dotnet/wpf · error · ArgumentNullException

throw new ArgumentNullException(nameof(assemblyName));

Error message

throw new ArgumentNullException(nameof(assemblyName));

What it means

The NamespaceMapEntry constructor throws ArgumentNullException when assemblyName is null. The assembly name is required to locate the CLR types that back the mapped XML namespace, so a null value makes the entry unusable.

Solutions

1. Pass the assembly's simple name string as the second constructor argument
2. Derive the name via Assembly.GetName().Name when only an Assembly object is available
3. Validate the mapping instruction/config entry contains an Assembly value before constructing

Example fix

// before
var map = new NamespaceMapEntry(nsUri, mappingInfo.Assembly, clrNs); // may be null
// after
string asmName = mappingInfo.Assembly ?? typeof(KnownType).Assembly.GetName().Name;
var map = new NamespaceMapEntry(nsUri, asmName, clrNs);

if (string.IsNullOrEmpty(assemblyName))
    assemblyName = typeof(FrameworkElement).Assembly.GetName().Name; // or explicit failure
var entry = new NamespaceMapEntry(xmlNamespace, assemblyName, clrNamespace);

bool HasAssemblyName(string assemblyName) => !string.IsNullOrEmpty(assemblyName);

try
{
    var entry = new NamespaceMapEntry(xmlNamespace, assemblyName, clrNamespace);
}
catch (ArgumentNullException ex) when (ex.ParamName == "assemblyName")
{
    log.LogError(ex, "Mapping for {Ns} lacks an Assembly attribute", xmlNamespace);
}

When it happens

Trigger: Constructing new NamespaceMapEntry(xmlNamespace, null, clrNamespace) — often when the assembly name is derived from a mapping instruction or config entry that was missing.

Common situations: Designer/tooling code registering namespace maps from <?Mapping ...?> data with missing Assembly= attributes; assembly renames leaving old mapping metadata pointing at nothing.

Related errors

• throw new ArgumentNullException(nameof(xmlNamespace));
• ArgumentNullException(nameof(assemblyName))
• ArgumentNullException(nameof(assemblyNames))
• ArgumentNullException(nameof(assemblyPath))
• ArgumentNullException(nameof(clrNamespace))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/0d835500e8f9c5dc. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlTypeMapper.cs:4233

        /// 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>
        /// <param name="assemblyPath">Path to use when loading assembly.  This may be null.</param>
         internal NamespaceMapEntry(
                string xmlNamespace,

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