dotnet/wpf · error · ArgumentNullException

throw new ArgumentNullException(nameof(value));

Error message

throw new ArgumentNullException(nameof(value));

What it means

The XmlNamespace property setter on XamlTypeMapper's mapping entry type rejects null values with ArgumentNullException. The setter is write-once: it only assigns the backing field when it is still null, but null values are rejected outright to keep the namespace mapping well-formed.

Solutions

1. Assign a non-null namespace string (e.g. 'http://schemas.microsoft.com/winfx/2006/xaml/presentation')
2. Guard with string.IsNullOrEmpty before assignment
3. If clearing the mapping is intended, replace the whole mapping object instead of nulling a property

Example fix

// before
entry.XmlNamespace = config.XmlNamespace; // may be null
// after
if (config.XmlNamespace != null) entry.XmlNamespace = config.XmlNamespace;

if (ns != null) entry.XmlNamespace = ns;

static bool IsValidNamespace(string s) => !string.IsNullOrEmpty(s);

try { entry.XmlNamespace = value; }
catch (ArgumentNullException ex) { Log.Warn("XmlNamespace cannot be null: " + ex.Message); }

When it happens

Trigger: Assigning null to the XmlNamespace property of a XamlTypeMapper mapping entry (e.g. mapper.XmlNamespace = someString) where someString is null.

Common situations: Deserialization or config parsing that produced a null namespace string; test code probing the setter's behavior.

Related errors

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/e1b974d904fe84c8. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlTypeMapper.cs:4274

            _assemblyPath = assemblyPath;
        }

        #endregion Constructors


        #region Properties

        /// <summary>
        /// Xml namespace specified in the constructor
        /// </summary>
        public string XmlNamespace
        {
            get { return _xmlNamespace; }
            set
            {
                if (value == null)
                {
                    throw new ArgumentNullException(nameof(value));
                }
                if (_xmlNamespace == null)
                {
                    _xmlNamespace = value;
                }
            }
        }

        /// <summary>
        /// AssemblyName specified in the constructor
        /// </summary>
        public string AssemblyName
        {
            get { return _assemblyName; }
            set
            {
                if (value == null)
                {

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