unoplatform/uno · error · Exception

Type is unsupported for indexing.

Error message

Type is unsupported for indexing.

What it means

Thrown by the Roslyn-based x:Bind expression visitor when the type of the indexed property/field does not expose a matching indexer property. The parser looks for an IPropertySymbol with IsIndexer=true whose single parameter is either System.String (for string-literal indices) or System.Int32 (for numeric-literal indices). If none is found on the type or its base types, this exception fires.

Source

Thrown at src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/Utils/XBindExpressionParser.cs:344

				var lastType = _lastAccessed.Symbol switch
				{
					IPropertySymbol property => property.Type,
					IFieldSymbol field => field.Type,
					_ => throw new Exception("Only properties and fields can be indexed.")
				};

				var expectedIndexerParameterType = index is LiteralExpressionSyntax indexLiteral && indexLiteral.IsKind(SyntaxKind.StringLiteralExpression)
					? SpecialType.System_String
					: SpecialType.System_Int32;
				var indexer = lastType.GetMemberIncludingBaseTypes(expectedIndexerParameterType, (m, arg) => m is IPropertySymbol { IsIndexer: true } p && p.Parameters[0].Type.SpecialType == arg);
				if (indexer is not null)
				{
					_lastAccessed = (indexer, IsTopLevelContext: false);
				}
				else
				{
					throw new Exception("Type is unsupported for indexing.");
				}

				_builder.Append('[');

				VisitLiteralExpression(index);

				_builder.Append(']');
			}

			public override void VisitArgumentList(ArgumentListSyntax node)
			{
				if (Failed)
				{
					return;
				}

				MainExpression.Arguments = new();

View on GitHub (pinned to 0418340488)

Solutions

1. Ensure the target type exposes a public indexer property (this[string] or this[int]).
2. If the type only has method-based access, add an indexer property or expose a computed property.
3. Match the index type to the available indexer — use a string literal for string indexers, an integer for int indexers.

Example fix

// before: MyCollection has no indexer
<Text Text="{x:Bind MyCollection[0]}" />
// after: add indexer to MyCollection
// public string this[int index] => _items[index];
<Text Text="{x:Bind MyCollection[0]}" />

// Verify the indexed type exposes a public indexer matching the index type:
// For int index: public T this[int index]
// For string index: public T this[string key]
// Check via reflection in a test:
// typeof(MyType).GetProperties().Any(p => p.GetIndexParameters().Length == 1
//   && p.GetIndexParameters()[0].ParameterType == typeof(int));

When it happens

Trigger: Indexing a property/field whose type does not have a string or int indexer — e.g. indexing a struct or a custom collection that does not expose an indexer property.

Common situations: Binding to a custom collection type that only has methods (Add/Get) but no 'this[]' indexer; indexing a value type or a non-indexable type; the index type does not match (e.g. using a string index on a type that only has an int indexer, or vice versa).

Related errors

• x:Bind indexing expects a single literal argument
• Only properties and fields can be indexed.
• Missing parenthesis?
• Expected attached property path to be member access.
• Expected ')' in attached property syntax.

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/9c90bb753f6e24a9. Report an issue: GitHub.