APISet

APISet is a Windows mechanism that provides an indirect naming layer between public API surface names and the actual DLLs that implement those APIs. It introduces a namespace of API set names, many of which appear as DLLs with names beginning api-ms-win-, used by the Windows loader during module binding. The goal is to decouple the exported API surface from a specific binary, enabling modular updates and better compatibility across Windows versions.

When a binary imports an API set name, the Windows loader consults the API set schema to resolve the API set to one or more concrete DLLs that export the requested functions. The mapping can vary by OS version, while the calling code remains the same. The API set data are consulted at load time, and the resolved DLLs are loaded to satisfy the imports. The mapping is maintained by a component such as ApiSetSchema.dll, which stores the relationships between API set names and their target binaries.

API set entries define a mapping from a logical API set namespace to one or more target DLLs, usually system components such as kernel32.dll, kernelbase.dll, or other Windows libraries. The API set names follow a convention like api-ms-win-<component>-<feature>-l<major>-<minor>-<patch>.dll. This indirection helps Windows update and componentization without breaking applications that reference the API set names directly.

Originating as part of Windows’ modularization and side-by-side architecture, API sets are a background feature of the Windows loader. They are generally transparent to end users and application developers, though they can affect debugging and static analysis tools, which may show API set DLLs instead of the concrete binaries.