dotnet/wpf · error · ArgumentNullException

type

Error message

type

What it means

The public XamlTypeInvoker(XamlType type) constructor requires a non-null XamlType; passing null throws ArgumentNullException with parameter name 'type'. An invoker cannot operate without its owning XamlType.

Solutions

1. Null-check the XamlType before constructing; handle the unresolved-type case explicitly.
2. Use the type's existing invoker: XamlType.Invoker already returns a correctly constructed XamlTypeInvoker.
3. Use XamlTypeInvoker.UnknownInvoker for unknown-type scenarios instead of constructing an invoker.

Example fix

// before
var invoker = new XamlTypeInvoker(schemaContext.GetXamlType(clrType)); // throws if null
// after
var xamlType = schemaContext.GetXamlType(clrType);
if (xamlType is null) { /* type not mapped */ return; }
var invoker = xamlType.Invoker;

if (xamlType is null) return; // type not resolvable in schema context

static bool TryGetInvoker(XamlType? t, out XamlTypeInvoker? invoker) { invoker = t?.Invoker; return invoker is not null; }

try { var invoker = new XamlTypeInvoker(xamlType); }
catch (ArgumentNullException ex) when (ex.ParamName == "type") { /* type lookup failed */ }

When it happens

Trigger: Calling `new XamlTypeInvoker(null)`, typically when a preceding lookup like schemaContext.GetXamlType(clrType) or XamlType returned null and the result flows into the constructor.

Common situations: Custom type mapping/extension code constructing invokers for types resolved at runtime; assemblies removed or renamed so the type lookup silently returns null.

Related errors

• member
• ArgumentNullException
• ArgumentNullException(nameof(assemblyNames))
• ArgumentNullException(nameof(contentType))
• ArgumentNullException(nameof(loaderType))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/698bd5f314695a4a. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Schema/XamlTypeInvoker.cs:31

    public class XamlTypeInvoker
    {
        private static XamlTypeInvoker s_Unknown;

        private Dictionary<XamlType, MethodInfo> _addMethods;
        internal MethodInfo EnumeratorMethod { get; set; }
        private XamlType _xamlType;

        private Action<object> _constructorDelegate;

        private ThreeValuedBool _isPublic;

        protected XamlTypeInvoker()
        {
        }

        public XamlTypeInvoker(XamlType type)
        {
            _xamlType = type ?? throw new ArgumentNullException(nameof(type));
        }

        public static XamlTypeInvoker UnknownInvoker
        {
            get
            {
                if (s_Unknown is null)
                {
                    s_Unknown = new XamlTypeInvoker();
                }

                return s_Unknown;
            }
        }

        public EventHandler<XamlSetMarkupExtensionEventArgs> SetMarkupExtensionHandler
        {
            get { return _xamlType?.SetMarkupExtensionHandler; }

View on GitHub (pinned to 81131a70a4)