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I should add that the purpose of them adding this capability isn't to set up a new cross-platform UI suite. It is just to lure enterprise shops to .NET core by supporting more of their .NET Framework workloads
Then again they'd also lose the Windows vendor lock-in, which might still be more valuable to them.
Then again they could sell their development toolchain and gain developer sympathy.
Think if it this way. Someone could write a .NET Core wrapper round the MacOS GUI libraries and platform APIs, but you couldn’t run that on Windows.
The "desktop app packs" for WPF and WinForms Microsoft has made very clear that they will not support anything other than Windows. (They also won't directly be a part of .NET Core according to diagrams, but packages installed on top of it.) It's also unclear if they can be open sourced (they probably cannot be), and if they can't be open sourced it is unlikely a community-led effort could easily work on making them cross-platform.
WinForms work quite nice on Linux with Mono. Won't they cooperate?
That said, this may still be a great opportunity for .NET Core to encourage trying Mono in its documentation for enterprises looking for cross-platform support. (Hopefully they already knew about Mono by this point, but an extra push won't hurt.