> From a user perspective, "dotnet run" does add a new scripting dialect.

By dialect I mean that in CSX, the C# language has subtle differences in behavior compared to CS files (e.g., top-level methods are compiled differently). In 'dotnet run app.cs', the C# is exactly the same, hence it can be converted to a project (and we have a command for that - 'dotnet project convert').

> It's just a preprocessor - not a change to the C# syntax.

Should #:load work in normal C# files as well then? If yes, we would have two ways to include files - the normal way (include them in the project) or via #:load. If not, how would 'dotnet project convert' work? (It could paste the text but that's not very DRY if it's imported at multiple places.)

That's in short why #:load doesn't play very nicely with existing C# (it's a different paradigm of including other files). So the current proposal is to just make files in the same folder visible (just like they would be for a .csproj), because that's consistent with current C#, 'dotnet project convert' works, etc. But this proposal is not final yet, there are other options including what you suggest - see https://github.com/dotnet/sdk/issues/49193.