OOPS - Object Oriented Programming Languages & Systems.
Everything in the world is an object. The type of the
object may vary. In OOPS, we get the power to create
objects of our own, as & when required.

Class - A class is an organized store-house in
object-oriented programming that gives coherent
functional abilities to a group of related code. It is
the definition of an object, made up of software code.
Using classes, we may wrap data and behaviour together
(Encapsulation). We may define classes in terms of
classes (Inheritance). We can also override the
behaviour of a class using an alternate behaviour
(Polymorphism).

Using inhertance, we may assign different traits to different
classes. Yet, the child classes will inherit some common
traits from the base class. As in the figure above, the
classes for "Permanent" and "Wage Worker" will inherit
some common traits from the class of "Employee".

A class may contain class members like fields, properties,
events & methods with different types of access modifiers
like private, public, protected or friend, to process the
way they are accessed. In VB.NET, a class may be declared
as below...

Public Class SampleClass
    'define class members  
End Class  
                                                   
Key Concepts of .NET - To work with classes and modules,
the key concepts to know are definition, access, inheritance,
constructors, destructors, delegates, abstract classes & interfaces..

Class members - The different types of entities in a class,
like fields, properties, events & methods.

Object - An object is an instance of a class. This instance
may be used to access the members of the class. It is pretty
easy to define an object. See sample code below...

Dim objSampleObject as SampleClass

Structure - It is a bit similar to a class. Semantically,
structures are known as value types, while classes as reference types. We do'nt instantiate an object using the New keyword while working with a structure. We can not inherit from a structure.

Public Structure Student
  Public RollNo as Integer
  Public Name as String
End Structure

Dim objStud as Student
objStud.RollNo=31
objStud.Name="Monu"

Here, note that the object objStud is not exactly an instance,
it is a simple object (object of a structure) that is used to
access the members of this structure. It would have behaved
differently had it been an object of a class, as it would have
invoked the constructor associated with the class.

Public Class ClassCalc
   Public Function FnAdd(ByVal dblA as double, ByVal dblB _

                                as double) as Double
     FnAdd = dblA + dblB
   End Function
End Class

Now, lets make use of the method FnAdd defined in the class
above. To use it, first create an object of this class, say
objX. Using this object, we can invoke the methods of this
class. See code below...

Dim objX as ClassCalc
Dim dblResult as Double
dblResult = objX.FnAdd(4.56,2.35)

Property - A property is a thing that describes the features of an object. A property is a piece of data contained within a class that has an exposed interface for reading/writing. Looking at that definition, you might think you could declare a public variable in a class and call it a property. While this assumption is somewhat valid, the true technical term for a public variable in a class is a field. The key difference between a field and a property is in the inclusion of an interface.

We make use of Get and Set keywords while working with
properties. We prefix the variables used within this code block with an underscore. Value is a keyword, that holds the value
which is being retrieved or set.

Private _Color As String
     Public Property Color()
            Get
                 Return _Color
            End Get
     Set(ByVal Value)
                 _Color = Value
     End Set
End Property

Event - An action that an object does. When something happens, we say an event has happened. For example, when a button is clicked, we say it is the click( ) event. When a mouse hovers on an image, we say the mouseover( ) event has taken place.

Access Modifiers - Keywords used to vary the way members of a class are used. Following are different types...

1) Public - These classes can be used anywhere in the code.
There are no restrictions.

Available only to code outside our class

2) Private - These classes are accessible only within their
declaration contexts. This includes nested procedures. When a variable is declared Public inside a Private class, it is
accessible only from inside that class.

Available only to code inside our class

3) Protected - These classes extend the accessibility of their
members to their child classes (means, to the classes that derive from them). They extend their members to both themselves & their child classes.

Available only to classes that inherit from our class

4) Friend - Friend access means that elements are accessible only within the program. Friend is the default access modifer for any class that does not have a modifier.

Available only to code within our project/component

5) Protected Friend - Available only to classes that inherit from our class (in any project) or to code within our project/component. This is a combination of Protected and Friend.

Default - A Default property is a single property of a class that can be set as the default. This allows developers that use your class to work more easily with your default property because they do not need to make a direct reference to the property. Default properties cannot be initialized as Shared or Private and all must be accepted at least on argument or parameter. Default properties do not promote good code readability, so use this option sparingly.

Overloads - The Overloads property allows a function to be
described using deferent combinations of parameters. Each
combination is considered a signature, thereby uniquely defining an instance of the method being defined. You can define a function with multiple signatures without using the keyword Overloads, but if you use the Overloads keyword in one, you must use it in all of the function's Overloaded signatures.

