What causes "unknownTypeNamespace" in unoplatform/uno?

Pass a non-null namespace string (use string.Empty if the type has no explicit namespace, matching the protected XamlType overload).

unoplatform/uno · error · ArgumentNullException

unknownTypeNamespace

Error message

unknownTypeNamespace

What it means

The XamlType(string unknownTypeNamespace, string unknownTypeName, ...) constructor throws ArgumentNullException('unknownTypeNamespace') when the namespace argument is null. This overload represents a type that cannot be resolved at parse time (an 'unknown' XamlType), but it still requires a non-null namespace string to populate PreferredXamlNamespace.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml/XamlType.cs:87

			} else if ((xt = XamlLanguage.AllTypes.FirstOrDefault (t => t.UnderlyingType == type)) != null) {
				Name = xt.Name;
				PreferredXamlNamespace = XamlLanguage.Xaml2006Namespace;
			} else {
				Name = GetXamlName (type);
				PreferredXamlNamespace = schemaContext.GetXamlNamespace (type.Namespace) ?? String.Format (CultureInfo.InvariantCulture, "clr-namespace:{0};assembly={1}", type.Namespace, type.Assembly.GetName ().Name);
			}
			if (type.IsGenericType) {
				TypeArguments = new List<XamlType> ();
				foreach (var gta in type.GetGenericArguments ())
					TypeArguments.Add (schemaContext.GetXamlType (gta));
			}
		}

		public XamlType (string unknownTypeNamespace, string unknownTypeName, IList<XamlType> typeArguments, XamlSchemaContext schemaContext)
			: this (schemaContext, null)
		{
			if (unknownTypeNamespace == null)
				throw new ArgumentNullException ("unknownTypeNamespace");
			if (unknownTypeName == null)
				throw new ArgumentNullException ("unknownTypeName");
			if (schemaContext == null)
				throw new ArgumentNullException ("schemaContext");

			type = typeof (object);
			Name = unknownTypeName;
			PreferredXamlNamespace = unknownTypeNamespace;
			TypeArguments = typeArguments != null && typeArguments.Count == 0 ? null : typeArguments;
//			explicit_ns = unknownTypeNamespace;
		}

		protected XamlType (string typeName, IList<XamlType> typeArguments, XamlSchemaContext schemaContext)
			: this (String.Empty, typeName, typeArguments, schemaContext)
		{
		}

		XamlType (XamlSchemaContext schemaContext, XamlTypeInvoker invoker)

View on GitHub (pinned to 0418340488)

Solutions

Pass a non-null namespace string (use string.Empty if the type has no explicit namespace, matching the protected XamlType overload).
Validate the namespace is non-null at the source that produces it before constructing the unknown XamlType.
If the namespace is genuinely unavailable, surface a parse error to the user rather than constructing an unknown type with null.

Example fix

// before
var xt = new XamlType(null, typeName, typeArgs, sctx);

// after
var xt = new XamlType(namespace ?? string.Empty, typeName, typeArgs, sctx);

Defensive patterns

Strategy: validation

Validate before calling

var ns = unknownTypeNamespace ?? string.Empty;
var xt = new XamlType(ns, unknownTypeName, typeArguments, schemaContext);

Prevention

Coalesce null namespaces to string.Empty when constructing unknown XamlTypes, matching the protected overload's behavior.
Validate parse output so an absent namespace becomes an explicit empty string, not null.

When it happens

Trigger: Building an unknown XamlType for a forward-declared type whose XML namespace was not supplied, or passing a namespace variable that resolved to null during XAML parsing.

Common situations: XAML parsers constructing placeholder types for unresolved xmlns; tooling that synthesizes XamlType instances from incomplete metadata; tests omitting the namespace argument.

Related errors

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/0d6fda3e42bdef1e. Report an issue: GitHub.