04 Feb C# Classes and Objects

In this lesson, we will understand what is a class and object in C#. With that, we will also see some examples of creating classes and objects. We have covered the following topics:

• What is a Class
• What is an Object
• How to create an Object
• Example – Classes & Objects

What is a class

In C#, a class is a template for an object, whereas an object is an instance of a class. Consider, a class as a blueprint.

To create a class is to declare in C#. At first, to declare a class, use the class keyword in C#. The class is a reserved word in C#.

Let us now see how to define a class in C#. In the below example, the class members are length and width:

class Rectangle {
  //  The data members or class members or member variables or fields
  double length;
  double width;
}

What is an Object

An object is an instance of a class i.e., an object is created from a class. Object represents real-life entities, for example, a Bike is an object. Object has State, Behavior, and Identity:

• Identity: Name of Bike
• State (Attributes): The data of object Bike i.e., Color, Model, Weight, etc.
• Behavior (Methods): Behavior of object Bike like to Drive, Brake

Let us now see the representation, with Bike as an object:

Create an Object

An object is a runtime entity with state and behavior. To create an object in C#, we use the new operator. The syntax is as follows:

ClassName obj = new ClassName()

Above, the obj is our object created using the new operator.

Let us see the above syntax and create three objects in C#:

// Objects of class Rectangle
Rectangle rct1 = new Rectangle();
Rectangle rct2 = new Rectangle();
Rectangle rct3 = new Rectangle();

Example: Classes and Objects

Let us see the complete example to create a class and object. In this example, we will also learn how to access the fields of the class using the dot syntax:

using System;

namespace Demo
{
  class Rectangle
  {
     /// The data members or class members or member variables or fields
     double length;
     double width;

    static void Main(string[] args)
    {
     // Creating an object rct
     // Declaring rct of type Rectangle
     Rectangle rct = new Rectangle();
     
     double area;
     
     // Accessing fields using the dot syntax 
     // Set the values of length and width
     rct.length = 10;
     rct.width = 12;
     
     Console.WriteLine("Length of Rectangle = {0}",rct.length);
     Console.WriteLine("Width of Rectangle = {0}",rct.width); 
     
     // Calculating Area of Rectangle     
     area = rct.length * rct.width;
 
     Console.WriteLine("\nArea of Rectangle = {0}",area);
    }
  }
}

Output

Length of Rectangle = 10
Width of Rectangle = 12

Area of Rectangle = 120