[to11mtm]

Almost. :)

Right now there is Xamarin Forms which has a community maintained Linux layer.

You can use that today if you write a Xamarin forms app.

From the Microsoft side of things, no, you can't just write a winforms app and use it in Linux.

However, they have announced MAUI which basically lets you write XML/XAML UI Declarations that are platform agnostic and can be translated into the various layers (such as Xamarin Forms.) I believe that's supposed to be a thing sometime late this year or early next (don't quote me on that!)

Other projects to look into are AvaloniaUI and Uno. Those are community projects that try to give you cross platform UI today.