unoplatform/uno · error · ArgumentNullException

typeArguments array contains one or more null XamlTypeName

Error message

typeArguments array contains one or more null XamlTypeName

What it means

Thrown by the XamlTypeName(string,string,IEnumerable<XamlTypeName>) constructor when the supplied typeArguments collection contains at least one null element. XamlTypeName models a fully-qualified XAML type with optional generic type arguments; a null entry in that list would produce malformed type strings and corrupt the XAML node stream, so the constructor rejects it eagerly.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml.Schema/XamlTypeName.cs:222

				var l = new List<XamlTypeName> ();
				l.AddRange (xamlType.TypeArguments.Select (x => new XamlTypeName (x)));
				TypeArguments = l;
			}
		}
		
		public XamlTypeName (string xamlNamespace, string name)
			: this (xamlNamespace, name, null)
		{
		}

		public XamlTypeName (string xamlNamespace, string name, IEnumerable<XamlTypeName> typeArguments)
			: this ()
		{
			Namespace = xamlNamespace;
			Name = name;
			if (typeArguments != null) {
				if (typeArguments.Any (t => t == null))
					throw new ArgumentNullException ("typeArguments", "typeArguments array contains one or more null XamlTypeName");
				var l = new List<XamlTypeName> ();
				l.AddRange (typeArguments);
				TypeArguments = l;
			}
		}

		public string Name { get; set; }
		public string Namespace { get; set; }
		public IList<XamlTypeName> TypeArguments { get; private set; }

		public override string ToString ()
		{
			return ToString (null);
		}

		public string ToString (INamespacePrefixLookup prefixLookup)
		{
			if (Namespace == null)

View on GitHub (pinned to 0418340488)

Solutions

1. Filter nulls out of the typeArguments collection before passing it: typeArguments.Where(t => t != null).
2. Fix the upstream code that produces null XamlTypeName entries (e.g. a mapping dictionary lookup returning null) rather than just filtering downstream.
3. Pass null as typeArguments when you have no generic parameters, instead of an empty or partially-null list.

Example fix

// before
var args = types.Select(t => MapOrNull(t));
var name = new XamlTypeName(ns, typeName, args);

// after
var args = types.Select(t => MapOrNull(t)).Where(t => t != null);
var name = new XamlTypeName(ns, typeName, args);

var filtered = typeArguments?.Where(t => t != null).ToList();
var name = new XamlTypeName(xamlNamespace, nameStr, filtered);

static IEnumerable<XamlTypeName> SafeArgs(IEnumerable<XamlTypeName> args)
    => args?.Where(t => t != null) ?? Enumerable.Empty<XamlTypeName>();

try { var tn = new XamlTypeName(ns, n, args); }
catch (ArgumentNullException ex) when (ex.ParamName == "typeArguments")
{ /* log and filter nulls, retry */ }

When it happens

Trigger: Constructing an XamlTypeName with a non-null typeArguments enumerable that itself contains a null entry, e.g. new XamlTypeName("ns","List", new XamlTypeName[]{ null }). Also fires when a LINQ Select producing type arguments yields null for any element (e.g. mapping an unresolved type).

Common situations: Programmatic XAML schema construction where generic type arguments are resolved from metadata at runtime and some resolution returns null. Copying a type-argument list from a partially-populated dictionary. Reflection-driven XAML generation where a reflected generic parameter could not be mapped to an XAML namespace.

Related errors

• Either property getter or setter must be non-null.
• member
• instance
• Namespace must be set before calling ToString method.
• Name must be set before calling ToString method.

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/463a5d7229297446. Report an issue: GitHub.