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Unit 1: Introduction to Visual Basic

1.4.1 Classes Notes

Class defines the operations an object can perform (methods, events, or properties) and defines
a value that holds the state of the object (fields). Although a class generally includes both
definition and implementation, it can have one or more members that have no implementation.
An instance of a class is an object. You access an object’s functionality by calling its methods and
accessing its properties, events, and fields.
The Table 1.1 provides a description of some of the characteristics that the runtime allows a class
to have. (Additional characteristics that are available through Attribute classes are not included
in this list.) Your language might not make all these characteristics available.
Table 1.1: Characteristics of a Class

Characteristic Description
Sealed Specifies that another type cannot be derived from this type.
Implements Indicates that the class uses one or more interfaces by providing implementations
of interface members.
Abstract Specifies that you cannot create an instance of the class. To use it, you must
derive another class from it.
Inherits Indicates that instances of the class can be used anywhere the base class is
specified. A derived class that inherits from a base class can use the
implementation of any virtual methods provided by the base class, or derived
class can override them with its own implementation.
Exported or Indicates whether a class is visible outside the assembly in which it is defined.
not exported Applies only to top-level classes.
Source: http://msdn.microsoft.com/en-us/library/2s9w552e%28v=vs.71%29.aspx
Nested classes also have member characteristics. Class members that have no implementation
are abstract members. A class that has one or more abstract members is itself abstract; new
instances of it cannot be created. Some languages that target the runtime allow you to mark a
class as abstract even if none of its members are abstract. You can use an abstract class when
you need to encapsulate a basic set of functionality that derived classes can inherit or override
when appropriate. Classes that are not abstract are referred to as concrete classes. A class can
implement any number of interfaces, but it can inherit from only one base class. All classes
must have at least one constructor, which initializes new instances of the class. Each language
that supports the runtime provides a way to indicate that a class or class member has specific
characteristics. When you use the syntax required by your language, the language ensures that
the characteristics of the class and its members are stored (as metadata) along with the
implementation of the class.

1.4.2 Interfaces

An interface is basically a class definition. It itself cannot be instantiated, rather it is implemented
by other classes. Interfaces can be defined and implemented in VB6 albeit with some workarounds
and limitations. For example, in VB6 you cannot use one variable alone to access the methods of
the interface and of the implementing class–you have to declare two variables, one as the
interface and the other as the implementing class and point both of them to the same instance.
VB.NET has a different and more straightforward implementation of interfaces. Like classes,
interfaces can be declared in any class file and are declared with a Interface End Interface statement
block:

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