The COM, OLE, and ActiveX Relationship

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COM is the backbone of each and every ActiveX and OLE technology. Pick up your current issue of Microsoft Systems Journal, and I'd bet a paycheck that at least one article discusses a new set of COM interfaces, COM service, or some other COM advancement. The point here (other than I can keep my paycheck to myself) is that COM technologies are bombarding us left and right. These days, any new software item coming from those kind folks in Redmond is delivered as a set of COM extensions-not a C-based API.

Before we can really appreciate the full set of COM-based technologies examined throughout this book, a brief (and painless) history lesson is in order, beginning with OLE 1.0.

OLE 1.0

OLE 1.0 was a 16-bit technology that appeared on the scene circa 1991. This technology enabled software applications to share information through linking and embedding. As you may know, certain applications may export visual objects to other applications. A host application may embed this object inside itself (which produced a copy of the original data) or reference it through a link (which is a connection to the original data source). The classic example of this technology is an MS Word document containing an embedded MS Excel spreadsheet. Under the hood, OLE 1.0 was not based on the COM we know today, but rather the clunky Dynamic Data Exchange (DDE) protocol. At this time, OLE was a technology specifically used for object linking and embedding.

OLE 2.0

Around 1993, 16-bit OLE 2.0 was released for the Windows 3.x OS. This marked a huge directional change for OLE, as the underpinnings of DDE were beginning to be stripped away and replaced with the COM infrastructure. With the release of Windows NT 3.51, OLE 2.0 was moved into 32 bits. The major additions to the existing 16-bit technology set was the full support for Unicode string handling, as well as tweaking COM itself to run completely on top of Microsoft's variation of the Open Software Foundation's Remote Procedure Call (OSF RPC) paradigm, rather than the older DDE paradigm.

Linking and embedding technology was still a core service of OLE 2.0-however, it did not stop there. OLE 2.0 introduced a whole slew of COM-related technologies. The key to OLE 2.0 was the extendable COM architecture. Given this, we will never see an OLE 3.0. Instead, we'll see more COM-related technologies. Thus, the terms OLE 1.0 and OLE 2.0 are not much more than historical footnotes in COM's family history.

ActiveX

ActiveX is the current blanket name for any COM-based technology. At one time, ActiveX referred to only web-specific COM technologies; however, these days most things COM are dubbed "ActiveX" something or other (ActiveX controls, ActiveX documents, ActiveX servers, and so on). To make things a bit more confusing, Microsoft occasionally uses the legacy term "OLE" to name some newer COM-based services, such as OLE DB (which really should have been ActiveX-DB to be consistent). We will see many ActiveX services pop up throughout the remainder of this book. Just realize that all ActiveX/OLE technologies are built off the COM protocol.

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