I have been developing Windows Forms programs for few years. I am now looking into .NET Core (including ASP.NET Core MVC). I am searching for the new GUI desktop technology.
It is now possible to use Qt, QtQuick, and QML with .NET Core, using Qml.Net.
It is highly performant (not "P/Invoke chatty"), fully featured and works across Linux, OS X, and Windows.
Check out my blog post to see how it compares to the other options out there currently.
PS: I'm the author.
For creating a console-based UI, you can use gui.cs. It is open-source (from Miguel de Icaza, creator of Xamarin), and runs on .NET Core on Windows, Linux, and macOS.
It has the following components:
Sample screenshot
Windows Forms (and its visual designer) have been available for .NET Core (as a preview) since Visual Studio 2019 16.6. It's quite good, although sometimes I need to open Visual Studio 2019 16.7 Preview to get around annoying bugs.
See this blog post: Windows Forms Designer for .NET Core Released
Also, Windows Forms is now open source: https://github.com/dotnet/winforms