What is the difference between a class and an object in C#?

When you define a class, you define a blueprint or template for a data type. The object is an instance of that class − a concrete implementation created from the blueprint. A class defines the structure and behavior, while objects are the actual entities that hold data and perform actions.

The methods and variables that constitute a class are called members of the class. To access class members, you use the dot (.) operator after the object name. The dot operator links the name of an object with the name of a member.

Class vs Object

Syntax

Following is the syntax for creating a class and instantiating objects −

// Class definition
class ClassName {
   // fields, properties, methods
}

// Object instantiation
ClassName objectName = new ClassName();

Following is the syntax for accessing class members using the dot operator −

objectName.fieldName = value;     // Access field
objectName.MethodName();          // Call method

Example

using System;

class Box {
   private double length;
   private double breadth;
   private double height;

   public void setLength(double len) {
      length = len;
   }

   public void setBreadth(double bre) {
      breadth = bre;
   }

   public void setHeight(double hei) {
      height = hei;
   }

   public double getVolume() {
      return length * breadth * height;
   }
}

class Program {
   static void Main(string[] args) {
      // Creating two objects from the same class
      Box Box1 = new Box();
      Box Box2 = new Box();
      double volume;

      // Box1 specifications
      Box1.setLength(6.0);
      Box1.setBreadth(7.0);
      Box1.setHeight(5.0);

      // Box2 specifications
      Box2.setLength(12.0);
      Box2.setBreadth(13.0);
      Box2.setHeight(10.0);

      // Calculate and display volumes
      volume = Box1.getVolume();
      Console.WriteLine("Volume of Box1: {0}", volume);

      volume = Box2.getVolume();
      Console.WriteLine("Volume of Box2: {0}", volume);
   }
}

The output of the above code is −

Volume of Box1: 210
Volume of Box2: 1560

Using Constructor for Object Initialization

using System;

class Box {
   private double length, breadth, height;

   public Box(double l, double b, double h) {
      length = l;
      breadth = b;
      height = h;
   }

   public double getVolume() {
      return length * breadth * height;
   }

   public void displayInfo() {
      Console.WriteLine("Dimensions: {0} x {1} x {2}", length, breadth, height);
   }
}

class Program {
   static void Main(string[] args) {
      Box smallBox = new Box(3.0, 4.0, 5.0);
      Box largeBox = new Box(10.0, 15.0, 8.0);

      smallBox.displayInfo();
      Console.WriteLine("Volume: {0}", smallBox.getVolume());

      largeBox.displayInfo();
      Console.WriteLine("Volume: {0}", largeBox.getVolume());
   }
}

The output of the above code is −

Dimensions: 3 x 4 x 5
Volume: 60
Dimensions: 10 x 15 x 8
Volume: 1200

Conclusion

A class is a blueprint that defines the structure and behavior, while an object is a concrete instance of that class with actual data. You can create multiple objects from a single class, each maintaining its own state and capable of performing the defined operations independently.