> obviously system.windows.forms could easily be implemented elsewhere
It can, but not easily. As OP has said, it is a wrapper around Win32, and not an opaque one - it literally has stuff like e.g. the Message struct with members like HWnd and LParam.
Mono did try at one point, but they kept hitting edge cases where this kind of stuff would break things. Eventually they gave up and just wrapped Wine. So, yes, if you really really want to run WinForms on Linux, Mono is where it's at. But ... why?
well maybe because there exists countless gui applications that uses it, and furthermore, if you say its crossplatform, you cant say that with a straight face unless you also support gui? fact of the matter is, microsoft was perfectly happy to preach their .net shit as cross platform to try extend into non-windows usage, except for gui stuff, as thats "for windows"
We are talking about WinForms specifically. If you want crossplatform GUI, then don't write it in WinForms - write it in Gtk#, or Avalonia, or Uno, or ...
Note that C++, Rust, Go all don't have any kind of standard GUI support out of the box at all.
It can, but not easily. As OP has said, it is a wrapper around Win32, and not an opaque one - it literally has stuff like e.g. the Message struct with members like HWnd and LParam.
Mono did try at one point, but they kept hitting edge cases where this kind of stuff would break things. Eventually they gave up and just wrapped Wine. So, yes, if you really really want to run WinForms on Linux, Mono is where it's at. But ... why?