dotnet/wpf · error · ArgumentNullException

ArgumentNullException(nameof(arrayType))

Error message

ArgumentNullException(nameof(arrayType))

What it means

ArrayExtension's Type-taking constructor throws ArgumentNullException when the arrayType argument is null. The extension cannot represent an array without a type, so the library rejects null eagerly at construction time rather than failing later during ProvideValue. This is a standard argument-validation guard in System.Xaml markup extensions.

Solutions

1. Pass a valid System.Type instance to the ArrayExtension(Type) constructor.
2. If the Type comes from a dynamic lookup, check it for null before constructing the extension and handle the failed lookup separately.
3. Use the parameterless ArrayExtension() constructor and set the Type property afterward if you intentionally defer type assignment.

Example fix

// before
var ext = new ArrayExtension(ResolveType("my:Widget[]")); // ResolveType may return null
// after
var t = ResolveType("my:Widget[]") ?? throw new InvalidOperationException("Array element type could not be resolved");
var ext = new ArrayExtension(t);

if (arrayType is null) throw new ArgumentException("arrayType must be a resolved Type", nameof(arrayType));
var ext = new ArrayExtension(arrayType);

bool IsValidArrayType(Type t) => t is not null;

try { var ext = new ArrayExtension(arrayType); }
catch (ArgumentNullException ex) when (ex.ParamName == "arrayType") { /* handle unresolved type */ }

When it happens

Trigger: Calling new ArrayExtension((Type)null) — directly or via reflection/activator from a XAML parser or serializer passing a null Type.

Common situations: Reflective construction of markup extensions where the Type was resolved dynamically (e.g. by assembly-qualified name lookup) and the lookup returned null; code generation tools emitting constructor calls with unresolved types.

Related errors

• ArgumentNullException(nameof(member))
• ArgumentNullException(nameof(contentType))
• ArgumentNullException(nameof(loaderType))
• ArgumentNullException(nameof(newNamespace))
• ArgumentNullException(nameof(oldNamespace))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/ca1a6964945782fb. Report an issue: GitHub.

Appendix: source

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

    [ContentProperty("Items")]
    [MarkupExtensionReturnType(typeof(Array))]
    public class ArrayExtension : MarkupExtension
    {
        private readonly ArrayList _arrayList = new ArrayList();

        /// <summary>
        /// Constructor that takes no parameters. This creates an empty array.
        /// </summary>
        public ArrayExtension()
        {
        }

        /// <summary>
        /// Constructor that takes one parameter. This initializes the type of the array.
        /// </summary>
        public ArrayExtension(Type arrayType)
        {
            Type = arrayType ?? throw new ArgumentNullException(nameof(arrayType));
        }

        /// <summary>
        /// Constructor for writing
        /// </summary>
        /// <param name="elements">The array to write</param>
        public ArrayExtension(Array elements)
        {
            ArgumentNullException.ThrowIfNull(elements);

            _arrayList.AddRange(elements);
            Type = elements.GetType().GetElementType();
        }

        /// <summary>
        /// Called to Add an object as a new array item. This will append the
        /// object to the end of the array.
        /// </summary>

View on GitHub (pinned to 81131a70a4)