dotnet/maui · error · NotSupportedException

Unsupported path part type: {nextPart.GetType()}

Error message

Unsupported path part type: {nextPart.GetType()}

What it means

An internal source-generator invariant in Maui's BindingSourceGen. AccessExpressionBuilder.ExtendExpression emits a C# access expression for each part of a compiled binding path and exhaustively pattern-matches the known IPathPart records (Cast, ConditionalAccess, IndexAccess with int/Enum/string index, MemberAccess). The default arm throws NotSupportedException only if a path part of an unrecognized type reaches it, which indicates a generator gap or version skew rather than user error.

Source

Thrown at src/Controls/src/BindingSourceGen/AccessExpressionBuilder.cs:23

public static class AccessExpressionBuilder
{
	/// <summary>
	/// Extends an expression by accessing the next part of a binding path.
	/// This is used for getter paths (reading values), not for setter assignments.
	/// </summary>
	public static string ExtendExpression(string previousExpression, IPathPart nextPart)
		=> nextPart switch
		{
			Cast { TargetType: var targetType } => $"({previousExpression} as {CastTargetName(targetType)})",
			ConditionalAccess conditionalAccess => ExtendExpression(previousExpression: $"{previousExpression}?", conditionalAccess.Part),
			IndexAccess { Index: int numericIndex } => $"{previousExpression}[{numericIndex}]",
			IndexAccess { Index: EnumIndex enumIndex } => $"{previousExpression}[{enumIndex.FullyQualifiedEnumValue}]",
			IndexAccess { Index: string stringIndex } => $"{previousExpression}[\"{stringIndex}\"]",
			MemberAccess { Kind: AccessorKind.Field, IsGetterInaccessible: true } memberAccess => $"{CreateUnsafeFieldAccessorMethodName(memberAccess.MemberName)}({previousExpression})",
			MemberAccess { Kind: AccessorKind.Property, IsGetterInaccessible: true } memberAccess => $"{CreateUnsafePropertyAccessorGetMethodName(memberAccess.MemberName)}({previousExpression})",
			MemberAccess memberAccess => $"{previousExpression}.{memberAccess.MemberName}",
			_ => throw new NotSupportedException($"Unsupported path part type: {nextPart.GetType()}"),
		};

	private static string CastTargetName(TypeDescription targetType)
		=> targetType.IsValueType ? $"{targetType.GlobalName}?" : targetType.GlobalName;
}

View on GitHub (pinned to f377ff1c5e)

Solutions

1. Simplify the binding path (remove casts/indices/method-like constructs) to isolate which part triggers it.
2. Align all Maui workload/package versions across the solution so source-generator and analysis assemblies match.
3. File a Maui issue with the exact Binding expression and types so the generator's switch can be extended.

When it happens

Trigger: A compiled binding path resolves to an IPathPart subtype not handled by the switch (e.g. a new index kind, a method-call node, or a path construct introduced by a newer Maui version before the generator was updated).

Common situations: Mixing Maui package versions across projects so the path-analysis and expression-building generators disagree; binding to an exotic expression after a Maui upgrade; a genuine generator bug for an unusual path.

Related errors

• Member '{member.MemberName}' with inaccessible accessor must
• IItemsViewSource is empty
• mainRenderer
• Unhandled expression type: '{0}'
• Unhandled binding type '{0}'

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/85e56c43b63e5abe. Report an issue: GitHub.