unoplatform/uno · error · ArgumentNullException

namespaceDeclaration

Error message

namespaceDeclaration

What it means

XamlWriterInternalBase.WriteNamespace throws ArgumentNullException('namespaceDeclaration') when a null NamespaceDeclaration is written. The declaration (prefix+namespace pair) is added to the namespaces list and forwarded to OnWriteNamespace, so a null entry would corrupt the namespace manager.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml/XamlWriterInternalBase.cs:154

		}

		public void WriteGetObject ()
		{
			manager.GetObject ();

			var xm = CurrentMember;

			var state = new ObjectState () {Type = xm.Type, IsGetObject = true};

			object_states.Push (state);

			OnWriteGetObject ();
		}

		public void WriteNamespace (NamespaceDeclaration namespaceDeclaration)
		{
			if (namespaceDeclaration == null)
				throw new ArgumentNullException ("namespaceDeclaration");

			manager.Namespace ();

			namespaces.Add (namespaceDeclaration);
			OnWriteNamespace (namespaceDeclaration);
		}

		public void WriteStartObject (XamlType xamlType)
		{
			if (xamlType == null)
				throw new ArgumentNullException ("xamlType");

			manager.StartObject ();

			var cstate = new ObjectState () {Type = xamlType};
			object_states.Push (cstate);

			OnWriteStartObject ();

View on GitHub (pinned to 0418340488)

Solutions

1. Filter out or reject null NamespaceDeclaration before calling WriteNamespace.
2. If you control the reader, ensure it never yields null namespace nodes.
3. Use XamlServices.Transform which forwards reader nodes through the writer's own WriteNode dispatcher.

Example fix

// before
writer.WriteNamespace(decl);

// after
if (decl != null) writer.WriteNamespace(decl);

if (namespaceDeclaration != null) writer.WriteNamespace(namespaceDeclaration);

bool IsValid(NamespaceDeclaration? d) => d != null && !string.IsNullOrEmpty(d.Namespace);

When it happens

Trigger: Calling WriteNamespace(null) directly, or passing a NamespaceDeclaration obtained from a reader that yielded null (e.g. a buggy custom XamlReader).

Common situations: Custom XamlReader implementations that emit null NamespaceDeclarations; pipeline code forwarding declarations without filtering nulls; tests asserting validation.

Related errors

• reader
• xamlType
• property
• typeName
• type

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/d04100dfccce7633. Report an issue: GitHub.