COM: Component Object Model Technologies

Microsoft COM (Component Object Model) technology in the Microsoft Windows-family of Operating Systems enables software components to communicate. COM is used by developers to create re-usable software components, link components together to build applications, and take advantage of Windows services. The family of COM technologies includes COM+, Distributed COM (DCOM) and ActiveX® Controls.

COM is used in applications such as the Microsoft Office Family of products. For example COM OLE technology allows Word documents to dynamically link to data in Excel spreadsheets and COM Automation allows users to build scripts in their applications to perform repetitive tasks or control one application from another.

Microsoft provides COM interfaces for many Windows application services such as Microsoft Message Queuing (MSMQ) Microsoft Active Directory (AD) and Windows Management and Instrumentation (WMI).

Microsoft recommends that developers use the .NET Framework rather than COM for new development.

How are COM and .NET related?

COM and .NET are complementary development technologies.
The .NET Common Language Runtime provides bi-directional, transparent integration with COM. This means that COM and .NET applications and components can use functionality from each system. This protects your existing investments in COM applications while allowing you to take advantage of .NET at a controlled pace.
COM and .NET can achieve similar results. The .NET Framework provides developers with a significant number of benefits including a more robust, evidence-based security model, automatic memory management and native Web services support. For new development, Microsoft recommends .NET as a preferred technology because of its powerful managed runtime environment and services.

Using COM from .NET and .NET from COM

For a technical discussion of using COM components from .NET see http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dndotnet/html/callcomcomp.asp.
For a technical discussion of using .NET components from COM see http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dndotnet/html/callnetfrcom.asp.

What is COM+?

COM+ is the name of the COM-based services and technologies first released in Windows 2000. COM+ brought together the technology of COM components and the application host of Microsoft Transaction Server (MTS). COM+ automatically handles difficult programming tasks such as resource pooling, disconnected applications, event publication and subscription and distributed transactions.
COM+ infrastructure also provides services to .NET developers and applications through the System.EnterpriseServices namespace of the .NET Framework. More information on creating .NET components that use Enterprise Services is available on the MSDN web site.

I want to build COM or COM+ applications. Where should I go?

The best resource for COM developers is the Microsoft Developer Network (MSDN) at http://msdn.microsoft.com/. The MSDN Library contains information for developers on the Microsoft platform including an overview of component development using COM-based technologies.

Is COM still supported? Is it going away?

COM is a feature of Windows. Each version of Windows has a support policy described in the Windows Product Lifecycle. COM continues to be supported as part of Windows. COM is a feature of the next version of Windows, Windows Vista.

Is COM+ or DTC still supported? Are they going away?

COM+ and DTC are features of Windows. Each version of Windows has a support policy described in the Windows Product Lifecycle. These technologies continue to be supported as part of Windows editions they were delivered with. COM+ and DTC are features of the coming version of Windows, Windows Vista.