What problem does it solve? Upgrading a project from .NET 8 to .NET 9 introduces dozens of breaking changes across the runtime, C# 13 compiler, ASP.NET Core, EF Core, and SDK tooling. Manually identifying which changes apply and how to fix them is error-prone and time-consuming. ## Core Features & Use Cases - Systematic Migration Workflow: Six-step process covering project assessment, TargetFramework update, build error resolution, behavioral change review, infrastructure updates, and verification. - Breaking Change References: Ten detailed reference documents organized by technology area (core libraries, C# compiler, ASP.NET Core, EF Core, cryptography, serialization/networking, WinForms/WPF, containers/interop, deployment/runtime, SDK/MSBuild) loaded on demand based on project type. - Concrete Fixes with Code Samples: Each breaking change includes impact level, before/after code examples, and mitigation guidance, covering SYSLIB0054-SYSLIB0057 obsoletions, params span overload resolution, BinaryFormatter removal, and EF Core 9 migration changes. - Use Case: You have an ASP.NET Core 8 API using EF Core and HttpClientFactory. This skill updates the TFM to net9.0, bumps package versions to 9.0.x, fixes the SocketsHttpHandler InvalidCastException, replaces non-deterministic HasData seed values, and updates 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.
