Chapter 21 -- XML with ADO 2.5 for Our Components

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Chapter 21

The main purpose of this chapter is to show you how to use the new XML functionality built into Microsoft ActiveX Data Objects (ADO) version 2.5. This new version comes with Microsoft Windows 2000, but you can also download ADO 2.5 from Microsoft's Web site to use it with Microsoft Windows NT 4, Windows 98, and even Windows 95. You can download the new version of ADO by downloading Microsoft Data Access Components (MDAC) 2.5 at http://www.microsoft.com/data/download.htm.

Earlier versions of ADO have been included in a multitude of Microsoft products, such as Visual Studio, Visual Basic, Office, SQL Server, and others. This widespread availability of such an important technology as ADO has caused problems for users and for developers. The exact version of ADO a user has in his or her workstation might depend on which set of products this specific user has installed and on the order in which he or she has installed them. The application you have developed for a number of users is liable to work well for some of them and less well for others. This problem isn't unique to ADO; it's a general DLL problem that even has a name. People call it DLL Hell or Developer's Hell, and we'll explain it briefly.

When a user has installed an application written by you, the developer, the user's computer is set in a perfect state to run your application. This happy condition lasts until the user installs another application. The more applications the user installs, the less likely it is that your application will continue to run well. There's no change in your application; in principle, it still works. The changes are all in the set of DLLs other applications might have made it use. That's why this situation is called DLL Hell. It's also your personal hell, which is why it's also called Developer's Hell.

When developing Windows 2000, Microsoft has attacked the DLL Hell problem in several ways. This is not the place to tell you much about the range of Microsoft strategies, but you should check out the smart shortcuts installed by Windows Installer, included in Windows 2000. Let us just briefly tell you how they work.

If the user deletes or renames a DLL needed by an application, Windows Installer can refresh the installation by reinstalling the DLL. This can happen without the user noticing it. Windows Installer also makes it possible to install different versions of the same DLL "side by side"—you can marry an application to a specific version of a DLL and install that version of the DLL in the same folder in which you install the application. This way Windows 2000, Windows Installer, and your application together can guarantee that whatever happens at large in the customer's installation, your application runs with the DLL versions it should.

To reduce DLL Hell, Microsoft has also decided not to include system software such as ADO in a lot of different products anymore. The only way to get a new version of such software will be to install a new version of the operating system. So the next time you install a new version of Office, Visual Studio, or SQL Server, your installed version of ADO should remain unchanged. Without a doubt, this new Microsoft strategy is going to increase stability in installations and for installed applications.



Designing for scalability with Microsoft Windows DNA
Designing for Scalability with Microsoft Windows DNA (DV-MPS Designing)
ISBN: 0735609683
EAN: 2147483647
Year: 2000
Pages: 133

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