On October 21, 2026, Microsoft's #ifdef Windows blog introduced Microsoft.UI.Reactor, a framework for writing WinUI desktop apps with the Model-View-Update (MVU) pattern. The idea is to drop XAML, code-behind and ViewModels: a screen is a C# Render() method that returns an immutable description of the UI. On each state change, with state held in hooks such as UseState, Reactor diffs the new description against the previous one and updates only the controls that changed.
The post makes clear this is not a new UI platform: the output is still standard WinUI, and Reactor can coexist with XAML/MVVM screens in the same process. The project is open source under the MIT license, ships more than 60 Reactor-specific Roslyn analyzers and uses a C# port of Meta's Yoga flexbox layout engine. To try it, the sample uses the .NET 10 SDK, package version 0.1.0-preview.16 and the winapp CLI, which also offers experimental project templates.
The caveat is its status: Microsoft itself labels Reactor as experimental, with no support policy and no API guarantees, and says the label should stay for the next few months. For .NET developers on Windows it is worth following and trying in pilot projects, but it is not yet something to bet critical production work on.
Original source: #ifdef Windows (Microsoft Dev Blogs)
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