unoplatform/uno · error · ArgumentNullException

xmlns

Error message

xmlns

What it means

Thrown by XamlSchemaContext.GetPreferredPrefix when the xmlns argument is null. GetPreferredPrefix returns the preferred XML prefix for a namespace ('x' for Xaml2006, else a registered prefix or 'p'); a null namespace cannot be looked up, so it fails fast.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml/XamlSchemaContext.cs:197

					all_xaml_types = new Dictionary<string, List<XamlType>>();
					foreach (var ass in AssembliesInScope)
						FillAllXamlTypes(ass);
				}

				List<XamlType> l;
				if (all_xaml_types.TryGetValue(xamlNamespace, out l))
					return l;
				else
					return empty_xaml_types;
			}
		}

		public virtual string GetPreferredPrefix (string xmlns)
		{
			lock (gate)
			{
				if (xmlns == null)
					throw new ArgumentNullException("xmlns");
				if (xmlns == XamlLanguage.Xaml2006Namespace)
					return "x";
				if (prefixes == null)
				{
					prefixes = new Dictionary<string, string>();
					foreach (var ass in AssembliesInScope)
						FillPrefixes(ass);
				}
				string ret;
				return prefixes.TryGetValue(xmlns, out ret) ? ret : "p"; // default
			}
		}

		protected internal XamlValueConverter<TConverterBase> GetValueConverter<
			[DynamicallyAccessedMembers(XamlValueConverter<TConverterBase>.TConverterBaseRequirements)] TConverterBase
		> ([DynamicallyAccessedMembers(XamlValueConverter<TConverterBase>.TConverterBaseRequirements)] Type converterType, XamlType targetType)
			where TConverterBase : class
		{

View on GitHub (pinned to 0418340488)

Solutions

1. Pass a non-null namespace string to GetPreferredPrefix.
2. Guard nullable namespaces at the call site before invoking the method.
3. Default to XamlLanguage.Xaml2006Namespace when the intended namespace is the XAML language namespace.

Example fix

// before
var prefix = ctx.GetPreferredPrefix(ns); // ns may be null
// after
var prefix = ns != null ? ctx.GetPreferredPrefix(ns) : "p";

if (xmlns == null)
    throw new InvalidOperationException("xmlns must be non-null before calling GetPreferredPrefix.");
var prefix = ctx.GetPreferredPrefix(xmlns);

When it happens

Trigger: Calling `schemaContext.GetPreferredPrefix(null)` — usually a caller that passed a namespace obtained from an unresolved declaration.

Common situations: Serialization/round-tripping code forwarding a nullable namespace; namespace lookups that returned null and were not guarded.

Related errors

• xamlNamespace
• xamlTypeName
• schemaContext
• The {0} string passed in the colorString argument is not a r
• The {0} string passed in the colorString argument is not a r

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/643c9cbf83a5c84a. Report an issue: GitHub.