Two things are suggested: Avalonia and Uno.
Both Uno and Avalonia are Free and open source. It looks like the "XPF" spin has a steep cost. Both are XAML based ways to design GUI.Sometimes it is suggested that you use "Wine" to run native WPF on linux, but based on past experience with Wine, I want nothing to do with that.
I also hear about Unity for game development. They say it is free "to start" for personal use and for organizations with under $100K in revenue. They say Unity is based on Mono. They also say that Unity mostly works out of the box on Linux.
Tom's Computer Info / tom@mmto.org