Shared -The Shared keyword is used in an inherited or base class to define a property or method as being shared among all instances of a given class. If multiple instances of a class with shared properties or methods are loaded, the shared properties or methods will provide the same data across each instance of the class. When one class alters the value for a shared property, all instances of that class will reflect the change. Shared properties of all instances of the class
point to the same memory location.

Overridable -The Overridable keyword is used when defining a property or method of an inherited class, as overridable by the inheriting class. 

Overides - The Overides keyword allows the inheriting class to disregard the property or method of the inherited class and implements ts own code.                                         

NotOverridable - The NotOverridable keyword explicitly declares a property or method as not overridable by an inheriting class, and all properties are "not overridable" by default. The only real advantage to using this keyword is to make your code more readable. 

MustOverride - The MustOverride keyword forces the inheriting class to implement its own code for the property or method.

Shadows - The Shadows keyword will effectively hide all of the other methods in the baseclass. It is like forcefully getting rid of the overloading that has been done on the methods of the base class.
The Shadows keyword works like the Overloads keyword except that with shadows we do not have to follow rules such as implementing the same signature. The Shadows keyword does not require the consent (override ability) of the inherited class to replace the property or method's implementation code. A method does not have to be defined as overridable for the Shadows keyword to work. Read the example...

'This is the Base Class
Public Class Parent
    Public Sub MyProc(ByVal num As Integer)
        MsgBox("Number in Parent is " & num)
    End Sub

    Public Sub MyProc(ByVal st As String)
        MsgBox("String in Parent is " & st)
    End Sub
End Class

'This is the Child Class
Public Class Child
    Inherits Parent
    Overloads Sub MyProc(ByVal num As Integer)
 'overloads the method with the same parameter list
        MsgBox("Number in Child is " & num)
    End Sub

    Overloads Sub MyProc(ByVal ch As Char)      
 ' overloads the method
        MsgBox("Character in Child is " & ch)
    End Sub
End Class

When we execute the following code...
Dim c As New Child()

' prints out "String in Parent is Hello Wazzup!"
c.MyProc("Hello Wazzup!")   

' prints out "Number in Child is 12"
c.MyProc(12)                

' prints out "Character in DerivedClass is B"
c.MyProc(Chr(66))          

When we use Shadows keyword...
Public Class ChildNumber2
    Inherits Parent
    Shadows Sub MyProc(ByVal num As Integer)    
 ' hides all the different argument list
        MsgBox("Number in ChildNumber2 is " & num)
    End Sub
End Class

Dim c2 As New DerivedClass2()
c2.MyProc(7)   'only one method is exposed, rest of the
                      'methods are hidden

Constructor - When a class instance is created in our code,
a special method of that class, called the constructor, is called. Similarly, when the class is destroyed, the destructor
method is called. These are general terms and usually not the actual member names in most object-oriented languages. It is initialized using the keyword New, and is destroyed using the keyword Finalize. In .NET, we tend to forget using Finalize as
the instances(means the object) are automatically destroyed by the Garbage Collecter, when the object is not in use by he CLR(Common Language Runtime).

Dim objSampleObject as New SampleClass
' write the code here...
objsampleobject.Finalize

We can add parameters to the constructors. This was'nt allowed in VB6. We can overload the constructors, change the order of parameters, datatypes of parameters which ultimately change the way the constructor works whenever an instance of that class is invoked.

Also note that a constructor can have any access modifier. If
no argument is added in a constructor, VB.NET adds a no-argument constructor during compile time. It adds the Public Sub New( ) with the class declared, if no argument is passed in the constructor. The following code is added...

Public Class ClassA                            
  Public Sub New( )
End Sub

However, when the constructor is added with parameters, the following code is generated...

Public Class ClassA                            
  Public Sub New(ByVal SomeString as String )
End Sub

When a child class' object is declared, the child class
constructor & its parent class constructor, both are invoked.
Read example below for more clarity...

Public Class Books
  Public Sub New()
    System.Console.WriteLine("Book's constructor.")
  End Sub

  Public Sub myProc()
    System.Console.WriteLine("This is a book.")
  End Sub
End Class

Public Class Authors : Inherits Books
  Public Sub New()
    System.Console.WriteLine("Author's constructor.")
  End Sub
End Class

When the Authors class' constructor is invoked, like in the
following code, the Books class' no-argument constructor is also called.

Dim author As Authors
author = New Authors()

The result on the console will be...

Book's constructor.
Author's constructor.