly;
            if (assembly is null)
            {
                return result;
            }

            foreach (XmlNsDefinition nsDef in NsDefs)
            {

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