System.Runtime.InteropServices.ExtendedLayoutAttribute Compiler Support

The .NET runtime team is introducing a new attribute, System.Runtime.InteropServices.ExtendedLayoutAttribute, which will allow the runtime team to provide additional layout options for types, primarily for interop scenarios. To provide the user experience requested by the runtime team, the C# and VB compilers will have the following support for this attribute:

• If a type has the ExtendedLayoutAttribute applied, the compiler will emit the TypeAttributes.ExtendedLayout value in the type's TypeAttributes flags.
• If a type has the ExtendedLayoutAttribute applied, the compiler will not allow the StructLayoutAttribute to be applied to the type.
• (C# compiler only) If a type has the ExtendedLayoutAttribute applied, the compiler will not allow the InlineArrayAttribute to be applied to the type.
• Within the Roslyn compiler, the ITypeSymbol for a type with the ExtendedLayoutAttribute applied will have the following behavior:
  • The Layout property will return a TypeLayout instance with the LayoutKind set to Extended (1), Size set to 0 and Pack set to 0.
• A type that is embedded in an assembly (using the NoPia technology) will have the ExtendedLayoutAttribute preserved on the embedded type.

The Roslyn compiler will not have knowledge of the specific options available on the ExtendedLayoutAttribute, as these options will be defined by the runtime team and may expand over time. The compiler will not attempt to detect invalid field types for specific layout options. The runtime team may add analyzers in the future to handle these scenarios.

The compiler will only recognize the presence of the attribute and its implications for the type symbol's LayoutKind and the emitted attribute.