dotnet/wpf · error · ArgumentNullException

ArgumentNullException(nameof(typeName))

Error message

ArgumentNullException(nameof(typeName))

What it means

The TypeExtension(string) constructor stores the type name and immediately rejects null with ArgumentNullException, because a TypeExtension without a type name can never resolve to a Type. The name may still be validated later during ProvideValue, but null is rejected right here.

Solutions

1. Pass a non-null type name string, e.g. new TypeExtension("local:MyClass").
2. Guard at the call site: if (typeName != null) new TypeExtension(typeName).
3. If the type is known at compile time, use new TypeExtension(typeof(MyClass)) instead.
4. Fix the source of the null (config key, attribute parse) before construction.

Example fix

// before
var ext = new TypeExtension(attributes["type"]); // may be null
// after
var ext = new TypeExtension(attributes["type"] ?? throw new InvalidOperationException("x:Type name is required"));

if (string.IsNullOrEmpty(typeName)) throw new InvalidOperationException("TypeExtension requires a non-empty type name");

static bool IsValidTypeName(string? n) => !string.IsNullOrWhiteSpace(n);

try { var ext = new TypeExtension(typeName); } catch (ArgumentNullException) { /* handle missing type name from config/attributes */ }

When it happens

Trigger: Calling new TypeExtension(null), or passing a variable that is null (e.g. an unresolved attribute value from XAML or config).

Common situations: Reading x:Type values from configuration that is missing; string variables from reflection that were never assigned; deserialization gaps leaving the type name null.

Related errors

• ArgumentNullException(nameof(type))
• ArgumentNullException(nameof(value))
• XmlReader is null
• ArgumentNullException(nameof(schemaContext))
• ArgumentNullException(nameof(typeName))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/cd4031193e27c861. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Windows/Markup/TypeExtension.cs:34

    [MarkupExtensionReturnType(typeof(Type))]
    public class TypeExtension : MarkupExtension
    {
        private string _typeName;
        private Type _type;

        /// <summary>
        /// Default Constructor
        /// </summary>
        public TypeExtension()
        {
        }

        /// <summary>
        /// Constructor that takes a type name
        /// </summary>
        public TypeExtension(string typeName)
        {
            _typeName = typeName ?? throw new ArgumentNullException(nameof(typeName));
        }

        /// <summary>
        /// Constructor that takes a type
        /// </summary>
        public TypeExtension(Type type)
        {
            _type = type ?? throw new ArgumentNullException(nameof(type));
        }

        /// <summary>
        /// Return an object that should be set on the targetObject's targetProperty
        /// for this markup extension. TypeExtension resolves a string into a Type
        /// and returns it.
        /// </summary>
        /// <param name="serviceProvider">Object that can provide services for the markup extension.</param>
        /// <returns>The object to set on this property.</returns>
        public override object ProvideValue(IServiceProvider serviceProvider)

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