1. CIS16 Application Development and Programming using Visual Basic.net
Chapter ten
classes

2. Object-Oriented Programming Terminology
Object-oriented programming language
Uses objects to accomplish a program’s goal
an object is anything that can be seen, touched, or used. In other words, an object is nearly any thing
Class
A pattern or blueprint for an object
Instance
An object created from a class
Instantiated
The process of creating an object from a class
Attributes
Characteristics that describe an object

3. Object-Oriented Programming Terminology (cont.)
Behaviors
Methods and events that define how the object will act or react
Methods
Operations (actions) that an object is capable of performing
Events
Actions to which an object can respond
Encapsulation
To enclose in a capsule
A class encapsulates all attributes and behaviors of an object it instantiates

4. Creating a Class Two types of classes used in VB applications:
Built-in classes, such as the TextBox class
Programmer-defined classes
Class statement
Used to define a class
Defines attributes and behaviors of objects created from the class
After a class has been defined, it can be used to instantiate objects

5. Creating a Class (cont.)

6. Creating a Class (cont.)

7. Class Attributes - Variables
Variables and properties both represent values that you can access
Variables
A variable corresponds directly to a memory location.
When defined outside of a procedure are referred to as member variables
Two types: PUBLIC & PRIVATE
Private member variables
will not be visible to other programs in the application
Can only be used within the class they are defined in
Public member variables will be visible to the application
They will be exposed to it

8. Class Attributes - properties
A property is a data element defined in the class
It has a code block, called a property procedure, written between a Property and End Property statement
The code block contains a Get procedure, a Set procedure, or both
In addition to retrieving or storing the property's value, a Property Procedure can also perform custom actions, such as updating an access counter.

10. Public Property Procedures
A Public Property procedure allows an application to refer to a Private member variable in a class.
Property procedures contain blocks of code:
Get block: Retrieves the contents of the Private variable
Set block: Assigns a value to the Private variable
Blocks may be used individually or together

11. Using Public Properties in the VB program
Using Getters:
The Class property's Get procedure retrieves the value - do not explicitly call it by name
Use the property name following the equal (=) sign in an assignment statement
Dim ThisMoment As Date
ThisMoment = TimeOfDay
Using Setters:
The Class property's Set procedure stores a value - do not explicitly call it by name. Use the property name on the left side of an assignment statement.
TimeOfDay = #12:00:00 PM#

12. Class Constructors Constructor
A class method whose purpose is to initialize the class’s Private variables
Processed each time an object is created
Must be coded as a Sub procedure named New
A class can have more than one constructor
The names are the same, but the parameter Lists (signatures) must differ
A parameterized constructor is simply a constructor that has parameters
A parameterized constructor allows an application to specify the object’s initial values
Default constructor
A constructor without parameters

13. Class Constructors
Parameterized constructors are called Overloaded Constructors

14. Overloaded Methods Overloaded methods
Methods with the same name but different parameters

15. Auto-Implemented Properties
Auto-implemented properties enables you to specify the property of a class in one line of code
Visual Basic automatically creates a hidden Private variable that it associates with the property
It also automatically creates hidden Get and Set blocks
It provides a shorter syntax to use when creating a class

16. Auto-Implemented Properties (cont.)
Property Prop2 As String = "Empty"
Private _Prop2 As String = "Empty“
Public Property Prop2 As String
Get
Return _Prop2
End Get
Set(ByVal value As String)
_Prop2 = value
End Set
End Property