The Windows App SDK (formerly Project Reunion) is a Microsoft platform SDK for building modern Windows desktop applications. It provides a unified set of WinRT APIs and runtime components that bridge Win32 and UWP development, enabling features like app lifecycle management (single-instance, file/protocol activation), dynamic package dependencies (resolving framework packages at runtime), rich notifications (toast, push, badge), resource management via MRT Core (PRI files with qualifier-based selection), deployment APIs (package sets, validation), environment variable management, OAuth 2.0 authentication, and interop wrappers for storage pickers and camera capture. The architecture is modular: each feature area is implemented as a separate C++/WinRT component (DLL) that can be consumed individually via NuGet packages. The SDK supports both packaged (MSIX) and unpackaged (self-contained) apps, with a bootstrap DLL that initializes the runtime and hooks into the Windows package graph via Detours. Key entry points include MddBootstrapInitialize2 for runtime initialization, the WindowsAppRuntime DLL for COM class factories, and individual component APIs like AppNotificationManager, PushNotificationManager, and PackageDependency. External dependencies include the Windows SDK, C++/WinRT projections, the Windows Implementation Libraries (WIL), and Microsoft Detours for API interception. Data flows involve inter-process communication through shared memory (for activation redirection), named pipes (for OAuth callbacks), and COM out-of-process servers (for push notification long-running tasks). The codebase is heavily instrumented with TraceLogging telemetry and uses feature flags (TerminalVelocity) to gate experimental APIs.
Files is a modern, open-source Windows file manager built with WinUI 3 and .NET 10, featuring multitabbing, dual-pane navigation, file tags, cloud drive integration, FTP support, Git version control, and a widget-based home page. The architecture is a modular monolith with separate projects for the main app (Files.App), a server component (Files.App.Server), storage abstractions (Files.App.Storage, Files.Core.Storage), a source generator for CsWinRT interop (Files.Core.SourceGenerator), and custom UI controls (Files.App.Controls). The codebase makes heavy use of source-generated P/Invoke via CsWin32, Roslyn source generators for VTable functions and command managers, and the CommunityToolkit.Mvvm pattern with ObservableObject and dependency property source generation. The app supports MSIX packaging, sideload and Store distribution channels, and includes C++/COM components for custom open/save dialogs and a launcher. The last commit was on 2026-06-30, and the project has 44k stars with active CI/CD workflows.