Address and improve light-up scenarios for Windows APIs

[angelazhangmsft]

Background

With the new TFMs in .NET 5 and removal of winmd support, we need to clarify the story for light-up scenarios and how we advise both library and app developers to write adaptive code. The scenarios involve C# libraries and apps in .NET 5+ that want to call Windows SDK projection APIs (WinRT APIs).

We are proposing adding an analyzer for ApiInformation checks to protect light-up code.

We can use this deliverable-sized issue to track light-up support work.

• Change #pragma warning solution to suppress SupportedOSPlatform attribute #718
• Automated test for SupportedOSPlatform #678

Open Questions

• .NET 5 library authors want to light-up with WinRT APIs, but this imposes a TFM requirement on customers (referencing applications)
• Improve integration of ApiInformation checks with projected SupportedOSPlatform attributes from C#/WinRT, and improve guidance on using ApiInformation checks
• Project SupportedOSPlatform attributes downlevel, e.g. Windows 8
• Clarify combinations of the TFM, SupportedOSPlatformVersion, TPV/TPMV
• Integration with Windows App SDK APIs