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Classes and Objects

C# OOP

C# - What is OOP?

OOP stands for Object-Oriented Programming.

Procedural programming is about writing procedures or methods that perform operations on the data, while object-oriented programming is about creating objects that contain both data and methods.

Object-oriented programming has several advantages over procedural programming:

  • OOP is faster and easier to execute
  • OOP provides a clear structure for the programs
  • OOP helps to keep the C# code DRY "Don't Repeat Yourself", and makes the code easier to maintain, modify and debug
  • OOP makes it possible to create full reusable applications with less code and shorter development time

Tip: The "Don't Repeat Yourself" (DRY) principle is about reducing the repetition of code. You should extract out the codes that are common for the application, and place them at a single place and reuse them instead of repeating it.

C# - What are Classes and Objects?

Classes and objects are the two main aspects of object-oriented programming.

Look at the following illustration to see the difference between class and objects:

class

Fruit

objects

Apple

Banana

Mango

Another example:

class

Car

objects

Volvo

Audi

Toyota

So, a class is a template for objects, and an object is an instance of a class.

When the individual objects are created, they inherit all the variables and methods from the class.


C# Classes and Objects

Classes and Objects

You learned from the previous chapter that C# is an object-oriented programming language.

Everything in C# is associated with classes and objects, along with its attributes and methods. For example: in real life, a car is an object. The car has attributes, such as weight and color, and methods, such as drive and brake.

A Class is like an object constructor, or a "blueprint" for creating objects.

Create a Class

To create a class, use the class keyword:

Create a class named "Car" with a variable color:

class Car 
		{
		  string color = "red";
		}

When a variable is declared directly in a class, it is often referred to as a field (or attribute).

It is not required, but it is a good practice to start with an uppercase first letter when naming classes. Also, it is common that the name of the C# file and the class matches, as it makes our code organized. However it is not required (like in Java).

Create an Object

An object is created from a class. We have already created the class named Car, so now we can use this to create objects.

To create an object of Car, specify the class name, followed by the object name, and use the keyword new:

Example

Create an object called "myObj" and use it to print the value of color:

class Car 
		{
		  string color = "red";

		  static void Main(string[] args)
		{
		    Car myObj = new Car();
		    Console.WriteLine(myObj.color);
		  }
		}
1

Note that we use the dot syntax (.) to access variables/fields inside a class (myObj.color). You will learn more about fields in the next chapter.

Multiple Objects

You can create multiple objects of one class:

Example

Create two objects of Car:

class Car
		{
		string color = "red";
		static void Main(string[] args)
		{
		Car myObj1 = new Car();
		Car myObj2 = new Car();
		Console.WriteLine(myObj1.color);
		Console.WriteLine(myObj2.color);
		}
		}
2

Using Multiple Classes

You can also create an object of a class and access it in another class. This is often used for better organization of classes (one class has all the fields and methods, while the other class holds the Main() method (code to be executed)).

  • prog2.cs
  • prog.cs

prog2.cs

class Car 
		{
		  public string color = "red";
		}

prog.cs

class Program
		{
		static void Main(string[] args)
		{
		Car myObj = new Car();
		Console.WriteLine(myObj.color);
		}
		}
3

Did you notice the public keyword? It is called an access modifier, which specifies that the color variable/field of Car is accessible for other classes as well, such as Program.

You will learn much more about access modifiers and classes/objects in the next chapters.


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