Object Oriented Programming [OOPS] Concepts

What is a class?
A class is the generic definition of what an object is. A Class describes all the attributes of the object, as well as the methods that implement the behavior of the member object. In other words, class is a template of an object. For ease of understanding a class, we will look at an example. In the class Employee given below, Name and Salary are the attributes of the class Person. The Setter and Getter methods are used to store and fetch data from the variable.

Example

public class Employee
{
private String name;
private String Salary;
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public String getSalary ()
{
return Salary;
}
public void setSalary (String Salary)
{
this. Salary = Salary;
}
}

What is an Object?
An object is an instance of a class. It contains real values instead of variables. For example, let us create an instance of the class Employee called “John”.

Example

Employee John= new Employee();
Now we can access all the methods in the class “Employee” via object “John” as shown below.
John.setName(“XYZ”);

What are the Access Modifiers in C#?
Different Access Modifier are – Public, Private, Protected, Internal, Protected Internal

Public – When a method or attribute is defined as Public, it can be accessed from any code in the project. For example, in the above Class “Employee” getName() and setName() are public.
Private – When a method or attribute is defined as Private, It can be accessed by any code within the containing class only. For example, in the above Class “Employee” attributes name and salary can be accessed within the Class Employee Only. If an attribute or class is defined without access modifiers, it’s default access modifier will be private.
Protected – When attribute and methods are defined as protected, it can be accessed by any method in the inherited classes and any method within the same class. The protected access modifier cannot be applied to classes and interfaces. Methods and fields in a interface can’t be declared protected.
Internal – If an attribute or method is defined as Internal, access is restricted to classes within the current project assembly.
Protected Internal – If an attribute or method is defined as Protected Internal, access is restricted to classes within the current project assembly and types derived from the containing class.

Explain Static Members in C#?

If an attribute’s value had to be same across all the instances of the same class, the static keyword is used. For example, if the Minimum salary should be set for all employees in the employee class, use the following code.

private static double MinSalary = 30000;

To access a private or public attribute or method in a class, at first an object of the class should be created. Then by using the object instance of that class, attributes or methods can be accessed. To access a static variable, we don’t want to create an instance of the class containing the static variable. We can directly refer that static variable as shown below.

double var = Employee.MinSalary ;

What is Reference Type in C#?

Let us explain this with the help of an example. In the code given below,

Employee emp1;
Employee emp2 = new Employee();
emp1 = emp2;
Here emp2 has an object instance of Employee Class. But emp1 object is set as emp2. What this means is that the object emp2 is referred in emp1, rather than copying emp2 instance into emp1. When a change is made in emp2 object, corresponding changes can be seen in emp1 object.

Define Property in C#?

Properties are a type of class member, that are exposed to the outside world as a pair of Methods. For example, for the static field Minsalary, we will Create a property as shown below.

private double minimumSalary;
public static double MinSalary
{
get
{
return minimumSalary;
}
set
{
minimumSalary = value;
}
}

So when we execute the following lines code

double minSal = Employee.MinSalary;

get Method will get triggered and value in minimumSalary field will be returned. When we execute,

Employee. MinSalary = 3000;

set Method will get triggered and value will be stored in minimumSalary field.

Explain Overloading in C#?

When methods are created with the same name, but with different signature its called overloading. For example, WriteLine method in console class is an example of overloading. In the first instance, it takes one variable. In the second instance, “WriteLine” method takes two variable.

Console.WriteLine(x);
Console.WriteLine("The message is {0}", Message);

Different types of overloading in C# are

• Constructor overloading
• Function overloading
• Operator overloading

What is Constructor Overloading in C# .net?

In Constructor overloading, n number of constructors can be created for the same class. But the signatures of each constructor should vary. For example

public class Employee 
{
 public Employee() 
 { }
 public Employee(String Name) 
 { }
}

What is Function Overloading in C# .net ?

In Function overloading, n number of functions can be created for the same class. But the signatures of each function should vary. For example

public class Employee 
{
 public void Employee() 
 { }
 public void Employee(String Name) 
 { }
}

What is Operator Overloading in C# .net ?

