VisualStudio bug?

Question

In VS 2010 created a new WinForm project. Added a new UserControl WPF.

2 compile-time errors appeared:

Error 1 The type \'Sys

Answer 1

Was this project migrated from .NET 3.5 to NET 4.0 ? If yes, the interface now resides in System.Xaml and you need to reference it.