I thought it was the other way around? C# on the outside, legacy code connected on the inside? Anyways, I found that confusing. For the rest, visual studio most definitely uses WPF for much of the UI but also the chrome (e.g. see https://softwareengineering.stackexchange.com/questions/7532... and https://www.quora.com/As-of-2015-how-much-of-Microsoft-Visua... and https://blogs.msdn.microsoft.com/visualstudio/2010/02/16/wpf...).