Component Object Model

Component Object Model
Name	Component Object Model
Developer	Microsoft Corporation
Released	1993
Programming language	C, C++
Operating system	Microsoft Windows
License	Proprietary

Component Object Model

Component Object Model is a binary-interface standard introduced by Microsoft to enable inter-process and intra-process component interaction on Microsoft Windows platforms. It defines a language-neutral way for software components to expose functionality through interfaces so that clients such as applications, libraries, and services can use those components without compile-time coupling. The model influenced later technologies in distributed computing and component-based software engineering across organizations such as IBM, Sun Microsystems, and the Object Management Group.

Overview

COM was promulgated by Microsoft as part of strategies involving Microsoft Windows evolution, relationship with Windows NT, and enterprise software scenarios pursued by Microsoft Corporation. It grew alongside initiatives including OLE, ActiveX, and the Windows Registry paradigm and was referenced in discussions at conferences like COMDEX and engineering collaborations with partners such as Intel Corporation and Hewlett-Packard. The model underpinned many consumer and enterprise products from Microsoft Office automation to system services in Internet Explorer, and its design parallels debates in component models discussed at the Object Management Group and in academic venues such as the ACM SIGPLAN community.

Architecture and Principles

COM centers on binary interfaces identified by globally unique identifiers similar to practices used in Distributed Computing Environment and CORBA proposals championed by OMG. Core architectural elements include interface-based polymorphism resembling ideas from C++ virtual tables, object identity and lifetime management via reference counting, and component registration tied to the Windows Registry and location mechanisms used by Microsoft Installer and Windows Service Control Manager. The use of Interface Definition Language concepts echoes standards like RPC and the DCE architecture, while GUID usage follows patterns established in distributed systems by enterprises such as Digital Equipment Corporation and in standards overseen by organizations including IETF.

Programming and Interfaces

COM programming typically employs languages such as C and C++, with language bindings for platforms including Visual Basic and scripting hosts like Windows Script Host. Interfaces are expressed as abstract vtable contracts; the canonical methods IUnknown includes QueryInterface, AddRef, and Release, mirroring patterns used by projects at Bell Labs and teachings from researchers at Stanford University and Massachusetts Institute of Technology. Interface binary compatibility permits clients compiled by Microsoft Visual C++ or built with tools from Borland Software to interoperate, and tools such as the Microsoft IDL compiler and development environments like Visual Studio generate wrappers and proxy/stub code akin to middleware tools used in Sun Microsystems products.

Versioning and Compatibility

COM's approach to versioning relies on interface immutability and new interfaces for extended behavior, an approach comparable to strategies used by Sun Microsystems in the evolution of Java APIs and to practices at IBM for CICS extensions. Compatibility is managed through interface GUIDs and class CLSIDs registered in the Windows Registry, and through deployment technologies such as side-by-side assembly concepts discussed in relation to Windows XP and later adapted in Windows Vista component isolation efforts. Enterprise deployment patterns for COM echo migration tactics used by large vendors including Oracle Corporation and SAP SE when managing backward compatibility for installed bases.

Security and Deployment

COM components run with privileges determined by host processes such as explorer.exe or services controlled via the Windows Service Control Manager, raising security considerations familiar to architects at Microsoft Security Response Center and analysts from Symantec Corporation and Kaspersky Lab. Distributed COM (DCOM) added networked activation and marshaling, inheriting authentication and authorization mechanisms analogous to NTLM and Kerberos deployments used by Active Directory. Deployment and versioning challenges led to mitigations using technologies such as Windows Side-by-Side and installer strategies promoted by Windows Installer guidance and enterprise policies used by organizations including NASA and Department of Defense IT programs.

Implementations and Usage

Implementations of the standard are found in core Windows subsystems developed by Microsoft Corporation and in third-party frameworks from vendors such as Borland Software and open-source efforts like Wine to provide compatibility on Linux and other POSIX platforms. COM underlies automation servers in Microsoft Office, browser extensions in Internet Explorer, shell extensions in Windows Explorer, and in multimedia pipelines influenced by vendors like RealNetworks and Apple Inc. for plugin architectures. Cross-platform component models from companies such as IBM and Sun Microsystems were inspired by or contrasted with COM in product lines including WebSphere and Java EE.

Criticism and Limitations

Critics at academic venues like ACM conferences and engineers from organizations such as Mozilla and Free Software Foundation have cited complexity in reference counting, deployment fragility known as "DLL Hell", and tight coupling to Windows Registry semantics. The binary-stable interface requirement complicates refactoring compared to approaches promoted in Eclipse Foundation plugin systems and Java package versioning, and security critiques referenced by groups including CERT emphasize attack surfaces introduced by in-process COM servers hosted by browsers and office suites. Efforts by Microsoft Research and community projects attempted to address these issues, but many large-scale migrations favored service-oriented and managed-platform alternatives advocated by Google and Amazon Web Services.

Category:Microsoft technologies