.Net Core on Windows has support for loading assemblies that reference COM interfaces and the win32 API, along with other things that aren’t supported elsewhere like C++/CLI.

That’s why loading System.Windows.Forms still works, it’s not part of .Net 5+, but it can still load the assemblies on Windows (they still use GDI, etc under the hood).

No, Microsoft's .NET only supports WinForms on Windows. They do have an official cross platform GUI toolkit in MAUI, but it strangely does not support Linux.

For this, having wine in charge makes sense - though, as with many things wine you could be waiting a while.

I've had pretty good success running WinForms apps in wine, and just straight up installing latest version of dotnet in wine.

Works better in proton from my experience though, but I think that is due to making unity games run better

With commercial applications that want to just take their existing code and have it run on Linux with only a couple lines changed, Avalonia XPF will do that

I had to check; you're right, no official support for Linux. Windows, macOS, iOS, Android. You can kludge Linux in with hacks.

If true this would be huge. I got burned on the whole silverlight, Universal Windows Platform, WPF etc. All these new and improved solutions had all sorts of issues, no designer, no or weaker accessibility stories, bloated, slow etc etc. C# + Winforms would be appealing. Some of the performance with larger datasets in the new solutions (tables etc) was just surprising. I really feel like Microsoft got so distracted chasing phones, tables, touch etc they forget just basic line of business application development which they could and should have owned.