OOP in C# Classes and Objects.

C#
LECTURE#9
Abid Kohistani
GC Madyan Swat

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.

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
Apple
objects
Vehicle Class
Audie object

What are Classes and Objects?
◦ 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.
◦ You will learn much more about classes and objects in the next chapter.

Classes and Objects
◦ 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 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";
}
Example
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:Example
class Car
{
static void Main(string[] args)
{
Car myObj = new Car();
Console.WriteLine(myObj.color);
}
}
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.
You can create multiple objects
of one class:
class Car
{
{
Car myObj1 = new Car();
Car myObj2 = new Car();
Console.WriteLine(myObj1.color);
Console.WriteLine(myObj2.color);
}
}

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)).
Example
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.
Class calling car class object
class Car
{
public string color = "red";
}
class Program
{
{
}
}

Class Members
Fields and methods inside classes are often referred to as "Class Members":
Create a Car class with three class members: two fields and one method.
Example
// The class
class MyClass
{
// Class members
string color = "red";// field
int maxSpeed = 200;// field
public void fullThrottle() // method
{
Console.WriteLine("The car is going as fast as it can!");
}
}
In the previous chapter, you learned that variables inside a class are called fields, and
that you can access them by creating an object of the class, and by using the
dot syntax (.).
The following example will create an object of the Car class, with the name myObj.
Then we print the value of the fields color and maxSpeed:
Fields
class Car
{
int maxSpeed = 200;
{
Console.WriteLine(myObj.maxSpeed);
}
}

Example with blank fields
class Car
{
string model;
string color;
int year;
{
Car Ford = new Car();
Ford.model = "Mustang";
Ford.color = "red";
Ford.year = 1969;
Car Opel = new Car();
Opel.model = "Astra";
Opel.color = "white";
Opel.year = 2005;
Console.WriteLine(Ford.model);
Console.WriteLine(Opel.model);
}
}

Object Methods
Methods normally belongs to a class, and they define how an object of a class behaves.
Just like with fields, you can access methods with the dot syntax. However, note that the method must be public. And remember that we use
the name of the method followed by two parentheses ( ) and a semicolon ; to call (execute) the method:
Why did we declare the method as public, and not
static, like in the examples from the C# Methods
Chapter?
The reason is simple: a static method can be accessed
without creating an object of the class, while public
methods can only be accessed by objects.
class Car
{
string color; // field
int maxSpeed; // field
public void fullThrottle() // method
{
}
{
myObj.fullThrottle(); // Call the method
}
}

Use Multiple Classes
Remember from the last chapter, that we can use multiple classes for better organization (one for fields and methods, and another one for
execution). This is recommended:
The public keyword is called an access modifier, which specifies that the fields of
Car are accessible for other classes as well, such as Program..
class Car
{
public string model;
public string color;
public int year;
public void fullThrottle()
{
}
}
class Program
{
{
Car Ford = new Car();
Ford.model = "Mustang";
Ford.color = "red";
Ford.year = 1969;
Car Opel = new Car();
Opel.model = "Astra";
Opel.color = "white";
Opel.year = 2005;
Console.WriteLine(Ford.model);
Console.WriteLine(Opel.model);
}
}
Example

OOP in C# Classes and Objects.

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