Xamarin is all about "wrappers for APIs".
Here is another interesting one, a 3D game engine wrapper in C#
UrhoSharp - Xamarin "UrhoSharp is a cross-platform high-level 3D and 2D engine that can be used to create animated 3D and 2D scenes for your applications using geometries, materials, lights and cameras." ... It is a .NET binding to the Urho3D engine and allows developers to write cross platform code that can target Android, iOS, Windows and Mac with the same codebase and can render to both OpenGL and Direct3D systems. UrhoSharp is a game engine with a lot of functionality out of the box"
SkiaSharp provides a powerful C# API for doing 2D graphics powered by Google’s Skia library, the same library that powers Google Chrome, Firefox, and Android’s graphic stacks."
Skia gives us the same graphics output across all platforms, is actively maintained, and offers great features not found elsewhere, like filters and hardware accelerated graphics."
That may in fact be very good solution, like Google's PDFium PDF rendering library is.
Embrace and collaborate.
There are many more GUI tools portable across platforms, i.e.