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Component Object Model (COM) is a Microsoft platform for software componentry introduced by Microsoft in 1993. It is used to enable interprocess communication and dynamic object creation in any programming language that supports the technology. COM is often used in the software development world as an umbrella term that encompasses the OLE, ActiveX, COM+ and DCOM technologies. Although COM was introduced in 1993, Microsoft did not begin emphasizing the name COM until 1997.

Although it has been implemented on several platforms, it is primarily used with Microsoft Windows. COM is expected to be replaced to at least some extent by the Microsoft .NET framework.

History

Anthony Williams, one of the more notable thinkers involved in the creation of the COM architecture, distributed a couple of internal papers in Microsoft that embraced the concept of software components; Object Architecture: Dealing With the Unknown - or - Type Safety in a Dynamically Extensible Class in 1988 and On Inheritance: What It Means and How To Use It in 1990. These provided the foundation of many, if not all, of the ideas behind the basics of COM.

From many of these ideas spawned Microsoft's first object-based framework, OLE, which is short for Object Linking and Embedding. OLE was built on top of dynamic data exchange (DDE) and designed specifically for compound documents. It was introduced with Word for Windows and Excel in 1991, and was later included with Windows, starting with version 3.1 in 1992. An example of a compound document is a spreadsheet embedded in a Word for Windows document; as changes are made to the spreadsheet within Excel, they appear automatically inside the Word document.

In 1991, Microsoft introduced Visual Basic Extensions (VBX) with Visual Basic 1.0.

In 1993, Microsoft released OLE 2 with COM as the underlying object model. While OLE 1 was focused on compound documents, COM and OLE 2 were designed to address software components in general. In 1994 OLE controls (OCX) were introduced as the successor to VBX controls. At the same time, Microsoft stated that OLE 2 would just be known as "OLE", and that OLE was no longer an acronym, but a name for all of the company's component technologies.

Later, in early 1996, Microsoft renamed some parts of OLE relating to the Internet ActiveX, and gradually renamed all OLE technologies to ActiveX, except the compound document technology that was used in Microsoft Office. Later that year, DCOM was introduced as an answer to CORBA.

Related technologies

COM was the major software development platform for Windows and, as such, influenced development of a number of supporting technologies.

COM+

With Windows 2000, a significant extension to COM named COM+ was introduced. At the same time, Microsoft de-emphasized DCOM as a separate entity.

An advantage of COM+ was that it could be run in "component farms", managed with the built-in Microsoft Transaction Server. A component, if coded properly, could be reused by new calls to its initializing routine without unloading it from memory. Components could also be distributed (called from another machine) as was previously only possible with DCOM.

DCOM

Main article: Distributed component object model

.NET

The COM platform has largely been superseded by the Microsoft .NET initiative, and Microsoft now focuses its marketing efforts on .NET. To some extent, COM is now deprecated in favour of .NET. Despite this, COM remains a viable technology with an important software base – for example the popular DirectX 3D rendering SDK is based on COM. Microsoft has no plans for discontinuing either COM or support for COM.

Several of the services that COM+ provides, such as transactions and queued components, are still important for enterprise .NET applications.

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