dotnet/wpf · error · ArgumentException

SR.ParserAttributeArgsLow with…

Error message

SR.ParserAttributeArgsLow with 'XmlnsCompatibleWithAttribute' (missing XmlnsCompatibleWithAttribute constructor arguments)

What it means

XmlnsCache throws ArgumentException when an [XmlnsCompatibleWithAttribute] found in a scanned assembly yields an empty OldNamespace or NewNamespace. The attribute requires both constructor arguments to be non-empty namespace strings, so a malformed usage in a referenced assembly makes the WPF XAML markup cache unable to build a valid compatibility mapping.

Solutions

Open the referenced assembly's source and provide both non-empty arguments to XmlnsCompatibleWithAttribute: [assembly: XmlnsCompatibleWith("old-clr-ns", "new-clr-ns")]
Identify the offending assembly by scanning referenced assemblies for XmlnsCompatibleWithAttribute usage and inspecting its OldNamespace/NewNamespace values
Remove the invalid XmlnsCompatibleWithAttribute entirely if the namespace compatibility mapping is not needed
Rebuild/reinstall the third-party assembly from a correct source version

Example fix

// before (AssemblyInfo.cs of referenced assembly)
[assembly: XmlnsCompatibleWith("")] // missing args

// after
[assembly: XmlnsCompatibleWith("clr-namespace:Old.Ns", "clr-namespace:New.Ns")]

Defensive patterns

Strategy: validation

Validate before calling

var attr = asm.GetCustomAttribute<XmlnsCompatibleWithAttribute>();
if (attr != null && (string.IsNullOrEmpty(attr.OldNamespace) || string.IsNullOrEmpty(attr.NewNamespace)))
    throw new InvalidOperationException($"Assembly {asm.GetName().Name} has an XmlnsCompatibleWithAttribute with empty namespace arguments.");

Prevention

Always supply both constructor arguments to XmlnsCompatibleWithAttribute
Keep namespace strings in named constants to avoid accidental empty literals
Audit AssemblyInfo files after refactors that rename namespaces

When it happens

Trigger: Calling AddReferencedAssemblies or GetMappingArray while scanning an assembly whose [assembly: XmlnsCompatibleWith(...)] attribute was declared with null/empty string arguments (e.g. XmlnsCompatibleWith("", "newNs") or omitted constructor args).

Common situations: Hand-edited AssemblyInfo.cs, misapplied attribute via reflection-emitted assemblies, or a third-party WPF assembly with a corrupt/incorrect XmlnsCompatibleWithAttribute declaration discovered when the XAML designer or markup compiler builds the xmlns compatibility table.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/092ecb92c2dfbfce. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XmlnsCache.cs:387

            {
#if PBTCOMPILER
                CustomAttributeData[] attributes = GetAttributes(asmList[asmIdx],
                                                "System.Windows.Markup.XmlnsCompatibleWithAttribute");
#else
                Attribute[] attributes = GetAttributes(asmList[asmIdx],
                                                typeof(XmlnsCompatibleWithAttribute));
#endif

                for(int attrIdx=0; attrIdx<attributes.Length; attrIdx++)
                {
                    string oldXmlns = null;
                    string newXmlns = null;

                    GetNamespacesFromCompatAttr(attributes[attrIdx], out oldXmlns, out newXmlns);

                    if (String.IsNullOrEmpty(oldXmlns) || String.IsNullOrEmpty(newXmlns))
                    {
                        throw new ArgumentException(SR.Format(SR.ParserAttributeArgsLow, "XmlnsCompatibleWithAttribute"));
                    }

                    if (_compatTable.ContainsKey(oldXmlns) &&
                        _compatTable[oldXmlns] != newXmlns)
                    {
                        throw new InvalidOperationException(SR.Format(SR.ParserCompatDuplicate, oldXmlns,
                                                                   _compatTable[oldXmlns]));
                    }
                    _compatTable[oldXmlns] = newXmlns;
                    _compatTableReverse[newXmlns] = oldXmlns;
                }
            }
        }

        // Table where Xml namespace is the key, and value is a list of assembly / clrns pairs
        private HybridDictionary _cacheTable = null;

        // Table where old namespaces are the key, and new namespaces are the value

View on GitHub (pinned to 81131a70a4)