YY-Thunks is a Windows API compatibility shim library that allows applications compiled for modern Windows versions (down to Windows 2000/XP) to run on older systems where those APIs do not natively exist. It achieves this through a sophisticated macro-based thunk framework: each API is declared with `__DEFINE_THUNK` which, at runtime, attempts to resolve the real export via `GetProcAddress` and falls back to a hand-coded emulation if missing. The library covers hundreds of APIs across dozens of system DLLs, including kernel32, bcrypt, user32, shell32, DNS, WinHttp, D3D11, and many more. The fallback implementations often reverse-engineer Windows internals, such as SRWLock/condition variable data structures, HSTRING layout, API-set namespace resolution, and threadpool dispatch. The repository also provides a companion tool (YY.Depends.Analyzer) for analyzing binary dependencies and identifying missing APIs, as well as a PE header patcher (MinimumRequiredVersionHelper) for lowering the reported minimum OS version in executables. The project is delivered as static .obj/.lib files and a NuGet package, integrated into the MSBuild pipeline. It is heavily tested with a custom unit test framework that can force fallback paths via `AwaysNullGuard` guards. While the codebase exhibits high craftsmanship in its emulation patterns, the sheer breadth and reliance on undocumented Windows internals introduce maintenance challenges. Some components (e.g., bcrypt.hpp) are monolithic, and hardcoded offsets or version-specific workarounds require continuous updates as Windows evolves.