ArgumentNullException(nameof(parserContext))

Error message

ArgumentNullException(nameof(parserContext))

What it means

The static ParserContext.ToXmlParserContext(ParserContext) converter requires a non-null parserContext and throws ArgumentNullException(nameof(parserContext)) when given null. It translates a XAML ParserContext into a System.Xml XmlParserContext (namespace manager, XmlLang, XmlSpace), which is impossible without a source context.

Solutions

Pass a valid ParserContext instance (use new ParserContext() if defaults suffice)
Null-check the context before calling ToXmlParserContext
Fix the producer of the ParserContext so it never returns null
Wrap in ArgumentNullException.ThrowIfNull(parserContext) guard at the call site

Example fix

// before
ParserContext pc = CreateParserContext(); // may be null
XmlParserContext xmlCtx = ParserContext.ToXmlParserContext(pc);
// after
ParserContext pc = CreateParserContext() ?? new ParserContext();
XmlParserContext xmlCtx = ParserContext.ToXmlParserContext(pc);

Defensive patterns

Strategy: validation

Validate before calling

ArgumentNullException.ThrowIfNull(parserContext);
var xmlCtx = ParserContext.ToXmlParserContext(parserContext);

Type guard

XmlParserContext? TryToXmlParserContext(ParserContext? ctx) => ctx is null ? null : ParserContext.ToXmlParserContext(ctx);

Try / catch

try
{
    var xmlCtx = ParserContext.ToXmlParserContext(parserContext);
}
catch (ArgumentNullException ex) when (ex.ParamName == "parserContext")
{
    xmlCtx = ParserContext.ToXmlParserContext(new ParserContext());
}

Prevention

Ensure ParserContext-producing code never returns null; default to new ParserContext()
Null-check before converting in custom XAML reader pipelines
Keep context construction and conversion close together to avoid null drift
Enable nullable reference types so nullability is enforced statically

When it happens

Trigger: Calling ParserContext.ToXmlParserContext(null), or passing a ParserContext variable that was never initialized or was set to null after a failed load, commonly in custom XAML reader/pipeline code.

Common situations: Custom XamlReader pipelines converting contexts between System.Xaml and System.Xml, error paths where a ParserContext factory returned null, and unit tests exercising edge cases.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b42a1f637690b8b9. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/ParserContext.cs:420

        /// </summary>
        /// <param name="parserContext">ParserContext to Convert</param>
        /// <returns>XmlParserContext</returns>
        public static implicit operator XmlParserContext(ParserContext parserContext)
        {
            return ParserContext.ToXmlParserContext(parserContext);
        }


        /// <summary>
        ///    Operator for Converting a ParserContext to an XmlParserContext
        /// </summary>
        /// <param name="parserContext">ParserContext to Convert</param>
        /// <returns>XmlParserContext</returns>
        public static XmlParserContext ToXmlParserContext(ParserContext parserContext)
        {
            if (parserContext == null)
            {
                throw new ArgumentNullException( nameof(parserContext));
            }

            XmlNamespaceManager xmlnsMgr = new XmlNamespaceManager(new NameTable());
            XmlSpace xmlSpace = System.Xml.XmlSpace.None;

            if (parserContext.XmlSpace != null && parserContext.XmlSpace.Length != 0)
            {
                TypeConverter typeConverter = TypeDescriptor.GetConverter(typeof(System.Xml.XmlSpace));
                if (null != typeConverter)
                {
                    try
                    {
                        xmlSpace = (System.Xml.XmlSpace)typeConverter.ConvertFromString(null, TypeConverterHelper.InvariantEnglishUS, parserContext.XmlSpace);
                    }
                    catch (System.FormatException)  // If it's not a valid space value, ignore it
                    {
                        xmlSpace = System.Xml.XmlSpace.None;
                    }

View on GitHub (pinned to 81131a70a4)