Windows USER
Windows USER provides a large part of the core user experience for Microsoft Windows. Historically, it was responsible for:






Gradually, as Windows has become larger and better factored, Windows USER has cooperated with other components to provide this functionality: Windows USER implements abstractions that are used to build Windows applications. These include
The HWND is the basic unit of visual display and input in the Windows developer platform. All visual elements either have their own HWNDs or live as part of a larger HWND. Many functions in Windows USER manipulate properties on HWNDs such as size, position and title. Each HWND also has a communication channel (WNDPROC - Window Procedure) for delivery of messages. Windows are managed as a tree, with the desktop window at the root of the hierarchy. Child windows are usually (but not always) visually contained within their parents. Applications and the operating system communicate with HWNDs using messages. A window message is the combination of a target HWND, a message code, and other details such as mouse position. By combining calls to Windows USER in the right way, a Windows application processes messages. The core Windows message loop (including calls to GetMessage, TranslateMessage and DispatchMessage) is at the core of any Windows application. Developer functionality related to Windows USER is provided in the C header file winuser.h. In 16-bit versions of Windows, Windows USER was implemented as a file called user.exe. The file extension here was a misnomer, as Windows USER was in fact always a dynamic link library. In 32-bit versions of Windows, the 32-bit version of Windows USER is called user32.dll and is located in the System32 directory, while compatibility with 16-bit applications is provided by having a modified version of user.exe also present. In 64-bit versions of Windows, the 64-bit implementation of Windows USER is called user32.dll and is located in the System32 directory, while a modified 32-bit version (also called user32.dll) is present in the SysWOW64 directory.
Developer experience
The component has existed in all versions of Windows, and includes functionality for window management, message passing, input processing and standard controls.