Appendix B -- Working with Interfaces and Classes

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Interfaces are the essence of most design patterns described in this book. Interfaces define the roles and determine the relationships of design pattern object participants. The design patterns featured in this book are implemented using Microsoft Visual Basic, which is possible only because the object-oriented language features in Visual Basic depend on COM and ActiveX. That said, interfaces are a cornerstone of the COM specification, which requires a formal abstraction of interface from implementation. It is therefore crucial to understand and appreciate how interfaces impact a system. This appendix briefly explains the following topics:

• Defining COM interfaces and classes in Visual Basic
• Freezing an interface contract using features provided in the Visual Basic integrated development environment (IDE) and using COM Interface Definition Language (IDL)
• Supporting Automation in ActiveX components created in Visual Basic for the benefit of scripting languages, such as Microsoft Visual Basic, Scripting Edition (VBScript) and Microsoft JScript, which are utilized by other technologies such as Active Server Pages (ASP)—Microsoft's scripting solution for a Web application that provides dynamic Web content

For more information on these topics, refer to Ted Pattison's book Programming Distributed Applications with COM and Microsoft Visual Basic 6.0 and the article "Visual Basic Design Time Techniques to Prevent Runtime Version Conflicts," both of which are cited in the bibliography.