unoplatform/uno · error · ArgumentNullException

xamlType

Error message

xamlType

What it means

Thrown by the XamlTypeName(XamlType xamlType) constructor when `xamlType` is null. The constructor copies the XamlType's PreferredXamlNamespace, Name, and TypeArguments into the new XamlTypeName, so a null source has nothing to copy; it is rejected immediately.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml.Schema/XamlTypeName.cs:200

				ret += ta.ToString (prefixLookup);
			}
			return ret;
		}

		// instance members

		public XamlTypeName ()
		{
			TypeArguments = empty_type_args;
		}

		static readonly XamlTypeName [] empty_type_args = Array.Empty<XamlTypeName>();

		public XamlTypeName (XamlType xamlType)
			: this ()
		{
			if (xamlType == null)
				throw new ArgumentNullException ("xamlType");
			Namespace = xamlType.PreferredXamlNamespace;
			Name = xamlType.Name;
			if (xamlType.TypeArguments != null && xamlType.TypeArguments.Count > 0) {
				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;

View on GitHub (pinned to 0418340488)

Solutions

1. Null-check the XamlType before constructing the XamlTypeName.
2. Fix the upstream type resolution so GetXamlType returns a non-null XamlType (verify assembly references and xmlns mappings).
3. Construct XamlTypeName from explicit (namespace, name) strings instead when you only have partial type info.

Example fix

// before
var name = new XamlTypeName(resolvedType); // resolvedType may be null

// after
if (resolvedType == null) {
    throw new InvalidOperationException("Could not resolve XamlType for type-name construction.");
}
var name = new XamlTypeName(resolvedType);

// before constructing
if (xamlType == null) throw new InvalidOperationException("XamlType resolution returned null; cannot build XamlTypeName.");

When it happens

Trigger: Calling new XamlTypeName(null); or new XamlTypeName(resolvedType) where resolvedType is null from a failed GetXamlType lookup.

Common situations: Type-resolution failures where GetXamlType returns null and the null propagates into XamlTypeName construction; code that assumes a type was always resolved.

Related errors

• type
• typeName
• namespaceResolver
• typeNameList
• prefixLookup

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/ad2e3d902027c29c. Report an issue: GitHub.