In OOP, you derive objects from classes. In this section, we'll take a closer look at what a class actually is.

Classes are one of the most important concepts in OOP. You can say they are the building blocks of OOP. A class can be described as the blueprint of an object.

A class is like a template or blueprint that tells us what properties and behaviors an instance of this class will have. In most circumstances, a class itself can't actually do anything—it is just used to create objects. Let's look at an example to demonstrate what I am saying. Let's say we have a Human class. Here, when we say Human, we don't mean any particular person, but we are referring to a human being in general. A human that has two hands, two legs, and a mouth, and which can also walk, talk, eat, and think. These properties and their behaviors are applicable...

We now know what a class is. Now let's look at what object refers to in OOP.

An object is an instance of a class. In other words, an object is an implementation of a class. For example, in our banking application, we have a Customer class, but that doesn't mean that we actually have a customer in our application. To create a customer, we have to create an object of the Customer class. Let's say that we have a customer called Mr. Jack Jones. For this customer, we have to create an object of the Customer class, where the name of the person is Jack Jones.

As Mr. Jones is our customer, this means that he also has an account in our bank. To create a bank account for Mr. Jones, we have to create an object of the BankAccount class.

In the previous code, you might have noticed that we created a few variables. A variable is something that varies, which means it is not constant. In programming, when we create a variable, the computer actually allocates a space in memory for it so that a value of the variable can be stored there.

Let's assign some values to the variables of the objects that we created in the previous section. We will first work with the customer1 object, as shown in the following code:

using System;

namespace Chapter2
{
    public class Code_2_2
    {
        static void Main(string[] args)
        {
            Customer customer1 = new Customer();
            customer1.firstName = "Molly";
            customer1.lastName = "Dolly";
            customer1.phoneNumber = "98745632";
            customer1.emailAddress = "mollydolly@email.com";

   ...

Let's talk about another important topic—namely methods. A method is a piece of code that is written in the code file and can be reused. A method can hold many lines of code, which will be executed when it is called. Let's take a look at the general form of a method:

access-modifier return-type method-name(parameter-list) {
    // method body
}

We can see that the first thing in the method declaration is an access-modifier. This will set the access permission of the method. Then, we have the return-type of the method, which will hold the type that the method will return, such as string, int, double, or another type. After that, we have the method-name and then brackets, (), which indicate that it is a method. In the brackets, we have the parameter-list. This can either be empty or can contain one or more parameters. Finally, we have curly brackets...

OOP is one of the most important programming methodologies nowadays. The whole concept depends on four main ideas, which are known as the pillars of OOP. These four pillars are as follows:

• Inheritance
• Encapsulation
• Polymorphism
• Abstraction

The word inheritance means receiving or deriving something from something else. In real life, we might talk about a child inheriting a house from his or her parents. In that case, the child has the same power over the house that his parents had. This concept of inheritance is one of the pillars of OOP. In programming, when one class is derived from another class, this is called inheritance. This means that the derived class will have the same properties...

This chapter covers classes and objects, the most important building blocks of OOP. These are the two things that we should learn before jumping into any other topics in OOP. It is important to make sure that these concepts are clear in our minds before moving on to other ideas. In this chapter, we learned about what a class is and why it's needed in OOP. We also looked at how to create a class in C# and how to define an object. After that, we looked at the relationship between classes and objects and how to instantiate a class and use it. We also talked about variables and methods in a class. Lastly, we covered the four pillars of OOP. In the next chapter, we will learn more about inheritance and class hierarchy.