VB.NET Classes - MSDN Documentation

Overview of Classes

Classes are the fundamental building blocks of object-oriented programming (OOP) in VB.NET. They serve as blueprints for creating objects, encapsulating data (fields or properties) and behavior (methods or functions) into a single unit.

A class defines the structure and behavior of objects. When you create an object from a class, it's called an instance of the class. Each instance can have its own unique set of data, but they all share the same behavior defined by the class.

Defining a Class

You define a class using the Class keyword, followed by the class name, and end it with the End Class statement.

Basic Class Definition:

                    
Public Class Car
    ' Fields (member variables)
    Public Make As String
    Public Model As String
    Public Year As Integer

    ' Constructor (special method for initializing objects)
    Public Sub New(make As String, model As String, year As Integer)
        Me.Make = make
        Me.Model = model
        Me.Year = year
    End Sub

    ' Method (behavior)
    Public Sub DisplayInfo()
        Console.WriteLine($"This is a {Me.Year} {Me.Make} {Me.Model}.")
    End Sub

    ' Property (controlled access to fields)
    Private _color As String
    Public Property Color() As String
        Get
            Return _color
        End Get
        Set(value As String)
            _color = value
        End Set
    End Property
End Class
                    
                

Class Members

Classes can contain various members:

Fields: Variables that hold data for the object (e.g., Make, Model).
Properties: Special methods that provide controlled access to a class's fields, often used for validation or calculated values.
Methods: Functions or Subroutines that define the actions an object can perform (e.g., DisplayInfo).
Constructors: Special methods named New that are called when an object is created, used for initialization.
Events: Mechanisms for objects to signal that something has happened.
Nested Classes: Classes defined within another class.

Creating Objects (Instantiating a Class)

You create an instance of a class using the New keyword.

Instantiating the Car Class:

                    
' Create an instance of the Car class
Dim myCar As New Car("Toyota", "Camry", 2023)

' Access members using the dot operator
myCar.Color = "Blue"
myCar.DisplayInfo() ' Output: This is a 2023 Toyota Camry.
Console.WriteLine($"The car's color is: {myCar.Color}") ' Output: The car's color is: Blue
                    
                

Access Modifiers

Access modifiers control the visibility and accessibility of class members:

Public: Accessible from anywhere.
Private: Accessible only within the class.
Protected: Accessible within the class and by derived classes.
Friend: Accessible within the same assembly (project).
Protected Friend: Accessible within the same assembly or by derived classes.
Internal: Same as Friend.

Key Concepts

Encapsulation: Bundling data and methods that operate on the data within a single unit (the class).
Abstraction: Hiding complex implementation details and exposing only necessary features.
Inheritance: Allowing a new class (derived class) to inherit properties and methods from an existing class (base class).
Polymorphism: The ability of an object to take on many forms, often through inheritance and method overriding.