[Sergio Pedri (Sergio0694)]

Summary

Add an opt-in (off by default) feature switch that lets projected runtime class RCWs without a native-exposed proxy be marshalled to native code as opaque CCWs, instead of throwing NotSupportedException. It is controlled by the new CsWinRTEnableOpaqueRcwMarshalling MSBuild property, which maps to the new CSWINRT_ENABLE_OPAQUE_RCW_MARSHALLING runtime switch.

Motivation

On some Windows versions, native XAML asks the .NET reference tracker host to wrap a projected RCW (for instance a Frame or a Grid set as Window.Content) through ComWrappers.GetOrCreateComInterfaceForObject. In CsWinRT 3.0 this reaches WindowsRuntimeMarshallingInfo.GetVtableInfo(), which throws for projected classes that don't have a native-exposed proxy, and XAML then fails fast.

With the switch enabled, those requests get an opaque CCW that exposes only the standard interfaces (IInspectable, IStringable, IWeakReferenceSource, IMarshal, IAgileObject, and so on). The RCW itself forwards any other QueryInterface request to its native object. Native-exposed types keep their own vtables, and the new behavior only applies to projected runtime classes.

The feature is pay-for-play: both fallbacks sit on paths that would otherwise throw, and are removed by ILLink and ILC when the switch is disabled, so there is no overhead by default.

Changes

• src/WinRT.Runtime2/Properties/WindowsRuntimeFeatureSwitches.cs: new [FeatureSwitchDefinition] property EnableOpaqueRcwMarshalling, backed by CSWINRT_ENABLE_OPAQUE_RCW_MARSHALLING (default false).
• nuget/Microsoft.Windows.CsWinRT.targets: new CsWinRTEnableOpaqueRcwMarshalling property (default false) and a trimmable RuntimeHostConfigurationOption.
• src/WinRT.Runtime2/InteropServices/TypeMapInfo/WindowsRuntimeMarshallingInfo.cs: when the switch is enabled, GetVtableInfo() reuses the shared opaque object vtables for projected classes, and GetRuntimeClassName() falls back to the full type name. New private IsProjectedClass helper.
• src/WinRT.Runtime2/InteropServices/Attributes/WindowsRuntimeNativeExposedTypeAttribute.cs: remarks now point to the new property as an alternative for opaque-only exposure.
• src/Tests/FunctionalTests/OpaqueRcwMarshalling/: new functional test for the enabled case (managed identity, standard and forwarded QueryInterface, GetIids, runtime class name, unwrapping through regular marshalling, and a native-exposed JsonArray keeping its own vtables).
• src/Tests/FunctionalTests/NativeExposedType/Program.cs: new check that the default (switch disabled) still fails with NotSupportedException.
• src/cswinrt.slnx and build/AzurePipelineTemplates/CsWinRT-FunctionalTest-Steps.yml: wire up the new functional test.
• docs/aot-trimming.md and docs/interop.md: document the property and the opaque CCW semantics and limitations.

Validation

• OpaqueRcwMarshalling passes (exit code 100) on x64 Native AOT and x86 trimmed JIT, and fails with the expected NotSupportedException when the switch is forced off.
• NativeExposedType passes (exit code 100), including the new disabled-default check.
• WinRT.Runtime builds clean in Release (warnings as errors), both the implementation and reference assembly variants.

Limitations

• The end-to-end packaged WinUI/XAML scenario was not validated here.
• Covariant cases (for example DependencyObjectCollection as an items source) still need [WindowsRuntimeNativeExposedType].
• Only ComWrappers-driven paths and projected runtime classes are affected.

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