dotnet/wpf · error · ArgumentNullException

throw new ArgumentNullException(nameof(xmlNamespace));

Error message

throw new ArgumentNullException(nameof(xmlNamespace));

What it means

The NamespaceMapEntry constructor validates its three string parameters and throws ArgumentNullException when xmlNamespace is null. A namespace map entry associates an XML namespace with an assembly and CLR namespace for XAML type resolution; the xmlNamespace is the primary key, so null is rejected.

Solutions

Pass the XML namespace URI string as the first constructor argument
Read the namespace from the parsed NamespaceDeclaration rather than assuming it
Fail fast at the caller: check string.IsNullOrEmpty(xmlNamespace) before constructing the map entry

Example fix

// before
var map = new NamespaceMapEntry(nsUri, asm, clrNs); // nsUri may be null
// after
if (nsUri == null)
    throw new InvalidOperationException("xmlns declaration missing for prefix");
var map = new NamespaceMapEntry(nsUri, asm, clrNs);

Defensive patterns

Strategy: validation

Validate before calling

if (string.IsNullOrEmpty(xmlNamespace))
    throw new ArgumentException("xmlNamespace must be a non-empty URI");
var entry = new NamespaceMapEntry(xmlNamespace, assemblyName, clrNamespace);

Type guard

bool CanBuildNamespaceMap(string xmlNamespace, string asm, string clrNs) =>
    !string.IsNullOrEmpty(xmlNamespace) && !string.IsNullOrEmpty(asm) && !string.IsNullOrEmpty(clrNs);

Try / catch

try
{
    var entry = new NamespaceMapEntry(xmlNamespace, assemblyName, clrNamespace);
}
catch (ArgumentNullException ex)
{
    log.LogError(ex, "Namespace map entry incomplete: {Param} is null", ex.ParamName);
}

Prevention

Verify xmlns declarations exist before creating map entries
Build map entries from parsed NamespaceDeclaration data, not raw strings
Validate all three arguments together in a helper before constructing

When it happens

Trigger: Constructing new NamespaceMapEntry(null, assemblyName, clrNamespace) — typically when the namespace URI came from an unparsed xmlns attribute or a lookup that returned null.

Common situations: Building custom XAML schema contexts in designers; programmatically registering namespace maps where xmlns declarations were missing from source XAML.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/efe7ef9663b1f880. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlTypeMapper.cs:4230

        #region Constructors

        ///<summary>
        /// NamespaceMapEntry default constructor
        ///</summary>
        public NamespaceMapEntry()
        {
        }

        /// <summary>
        /// Constructor
        /// </summary>
        /// <param name="xmlNamespace">The XML NamespaceURi</param>
        /// <param name="assemblyName">Assembly to use when resolving a Tag</param>
        /// <param name="clrNamespace">Namespace within the assembly</param>
        public NamespaceMapEntry(string xmlNamespace,string assemblyName,string clrNamespace)
        {
            if (xmlNamespace == null)
                throw new ArgumentNullException(nameof(xmlNamespace));

            if (assemblyName == null)
                throw new ArgumentNullException(nameof(assemblyName));

            if (clrNamespace == null)
                throw new ArgumentNullException(nameof(clrNamespace));

            _xmlNamespace = xmlNamespace;
            _assemblyName = assemblyName;
            _clrNamespace = clrNamespace;
        }

        /// <summary>
        /// Constructor
        /// </summary>
        /// <param name="xmlNamespace">The XML NamespaceURi</param>
        /// <param name="assemblyName">Assembly to use when resolving a Tag</param>
        /// <param name="clrNamespace">Namespace within the assembly</param>

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