OCX

OCX stands for Object Linking and Embedding Custom Control, and refers to a class of software components for Windows known as ActiveX controls. An OCX file is a dynamic-link library that contains COM objects, along with resources and metadata such as a type library, which describes the object's interfaces, methods, properties, and events. OCX controls are designed to be hosted inside container applications that support ActiveX, such as Microsoft Visual Basic, Visual Studio designer environments, or Internet Explorer.

OCX controls communicate through COM, exposing IUnknown and interface-based APIs. They are registered in the system registry with regsvr32; registration enables the operating system and host applications to locate and instantiate the control. During development, a control can implement design-time features and events; at run time, a container loads the control, creates instances, and wires events.

Historically, OCX controls were widely used in Windows desktop development in the 1990s and early 2000s to extend applications with reusable UI and functionality. With the advent of .NET, web technologies, and modern security models, ActiveX/OCX usage has declined, and many browsers and platforms restrict or block unsigned or untrusted controls. OCX components may be 32-bit or 64-bit; a given host process must use a matching bitness, requiring separate OCX files for each architecture.

Today, OCX remains in use in some legacy enterprise applications and in environments that rely on COM-based interoperability, but new development has largely moved to newer architectures and standards. See also: ActiveX, COM, regsvr32, Type Library.