We had seen function overloading in the previous example. For operator Overloading, we will have a look at the example given below. We had defined a class rectangle with two operator overloading methods.

class Rectangle
{
 private int Height;
 private int Width;

 public Rectangle(int w,int h)
 {
   Width=w;
   Height=h;
 } 
 public static bool operator >(Rectangle a,Rectangle b)
 {
   return a.Height > b.Height ;
 }
 public static bool operator <(Rectangle a,Rectangle b)
 {
   return a.Height < b.Height ;
 } 
}

Let us call the operator overloaded functions from the method given below. When first if condition is triggered, the first overloaded function in the rectangle class will be triggered. When second if condition is triggered, the second overloaded function in the rectangle class will be triggered.

public static void Main()
{
Rectangle obj1 =new Rectangle();
Rectangle obj2 =new Rectangle();

 if(obj1 > obj2)
 {
  Console.WriteLine("Rectangle1 is greater than Rectangle2");
 } 

 if(obj1 < obj2)
 {
  Console.WriteLine("Rectangle1 is less than Rectangle2");
 }
}

What is Data Encapsulation ?

Data Encapsulation is defined as the process of hiding the important fields from the end user. In the above example, we had used getters and setters to set value for MinSalary. The idea behind this is that, private field “minimumSalary” is an important part of our classes. So if we give a third party code to have complete control over the field without any validation, it can adversely affect the functionality. This is inline with the OOPS Concept that an external user should know about the what an object does. How it does it, should be decided by the program. So if a user set a negative value for MinSalary, we can put a validation in the set method to avoid negative values as shown below

set
{
 if(value > 0)
 {
  minSalary = value;
 }
}

Explain Inheritance in C# ?

In object-oriented programming (OOP), inheritance is a way to reuse code of existing objects. In inheritance, there will be two classes – base class and derived classes. A class can inherit attributes and methods from existing class called base class or parent class. The class which inherits from a base class is called derived classes or child class. For more clarity on this topic, let us have a look at 2 classes shown below. Here Class Car is Base Class and Class Ford is derived class.

class Car
{
 public Car()
 {
  Console.WriteLine("Base Class Car");
 }

 public void DriveType()
 {
  Console.WriteLine("Right Hand Drive");
 }
}

class Ford : Car
{
 public Ford()
 {
  Console.WriteLine("Derived Class Ford");
 }

 public void Price()
 {
  Console.WriteLine("Ford Price : 100K $");
 }
}

When we execute following lines of code ,

Ford CarFord = new Ford();
CarFord.DriveType();
CarFord.Price();

Output Generated is as given below.

Base Class Car
Derived Class Ford
Right Hand Drive
Ford Price : 100K $

What this means is that, all the methods and attributes of Base Class car are available in Derived Class Ford. When an object of class Ford is created, constructors of the Base and Derived class get invoked. Even though there is no method called DriveType() in Class Ford, we are able to invoke the method because of inheriting Base Class methods to derived class.

Can Multiple Inheritance implemented in C# ?

In C#, derived classes can inherit from one base class only. If you want to inherit from multiple base classes, use interface.

What is Polymorphism in C# ?

The ability of a programming language to process objects in different ways depending on their data type or class is known as Polymorphism. There are two types of polymorphism

• Compile time polymorphism. Best example is Overloading
• Runtime polymorphism. Best example is Overridin

Explain the use of Virtual Keyword in C# ?

When we want to give permission to a derived class to override a method in base class, Virtual keyword is used. For example. lets us look at the classes Car and Ford as shown below.

class Car
{
 public Car()
 {
  Console.WriteLine("Base Class Car");
 }
 public virtual void DriveType()
 {
  Console.WriteLine("Right Hand Drive");
 }
}

class Ford : Car
{
 public Ford()
 {
  Console.WriteLine("Derived Class Ford");
 }
 public void Price()
 {
  Console.WriteLine("Ford Price : 100K $");
 }
 public override void DriveType()
 {
  Console.WriteLine("Right Hand ");
 }
}

When following lines of code get executed

Car CarFord = new Car();
CarFord.DriveType();
CarFord = new Ford();
CarFord.DriveType();

Output is as given below.

Base Class Car
Right Hand Drive
Base Class Car
Derived Class Ford
Right Hand