dotnet/wpf · error · ArgumentNullException

ArgumentNullException(nameof(clrNamespace))

Error message

ArgumentNullException(nameof(clrNamespace))

What it means

XamlTypeMapper.AddMappingProcessingInstruction requires a non-null clrNamespace: the CLR namespace portion of the mapping (e.g. "System.Windows.Controls"). It throws ArgumentNullException when clrNamespace is null, because a mapping with no target CLR namespace is meaningless for type resolution.

Solutions

1. Supply the CLR namespace string, e.g. mapper.AddMappingProcessingInstruction(ns, "MyCompany.MyApp.Controls", "MyAssembly").
2. Parse the clr-namespace from the mapping URI and reject/handle mappings where it is missing before calling.
3. Catch ArgumentNullException at the mapping-registration boundary and log which mapping entry was malformed.

Example fix

// before
mapper.AddMappingProcessingInstruction(xmlns, clrNsFromUri, assembly); // clrNsFromUri == null
// after
if (string.IsNullOrEmpty(clrNsFromUri))
    throw new XamlParseException($"Mapping '{xmlns}' has no clr-namespace");
mapper.AddMappingProcessingInstruction(xmlns, clrNsFromUri, assembly);

if (string.IsNullOrEmpty(clrNamespace))
    throw new ConfigurationErrorsException($"Mapping '{xmlNamespace}' is missing its clr-namespace");

static bool TryGetClrNamespace(string mappingUri, out string clrNamespace)
{
    clrNamespace = null;
    const string prefix = "clr-namespace:";
    if (mappingUri == null || !mappingUri.StartsWith(prefix)) return false;
    int end = mappingUri.IndexOf(';');
    clrNamespace = end < 0 ? mappingUri.Substring(prefix.Length) : mappingUri.Substring(prefix.Length, end - prefix.Length);
    return clrNamespace.Length > 0;
}

try
{
    mapper.AddMappingProcessingInstruction(xmlNamespace, clrNamespace, assemblyName);
}
catch (ArgumentNullException ex) when (ex.ParamName == "clrNamespace")
{
    throw new ConfigurationErrorsException($"Mapping '{xmlNamespace}' has no clr-namespace", ex);
}

When it happens

Trigger: Calling mapper.AddMappingProcessingInstruction(xmlNamespace, null, assemblyName) — the clr-namespace portion is absent, e.g. parsing "clr-namespace:;assembly=X" or a mapping record built from a PI missing its clr-namespace part.

Common situations: Hand-edited XAML mapping processing instructions with empty clr-namespace, config-driven mapping tables missing the namespace column, or code generators emitting mappings with uninitialized namespace strings.

Related errors

• ArgumentNullException(nameof(assemblyName))
• ArgumentNullException(nameof(assemblyNames))
• ArgumentNullException(nameof(assemblyPath))
• ArgumentNullException(nameof(localName))
• ArgumentNullException(nameof(propertyName))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/d93640827187281d. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlTypeMapper.cs:143

        /// </param>
        /// <param name="clrNamespace">
        /// The "System.Windows.ComponentModel" argument in the mapping PI example.
        /// </param>
        /// <param name="assemblyName">
        /// The "PresentationFramework" argument in the mapping PI example.
        /// </param>
        public void AddMappingProcessingInstruction(
            string  xmlNamespace,
            string  clrNamespace,
            string  assemblyName )
        {
            if( null == xmlNamespace )
            {
                throw new ArgumentNullException(nameof(xmlNamespace));
            }
            if( null == clrNamespace )
            {
                throw new ArgumentNullException(nameof(clrNamespace));
            }
            if( null == assemblyName )
            {
                throw new ArgumentNullException(nameof(assemblyName));
            }

            // Parameter validation : Check for String.Empty as well?

            // Add mapping to the table keyed by xmlNamespace
            ClrNamespaceAssemblyPair pair = new ClrNamespaceAssemblyPair(clrNamespace, assemblyName);
            PITable[xmlNamespace] = pair;

            // Add mapping to the table keyed by assembly and clrnamespace
            string upperAssemblyName = assemblyName.ToUpper(
                                              TypeConverterHelper.InvariantEnglishUS);
            String fullName = $"{clrNamespace}#{upperAssemblyName}";

            _piReverseTable[fullName] = xmlNamespace;

View on GitHub (pinned to 81131a70a4)