dotnet/wpf · error · ArgumentNullException

ArgumentNullException(nameof(localName))

Error message

ArgumentNullException(nameof(localName))

What it means

XamlTypeMapper.GetType(xmlNamespace, localName) also requires a non-null localName (the type name without namespace prefix, e.g. "Button"). It throws ArgumentNullException for a null localName after validating xmlNamespace. Without a local name there is no type to look up in the namespace map.

Solutions

1. Validate/split the qualified name so localName is always non-null before calling GetType (e.g. name.Substring after the ':' separator, or the whole string when no prefix).
2. Skip elements whose local name is null/empty instead of resolving them.
3. Catch ArgumentNullException and surface a message identifying the malformed XAML element name.

Example fix

// before
Type t = mapper.GetType(xmlns, SplitLocalName(qname)); // may return null
// after
string localName = SplitLocalName(qname);
if (string.IsNullOrEmpty(localName))
    throw new XamlParseException($"Invalid element name '{qname}'");
Type t = mapper.GetType(xmlns, localName);

if (string.IsNullOrEmpty(localName))
    throw new XamlParseException($"Missing local name for element in namespace '{xmlNamespace}'");

static bool HasLocalName(string qualifiedName, out string localName)
{
    int i = qualifiedName?.IndexOf(':') ?? -1;
    localName = i >= 0 ? qualifiedName.Substring(i + 1) : qualifiedName;
    return !string.IsNullOrEmpty(localName);
}

try
{
    type = mapper.GetType(xmlNamespace, localName);
}
catch (ArgumentNullException ex) when (ex.ParamName == "localName")
{
    throw new XamlParseException("XAML element has an empty local name", ex);
}

When it happens

Trigger: Calling mapper.GetType(xmlNamespace, null) — the local type name is null, commonly when XAML node streams supply an empty/unnamed element or when GetTypeFromBaseString/GetTypeArgsType forwards an uninitialized name.

Common situations: Processing XAML where a start element name failed to parse, custom BAML-style node processing that splits qualified names incorrectly (prefix parsing yields null local part), or reflection-driven code generation passing uninitialized strings.

Related errors

• ArgumentNullException(nameof(xmlNamespace))
• throw new ArgumentNullException( nameof(element));
• throw new ArgumentNullException( nameof(typeName));
• ArgumentNullException(nameof(assemblyName))
• ArgumentNullException(nameof(assemblyNames))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/48856476ae12b3c4. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlTypeMapper.cs:109

        ///     you would call XamlTypeMapper.GetType("AvalonBase","Button");
        ///     <para/>
        ///     Note the XmlNamespace "AvalonBase" is the actual namespace value, not
        ///     the base: prefix.
        /// </remarks>
        /// <param name="xmlNamespace">NamespaceURI of tag</param>
        /// <param name="localName">localName of the Tag</param>
        /// <returns>Type for the object. If no type was found NULL is returned</returns>
        public Type GetType(
            string xmlNamespace,
            string localName)
        {
            if(null == xmlNamespace)
            {
                throw new ArgumentNullException( nameof(xmlNamespace));
            }
            if(null == localName)
            {
                throw new ArgumentNullException( nameof(localName));
            }

            TypeAndSerializer typeAndSerializer =
                GetTypeOnly(xmlNamespace,localName);

            return typeAndSerializer?.ObjectType;
        }

#if !PBTCOMPILER
        /// <summary>
        ///  Programmatic counterpart to the <?Mapping ... ?> XAML PI.  For example, <para/>
        ///    <?Mapping XmlNamespace="swc" ClrNamespace="System.Windows.ComponentModel" Assembly="PresentationFramework" ?>
        /// </summary>
        /// <param name="xmlNamespace">
        /// The "swc" argument in the mapping PI example.
        /// </param>
        /// <param name="clrNamespace">
        /// The "System.Windows.ComponentModel" argument in the mapping PI example.

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