Introduction
OOPS or Object Oriented Programming Structures. Some of the concepts have been always used in one or the other programming languages. For example you must have used structs in C which is a good example of encapsulation. There are four major pillar of OOPS. Let’s try to understand each one of them by taking some examples also:-
1.) Encapsulation

2.) Abstraction

3.) Polymorphism

4.) Inheritance

Encapsulation:-
Interview Definition: - Binding data and member functions together inside a single unit.
How to Encapsulate: - By creating types e.g. Classes and Struct
By using encapsulation we can create our own custom types by reusing the existing or primitive types.

Abstraction:-
Abstraction defines way to abstract or hide your data and members from outside world. Simply speaking Abstraction is hiding the complexities of your class or Struct or in a generic term Type from outer world. This is achieved by means of access specifiers.
Interview Definition: - Hiding the complexities of your type from outside world
How to abstract: - By using Access Specifiers
VB.Net has five access specifiers:-
Public — Accessible outside the class through object reference.
Private — Accessible inside the class only through member functions.
Protected — Just like private data but accessible in derived classes also through member functions.
Friend – Friend is Visible inside the assembly and accessible through objects.
Protected Friend – Protected Friend is Visible inside the assembly through objects and in derived classes outside the assembly through member functions.
Let’s try to understand by a practical example:-
Interview Tip:-
The default access specifier for a class is Friend while that of member of class is private

      Public Class Class1
        Private i As Integer
        'No Access specifier means private
        Public j As Integer
        ' Public
        Protected k As Integer
        'Protected data
        Friend m As Integer
        ' Internal means visible inside assembly
        Protected Friend n As Integer
        'inside assembly as well as to derived classes outside assembly
        Shared x As Integer
        ' This is also private
        Public Shared y As Integer
        'Static means shared across objects
        <DllImport("MyDll.dll")> _
        Public Shared Function MyFoo() As Integer
        End Function
        'extern means declared in this assembly defined in some other assembly
        Public Sub myFoo2()
            'Within a class if you create an object of same class then you can access all data members through object reference even private data too
            Dim obj As New Class1()
            obj.i = 10
            'Should be Error since we can’t access private data through object. But here it is accessible.:)
            obj.j = 10
            obj.k = 10
            obj.m = 10
            obj.n = 10
            ' obj.s =10; //Error Shared data can be accessed by class names only
            Class1.x = 10
            ' obj.y = 10; //Error Shared data can be accessed by class names only
            Class1.y = 10
        End Sub
    End Class

Now let’s try to copy the same code inside Main method and try to compile

<STAThread> _
Private Shared Sub Main()
'Access specifiers comes into picture only when you create object of class outside the class
Dim obj As New Class1()
' obj.i =10; //Error can’t access private data through object.
obj.j = 10
' obj.k=10; //Error can’t access protected data through object.
obj.m = 10
obj.n = 10
' obj.s =10; //Errror Static data can be accessed by class names only
Class1.x = 10
'Error can’t access private data outside class
' obj.y = 10; //Errror Static data can be accessed by class names only
Class1.y = 10
End Sub

What if Main is inside another assembly?

<STAThread> _
Private Shared Sub Main()
'Access specifiers comes into picture only when you create object of class outside the class
Dim obj As New Class1()
' obj.i =10; //Error can’t access private data through object.
obj.j = 10
' obj.k=10; //Error can’t access protected data through object.
' obj.m=10; // Error can’t access internal data outside assembly
' obj.n=10; // Error can’t access internal data outside assembly

' obj.s =10; //Errror Static data can be accessed by class names only
Class1.x = 10
'Error can’t access private data outside class
' obj.y = 10; //Errror Static data can be accessed by class names only
Class1.y = 10
End Sub