Microsoft has introduced the experimental C# framework Microsoft.UI.Reactor. Instead of XAML and code-behind, it uses an MVU approach that updates state and computes the UI from that state, while existing WinUI controls handle actual rendering. Small apps can be tried using .NET 10’s file-based structure, with package settings and code in one file. It is neither a new renderer nor a WinUI fork, and does not force replacement of XAML or MVVM. As a preview without a support policy or API stability guarantee, it is suited to limited experiments rather than wholesale migration of existing production apps.