dotnet/wpf · error · ArgumentNullException

throw new ArgumentNullException(nameof(prefix));

Error message

throw new ArgumentNullException(nameof(prefix));

What it means

XmlnsDictionary.AddNamespace throws ArgumentNullException when the prefix argument is null. The dictionary maps XML namespace prefixes to namespace URIs for WPF XAML markup processing, and a null prefix has no meaning, so the library rejects it immediately after checking the dictionary is not sealed.

Solutions

Ensure the prefix argument is a non-null string before calling Add/AddNamespace.
If the prefix may be absent, skip the Add call or substitute a sensible default (e.g. empty string for the default namespace).
Validate inputs when loading prefix mappings from external config before passing them in.

Example fix

// before
xmlnsDictionary.Add(prefix, namespaceUri);
// after
if (prefix == null) throw new InvalidOperationException("Prefix mapping missing from config");
xmlnsDictionary.Add(prefix, namespaceUri);

Defensive patterns

Strategy: validation

Validate before calling

if (prefix == null) throw new ArgumentException("prefix must be non-null", nameof(prefix));
xmlnsDictionary.Add(prefix, namespaceUri);

Type guard

bool IsValidPrefix(string p) => p != null;

Try / catch

try { xmlnsDictionary.Add(prefix, ns); }
catch (ArgumentNullException ex) { /* log missing prefix; skip mapping */ }

Prevention

Null-check prefix/URI pairs loaded from config before registering them
Use string.Empty explicitly for the default namespace instead of null
Centralize namespace registration in one helper that validates inputs

When it happens

Trigger: Calling XmlnsDictionary.Add(prefix, xmlNamespace) (which forwards to AddNamespace) with a null prefix string, e.g. dictionary.Add(null, "http://...") or passing an uninitialized variable as the prefix.

Common situations: Building custom XAML namespaces programmatically, XAML schema context setup code, or deserializing prefix mappings from config where a prefix entry is missing.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/8ce41a16298d7755. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XmlnsDictionary.cs:612

                throw new InvalidOperationException(SR.ParserDictionarySealed);
            }
        }

        /// <summary>
        /// Helper to Add Namespace - Checks for prefix from rear.
        ///  If exists : override it locally, if not Adds an entry.
        /// </summary>
        /// <param name="prefix">prefix to add</param>
        /// <param name="xmlNamespace">namespace uri string to add</param>
        private void AddNamespace(string prefix, string xmlNamespace)
        {
            CheckSealed();
            
            if (xmlNamespace == null)
                throw new ArgumentNullException(nameof(xmlNamespace));

            if (prefix == null)
                throw new ArgumentNullException(nameof(prefix));

            int lastScopeCount = _nsDeclarations[_lastDecl].ScopeCount;

            if (_lastDecl > 0)
            {
                // Check the local scope for the given prefix
                for (int thisDecl = _lastDecl-1; 
                     thisDecl >= 0 && _nsDeclarations[thisDecl].ScopeCount == lastScopeCount; 
                     thisDecl--)
                {
                    if (String.Equals(_nsDeclarations[thisDecl].Prefix, prefix))
                    {
                        // Redefine an existing namespace
                        _nsDeclarations[thisDecl].Uri = xmlNamespace;
                        return; 
                    }
                }

View on GitHub (pinned to 81131a70a4)