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Part 7: Programming and Converting Acces... > Exchanging Data with Automation and ...

Chapter 31. Exchanging Data with Automation and ActiveX Controls

Interprocess communication (IPC) has come a long way from its early origins in Windows dynamic data exchange (DDE). DDE made it possible to transfer data between Windows applications by setting up a DDE conversation between the two applications using the Windows Clipboard as an intermediary. Windows 3.1 made Object Linking and Embedding (OLE) 1.0 the preferred method of transferring data between OLE-enabled, 16-bit applications. These transfers are still made via the Clipboard with the Edit, Paste Special menu choice. OLE 2.x, implemented in 16-bit Access 2.0 and 32-bit Access 95, brought drag-and-drop operation to Clipboard-based OLE. OLE 2.x also introduced OLE Automation and programmable objects—the foundation for a Windows component architecture.

Access 2.0 was the first Microsoft application to support 16-bit OLE Controls (OCX), the intended replacement for Visual Basic Extension (VBX) custom controls. OLE Controls let Microsoft and third-party suppliers create special-purpose components to supplement the standard set of native controls offered by Access. Access 95 and Visual Basic 4.0 introduced a common set of 32-bit OLE Controls. Developers found creating OLE Controls with C++ to be a daunting task, and OCXs turned out to be much larger than their VBX counterparts. Despite these hurdles, a wide variety of OLE Controls became available for Access 95 and Visual Basic 4.0.

Microsoft's decision to "embrace and extend the Internet," announced on December 9, 1995, caused Microsoft to rethink its original OLE Control design specification. OLE Controls were much too large to download from the Internet on an as-required basis. Microsoft announced at its Professional Developers Conference on March 12, 1996, the specification for a lightweight version of OLE Controls—called ActiveX controls—as the major element of a broader ActiveX Technologies program. 32-bit ActiveX controls are 30 percent to 50 percent smaller than the 16-bit and 32-bit OLE Controls they replace.

This chapter introduces you to (OLE) Automation and ActiveX controls with emphasis on their implementation in Access 97. Automation is the technology that lets you program Jet's Data Access Objects (DAO) with VBA. Automation objects expose only properties and events. ActiveX controls extend Automation objects with events that connect to VBA event-handling subprocedures.


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