What problem does it solve? Upgrading a project from .NET 8 to .NET 9 introduces dozens of breaking changes across the compiler, runtime, ASP.NET Core, EF Core, and cryptography APIs. This Skill provides a systematic, step-by-step workflow that updates the target framework, fixes build errors, addresses behavioral changes, and updates infrastructure so the migrated project builds cleanly and passes tests. ## Core Features & Use Cases - Guided six-step migration workflow: Assess the project, update the TargetFramework to net9.0, resolve source-incompatible changes, address behavioral changes, update Dockerfiles and CI/CD pipelines, then verify with clean builds and tests. - Technology-aware reference loading: Ten detailed reference documents cover C# 13 compiler changes, core libraries, ASP.NET Core 9, EF Core 9, cryptography, serialization/networking, WinForms/WPF, containers/interop, and deployment/runtime, loaded only when relevant to the project. - Concrete fixes for high-impact changes: Handles BinaryFormatter removal, params span overload resolution, SYSLIB0054-SYSLIB0057 obsoletions, EF Core pending-model-changes exceptions, HttpClientFactory SocketsHttpHandler changes, and saturating float-to-int conversions. - Use Case: You have an ASP.NET Core 8 Web API using EF Core and HttpClientFactory. This Skill walks you through retargeting to net9.0, updating package versions, fixing the new DI validation errors in development, replacing obsolete X509Certificate2 constructors, and updating your Dockerfile base images. ## Quick Start Migrate my .NET 8 solution at ./src/MyApp.sln to .NET 9 and fix all resulting build errors and breaking changes.
