Writing Applications Using COM

COM is the Component Object Model, a Windows-based distributed object architecture designed to provide object interoperability using predefined routines called interfaces. COM applications use objects that are implemented by a different process or, if you use DCOM, on a separate machine. You can also use COM+, ActiveX and Active Server Pages.

COM is a language-independent software component model that enables interaction between software components and applications running on a Windows platform. The key aspect of COM is that it enables communication between components, between applications, and between clients and servers through clearly defined interfaces. Interfaces provide a way for clients to ask a COM component which features it supports at runtime. To provide additional features for your component, you simply add an additional interface for those features.

Using COM and DCOM

Various classes and wizards that make it easy to create COM, OLE, or ActiveX applications. You can create COM clients or servers that implement COM objects, Automation servers (including Active Server Objects), ActiveX controls, or ActiveForms. COM also severs as the basis for other technologies such as Automation, ActiveX controls, Active Documents, and Active Directories.

Using Delphi to create COM-based applications offers a wide range of possibilities, from improving software design by using interfaces internally in an application, to creating objects that can interact with other COM-based API objects on the system, such as the Win9x Shell extensions and DirectX multimedia support. Applications can access the interfaces of COM components that exist on the same computer as the application or that exist on another computer on the network using a mechanism called Distributed COM (DCOM).

For more information on COM and Active X controls, see Overview of COM technologiesCreating an ActiveX Control and Distributing a Client Application as an ActiveX Control.

For more information on DCOM, see Using DCOM connections.

Using MTS and COM+

COM applications can be augmented with special services for managing objects in a large distributed environment. These services include transaction services, security, and resource management supplied by Microsoft Transaction Server (MTS) on versions of Windows prior to Windows 2000) or COM+ (for Windows 2000 and later).

For more information on MTS and COM+, see Creating MTS or COM+ objects and Using transactional data modules.