dotnet/wpf · error · ArgumentNullException

ArgumentNullException(nameof(oldNamespace))

Error message

ArgumentNullException(nameof(oldNamespace))

What it means

The XmlnsCompatibleWithAttribute constructor throws ArgumentNullException when oldNamespace is null. This attribute declares that an old XML namespace is subsumed by a new one; the old namespace identifier is the core datum, so null is rejected at construction.

Solutions

1. Pass the full old namespace URI string, e.g. "http://schemas.example.com/2006/xaml".
2. Validate the config/source string for null before constructing the attribute.
3. Use string constants from a shared namespace definitions file to avoid typos/nulls.
4. Catch ArgumentNullException if attribute inputs are untrusted metadata.

Example fix

// before
var attr = new XmlnsCompatibleWithAttribute(GetOldNs(), newNs); // GetOldNs() null
// after
var oldNs = GetOldNs() ?? "http://schemas.example.com/2006/xaml";
var attr = new XmlnsCompatibleWithAttribute(oldNs, newNs);

if (string.IsNullOrEmpty(oldNamespace)) throw new ArgumentException("oldNamespace is required.");
var attr = new XmlnsCompatibleWithAttribute(oldNamespace, newNamespace);

try { attr = new XmlnsCompatibleWithAttribute(oldNs, newNs); }
catch (ArgumentNullException ex) { log.Error($"Bad xmlns compatibility metadata: {ex.ParamName}"); attr = null; }

When it happens

Trigger: Executing new XmlnsCompatibleWithAttribute(null, "http://new-namespace") from assembly attribute metadata or reflection-driven code.

Common situations: Generating compatibility attributes from config where the old-namespace entry is missing; merging assembly info files and dropping the old namespace value.

Related errors

• ArgumentNullException(nameof(newNamespace))
• ArgumentNullException(nameof(contentType))
• ArgumentNullException(nameof(loaderType))
• ArgumentNullException(nameof(clrNamespace))
• ArgumentNullException(nameof(prefix))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/9d69945352433df0. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Windows/Markup/XmlnsCompatibleWithAttribute.cs:31

    ///
    ///    "http://schemas.example.com/2003/mynamespace"
    ///
    /// is changed to
    ///
    ///    "http://schemas.example.com/2005/mynamespace"
    /// </summary>
    [AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
    [TypeForwardedFrom("WindowsBase, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35")]
    public sealed class XmlnsCompatibleWithAttribute : Attribute
    {
        /// <summary>
        /// Constructor
        /// </summary>
        /// <param name="oldNamespace">old Xml namespce</param>
        /// <param name="newNamespace">new xml namespace</param>
        public XmlnsCompatibleWithAttribute(string oldNamespace, string newNamespace)
        {
            OldNamespace = oldNamespace ?? throw new ArgumentNullException(nameof(oldNamespace));
            NewNamespace = newNamespace ?? throw new ArgumentNullException(nameof(newNamespace));
        }

        /// <summary>
        /// Old Xml Namespace
        /// </summary>
        public string OldNamespace { get; }

        /// <summary>
        /// New Xml Namespace
        /// </summary>
        public string NewNamespace { get; }
   }
}

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