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Thursday, January 19, 2012

Using VBA with Excel : COM

This blog post was Posted over 3 years ago at http://www.yousaytoo.com/using-vba-with-excel-a-manifestation-of-com/10713 I posted this article with the title
Using VBA with Excel : A manifestation of COM
Component Object Model provides excellent technique for code and component reusability. For eg. Excel can interact with microsoft word. API and macros are tools which provide high end techniques for this purpose. Even excel functions can be used. Microsoft now recommends .NET platform over COM. But its efficiency and fame can never be forgotten or underestimated. COM [COM 95] refers to both a specification and implementation developed by Microsoft Corporation which provides a framework for integrating components. This framework supports interoperability and reusability of distributed objects by allowing developers to build systems by assembling reusable components from different vendors which communicate via COM. By applying COM to build systems of preexisting components, developers hope to reap benefits of maintainability and adaptability.

COM defines an application programming interface (API) to allow for the creation of components for use in integrating custom applications or to allow diverse components to interact.

However, in order to interact, components must adhere to a binary structure specified by Microsoft. As long as components adhere to this binary structure, components written in different languages can interoperate Usage Considerations A number of issues must be evaluated when considering COM, DCOM, and COM+. They include -- Platform support. COM and DCOM are best supported on Windows 95 and NT platforms. However, Microsoft has released a version of COM/DCOM for MacOS that supports OLE-style compound documents and the creation of ActiveX controls. Software AG, a Microsoft partner, has released DCOM for some UNIX operating systems, concretely OS/390, HP-UX 11.0, SUN Solaris, AIX 4.2, 4.3, Tru64 Unix 4.0 and Linux. However, DCOM over non-Windows platforms has few supporters.

Until DCOM for alternate platforms has solidified, the technology is best applied in environments that are primarily Windows-based. Platform specificity of COM/DCOM components. Because COM and DCOM are based on a native binary format, components written to these specifications are not platform independent. Thus, either they must be recompiled for a specific platform, or an interpreter for the binary format must become available. Depending on your perspective, the use of a binary format may be either an advantage (faster execution, better use of native platform capabilities) or a disadvantage (ActiveX controls, unlike Java applets, are NOT machine independent). See Java for more information. Security. Because COM/DCOM components have access to a version of the Microsoft Windows API, "bad actors" can potentially damage the user's computing environment. In order to address this problem, Microsoft employs "Authenticode" [Microsoft 96] which uses public key encryption to digitally sign components. Independent certification authorities such as VeriSign issue digital certificates to verify the identity of the source of the component [VeriSign 97]. However, even certified code can contain instructions that accidentally, or even maliciously, compromise the user's environment. Support for distributed objects. COM/DCOM provides basic support for distributed objects.

There is currently no support for situations requiring real time processing, high reliability, or other such specialized component interaction. Stability of APIs. In October of 1996 Microsoft turned over COM/DCOM, parts of OLE, and ActiveX to the Open Group (a merger of Open Software Foundation and X/Open). The Open Group has formed the Active Group to oversee the transformation of the technology into an open standard. The aim of the Active Group is to promote the technology's compatibility across systems (Windows, UNIX, and MacOS) and to oversee future extension by creating working groups dedicated to specific functions. However, it is unclear how much control Microsoft will relinquish over the direction of the technology. Certainly, as the inventor and primary advocate of COM and DCOM, Microsoft is expected to have strong influence on the overall direction of the technology and underlying APIs.

Long-term system maintainability. Microsoft is actively supporting COM and DCOM technology and pushing it in distributed and Web-based directions. Microsoft is also trying to preserve existing investments in COM technology while introducing incremental changes. Microsoft, for example, has ensured backward compatibility of COM+. Although this affirmation is in general true, COM objects that access local information in the registry or in system folders may require modification. In general, the PC community has not been faced with the concern of very long-lived systems, and vendors often provide support only for recent releases.

Now i think this gives enuff primitive ideas on triggering your interests towards COM and VBA, in case you are into that domain. Take some time in reading more articles and clicking on some advertisement links too...

Mohd Anwar Jamal Faiz

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