Q: Are you also developing a GUI designer like in Visual Studio for
WinForms, WPF etc? A: Yes, we’re looking at how we can support at
least previewing WinForms/WPF/Xamarin Forms. We’re not sure we’re
going to make it for the first release though.
JetBrains already published the answer,
https://blog.jetbrains.com/dotnet/2016/11/28/introducing-rider-the-new-net-ide-webinar-recording-and-qa/
Generally speaking, they can easily support that on Windows (as open source IDE SharpDevelop already supports that). But it would be a less useful feature as Rider targets cross platform developers.
