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

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.