Programming with Objects: Class Libraries and Reusable Code

1. Programming with Objects: Class Libraries and Reusable Code

2. Objectives • Understand and employ the Visual Basic .NET classes, forms, components, modules, structures, and namespaces • Understand the notion of a class library and its usefulness in creating reusable code • Convert a Windows Forms application into a class library • Build a class library, and create a solution with multiple projects • Incorporate a component into a class library project, and use the component to exchange parameter data with a client program

3. Understanding Classes, Forms, Components, Modules, Structures, and Namespaces • Principle of structured programming • Large programs should be subdivided into smaller, meaningful logical functional units • Class • Defines the properties, fields, methods, and events that go into an object • A reference type • Objectis an instance of a class • Can inherit from another class • Is both inheritable and reusable • Form • A type of class • Has a graphical component and a code component • May serve as a container for other forms • Must be instantiated as an object before it can be used • Code portion of a Windows form is loaded at the same time and on the same computer as the GUI

4. Component • A class that conforms to a certain standard for interacting with other components • May or may not contain a graphical element • Can provide an easy way to communicate between projects in a solution • After component class has been instantiated, its methods are available

5. Module • A reference type, consisting of declarations and procedures only • Similar to a class in terms of serving as a container for code • Unlike a class in several respects • Cannot be instantiated nor inherited • Cannot be nested inside another structure • Variables and procedures are shared by default • Public variables and procedures • Accessible and scoped to the namespace that contains the module

6. Structure • An expansion of the user-defined type in previous releases of Visual Basic • Can contain different kinds of data items, each with its own accessibility • Is a value type (each variable based on a structure contains its own data) • Declared at the module or class level

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8. Namespace • Organizes the list of names that occur in an assembly • An assembly • The package that results from compilation • Can also contain lower-level namespaces • Reason for subdividing large project into namespaces • To avoid name collisions

9. Compiling a Project into a Class Library • Class library • Defines a class • Great way to create reusable objects • Compiles to a DLL file (dynamic link library) • DLL file • Cannot be executed directly • Must be called from inside another running program • Output from the compiler is known as an assembly • After you have compiled your project into a class library • Executable project (or client) that calls this library must identify both the class library’s DLL file and its namespace name

10. Summary • Class • Template or blueprint for an object • Form • Class that has both a graphical representation and a code segment • Component • Class designed for interaction with other components • Module • Contains code only and its data values are shared • Structure • Value type that defines fields, properties, methods, and events at the module level • Namespace • Defines a list of names (classes, structures, other namespaces, etc.) within an assembly • Class library • Defines a reusable program • Project compiled into a class library • Can receive ByVal or ByRef parameters from the calling (client) program • Method of passing parameters between programs • Through component class