unoplatform/uno · error · FormatException

Invalid type name list: '{0}'

Error message

Invalid type name list: '{0}'

What it means

Thrown by XamlTypeName.ParseList when TryParseList returns false. ParseList is the throwing wrapper; the FormatException signals the comma-separated type-name list is malformed or contains a name that fails to parse (bad prefix, unknown namespace, mismatched brackets).

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml.Schema/XamlTypeName.cs:105

				local = typeName.Substring (idx + 1);
				if (!XamlLanguage.IsValidXamlName (prefix))
					return false;
			}
			if (!XamlLanguage.IsValidXamlName (local))
				return false;
			string ns = namespaceResolver.GetNamespace (prefix);
			if (ns == null)
				return false;

			result = new XamlTypeName (ns, local, args);
			return true;
		}

		public static IList<XamlTypeName> ParseList (string typeNameList, IXamlNamespaceResolver namespaceResolver)
		{
			IList<XamlTypeName> list;
			if (!TryParseList (typeNameList, namespaceResolver, out list))
				throw new FormatException (String.Format (CultureInfo.InvariantCulture, "Invalid type name list: '{0}'", typeNameList));
			return list;
		}

		static readonly char [] comma_or_parens = new char [] {',', '(', ')'};

		public static bool TryParseList (string typeNameList, IXamlNamespaceResolver namespaceResolver, out IList<XamlTypeName> result)
		{
			if (typeNameList == null)
				throw new ArgumentNullException ("typeNameList");
			if (namespaceResolver == null)
				throw new ArgumentNullException ("namespaceResolver");

			result = null;
			int idx = 0;
			int parens = 0;
			XamlTypeName tn;

			List<string> l = new List<string> ();

View on GitHub (pinned to 0418340488)

Solutions

  1. Use TryParseList instead of ParseList and handle the false return with detail about which element failed.
  2. Ensure every prefix used across all list elements is mapped in the resolver.
  3. Validate list syntax: comma-separated `prefix:Name(args)` entries with balanced parentheses.
  4. Parse elements individually to isolate which one is malformed.

Example fix

// before
var list = XamlTypeName.ParseList(input, resolver);

// after
if (!XamlTypeName.TryParseList(input, resolver, out var list)) {
    throw new FormatException($"Cannot parse XAML type name list '{input}'; verify each element's prefix and namespace.");
}
Defensive patterns

Strategy: try-catch

Try / catch

try {
    var list = XamlTypeName.ParseList(input, resolver);
} catch (FormatException ex) {
    // parse elements individually to isolate which one is malformed
}

Prevention

When it happens

Trigger: Calling XamlTypeName.ParseList("ns:Foo, x:Int32", resolver) where one element fails to parse; unbalanced parentheses in a generic argument list spanning the comma; a list element with an unmapped prefix.

Common situations: Parsing multi-type generic argument lists built by hand; lists assembled from user input with inconsistent prefixes; resolver missing one of the prefixes used across the list.

Related errors

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/a687a4ff03956f1e. Report an issue: GitHub.