What steps will reproduce the issue? * Create some valid C# code file (e.g. Example.cs) * Change it's extension from .cs to something else (e.g. Example.notcs) * Add it as a compilable C# file to project (e.g. add explicit ᐸCompile Include="Example.notcs"/ᐳ to .csproj) * Reference a type from this file in other .cs file What is the expected result? The project builds, syntax highlighting/code analysis works as expected in other .cs files. What happens instead? The project does indeed build without issues, syntax highlighting/code analysis works in VS+R#, but is broken in Rider: all symbols defined in non-cs file are not resolved Workaround: Link the file in project as a .cs file (e.g. ᐸCompile Include="Example.notcs" Link="Example.cs"/ᐳ) Attachments: Sample solution with non-cs filed defined in C# project and C++ project. Screenshots: syntax highlighting in VS2019+R# and Rider2021.1.3 Longer story: A person asked in a chat how to fix syntax highlighting in his specific use-case. That use-case: reuse single file…