dotnet/wpf · error · ArgumentNullException

throw new ArgumentNullException( "elem" );

Error message

throw new ArgumentNullException( "elem" );

What it means

XmlAttributeProperties.LookupNamespace(DependencyObject elem, string prefix) throws ArgumentNullException when elem is null. The method walks the dependency-object parent chain to resolve an XML namespace prefix, so it needs a valid starting element.

Solutions

1. Verify the DependencyObject is created and non-null before calling LookupNamespace
2. Fail fast with your own error message if elem is null
3. If the element may legitimately be absent, skip the prefix lookup

Example fix

// before
var ns = XmlAttributeProperties.LookupNamespace(element, prefix); // element may be null
// after
var ns = element != null ? XmlAttributeProperties.LookupNamespace(element, prefix) : null;

if (element == null) return null;
var ns = XmlAttributeProperties.LookupNamespace(element, prefix);

static bool CanLookup(DependencyObject e) => e != null;

try { ns = XmlAttributeProperties.LookupNamespace(element, prefix); }
catch (ArgumentNullException ex) { Log.Warn("LookupNamespace needs a non-null element and prefix"); ns = null; }

When it happens

Trigger: Calling XmlAttributeProperties.LookupNamespace(null, "x") — e.g. passing a control that has not been created yet, or the result of a FindName lookup that returned null.

Common situations: XAML designer/parser extensions resolving prefixes during load before the element tree exists; data-binding code that resolves xmlns prefixes for dynamically generated attributes where the element reference is null.

Related errors

• throw new ArgumentNullException( "prefix" );
• throw new ArgumentNullException( "uri" );
• ArgumentNullException(nameof(assemblyName))
• ArgumentNullException(nameof(assemblyNames))
• ArgumentNullException(nameof(assemblyPath))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/e12c465871a8f7d6. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XmlAttributeProperties.cs:78

            XmlNamespaceMapsProperty =
                DependencyProperty.RegisterAttached("XmlNamespaceMaps", typeof(Hashtable), typeof(XmlAttributeProperties),
                                            new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.Inherits));
        }

#if Lookups
        /// <summary>
        /// Looks up the namespace corresponding to an XML namespace prefix
        /// </summary>
        /// <param name="elem">The DependencyObject containing the namespace mappings to be 
        ///  used in the lookup</param>
        /// <param name="prefix">The XML namespace prefix to look up</param>
        /// <returns>The namespace corresponding to the given prefix if it exists, 
        /// null otherwise</returns>
        public static string LookupNamespace(DependencyObject elem, string prefix)
        {
            if (elem == null)
            {
                throw new ArgumentNullException( "elem" ); 
            }
            if (prefix == null)
            {
                throw new ArgumentNullException( "prefix" ); 
            }

            // Walk up the parent chain until one of the parents has the passed prefix in 
            // its xmlns dictionary, or until a null parent is reached.
            XmlnsDictionary d = GetXmlnsDictionary(elem);
            while ((null == d || null == d[prefix]) && null != elem)
            {
                elem = LogicalTreeHelper.GetParent(elem);
                if (elem != null)
                {
                    d = GetXmlnsDictionary(elem);
                }
            }
            

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