C# OOPS Interview Questions and Answers
1) What is a class ?
A class is the generic definition of what an object is . A Class describes all the attributes of object, as well as the methods that implements the behavior of member object. That means, class is a template of an object. Easy way to understand a class is to look at an example . In the class Employee given below, Name and Salary are the attributes of the class Person , Setter and Getter methods are used to store and fetch data from the variable.
A class is the generic definition of what an object is . A Class describes all the attributes of object, as well as the methods that implements the behavior of member object. That means, class is a template of an object. Easy way to understand a class is to look at an example . In the class Employee given below, Name and Salary are the attributes of the class Person , Setter and Getter methods are used to store and fetch data from the variable.
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;
}
}
{
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;
}
}
2) What is an Object?
Object is an instance of a class, it contains real values instead of variables. For example, lets create an instance of class Employee called “Jhon”.
Employee sss = new Employee();
Now we can
access all the methods in the class “Employee” via object “sss” as shown
below.
sss.setName(“XYZ”);
3) What are the Access Modifiers in C# ?
Different Access Modifier are - Public, Private, Protected, Internal, Protected Internal
sss.setName(“XYZ”);
3) 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 project. For example in the above Class “Employee” , getName(), setName() etc are public.
Private - When a method or attribute is defined as Private , It can be accessed by any code within the containing type only. For example in the above Class “Employee” , attributes name and salary can be accessed with in the Class Employee Only. If an attribute or class are defined without access modifiers , its default access modifier will be private.
Protected - When an attribute and methods are defined as protected, it can be accessed by any method in inherited classes and any method within the same class. The protected access modifier cannot be applied to class and interfaces. Methods and fields in a interface cannot 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.
4) Define Static Members in C# ?
If an attributes value had to be same across all the instances of the same class , 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 attributes or methods in a class, at first an object of the
class should be created . Then using that instance of class , attributes or
methods can be accessed. To access a static variable, we don't want to create
an instance of the class having the static varibale . We can directly refer
that static variable as shown below.
double var = Employee.MinSalary ;
5) Define Reference Types in C# ?
Let us explain this with an example . For the code given below,
double var = Employee.MinSalary ;
5) Define Reference Types in C# ?
Let us explain this with an example . For the code given below,
Employee
emp1 ;
Employee emp2 = new Employee();
emp1 = emp2;
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 object emp2 is refereed in emp1 and not that emp2 is copied into emp1. When a change is made in emp2 object, corresponding changes can be seen in emp1 object.
6) Define
Property in C# ?
Properties are a type of class member , that are exposed to outside world as a pair of Methods.For example for the static field Minsalary, we will Create a property.
Properties are a type of class member , that are exposed to outside world as a pair of Methods.For example for the static field Minsalary, we will Create a property.
private
double minimumSalary ;
public static double MinSalary
{
get
{
return minSalary;
}
set
{
minSalary = value;
}
}
public static double MinSalary
{
get
{
return minSalary;
}
set
{
minSalary = 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 .
7) Define Overloading in C# ?
When methods are created with same name , but with different signature its called overloading.
For example , WriteLine method in console class is an example for overloading. In 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 type of overloading in C# are
1)Constructor overloading
2)Function overloading.
3)Operator overloading
8) Define Constructor Overloading in C# .net ?
In Constructor overloading, n number of constructors can be created for same class. But the signatures of each constructor should vary. For example
public class
Employee
{
public Employee()
{ }
public Employee(String Name)
{ }
}
{
public Employee()
{ }
public Employee(String Name)
{ }
}
9) Define Function Overloading in C# .net ?
In Function overloading, n number of functions can be created for same class. But the signatures of each function should vary. For example
public class
Employee
{
public void Employee()
{ }
public void Employee(String Name)
{ }
}
{
public void Employee()
{ }
public void Employee(String Name)
{ }
}
10) Define Operator Overloading in C# .net ?
We had seen function overloading in the previous example.For operator Overloading , we will have look at the example below. We define 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 ;
}
}
{
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 below. When first if condition is triggered, first overloaded function in the rectangle class will be triggered. When second if condition is triggered, 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");
}
}
{
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");
}
}
11) Define
Data Encapsulation in c# ?
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. 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 in line with 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 set method to avoid negative values as shown below
set
{
if(value > 0)
{
minSalary = value;
}
}
12) Define 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 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 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 is 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.
13) Define Multiple Inheritance in C# ?
In C# , derived classes can inherit from only one base class. If you want inherit multiple base classes, use interface.
14) What is Polymorphism in C#?
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
1) Compile time polymorphism. Best example is Overloading
2) Runtime polymorphism. Best example is Overriding
15) Define Virtual Keyword in C#?
When we want to give permission to 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
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. 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 in line with 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 set method to avoid negative values as shown below
set
{
if(value > 0)
{
minSalary = value;
}
}
12) Define 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 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 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 is 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.
13) Define Multiple Inheritance in C# ?
In C# , derived classes can inherit from only one base class. If you want inherit multiple base classes, use interface.
14) What is Polymorphism in C#?
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
1) Compile time polymorphism. Best example is Overloading
2) Runtime polymorphism. Best example is Overriding
15) Define Virtual Keyword in C#?
When we want to give permission to 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
16) Define
overriding in c# ?
To overide a base class method which is defined as virtual , Overide keyword is used. In the above example , method DriveType is overrided in derived class.
17) Define Method Hiding in C# ?
If the derived class doesn't want to use methods in base class , derived class can implement the same method in derived class with same signature. For example in the classes given below, DriveType() is implemented in the derived class with same signature. This is called Method Hiding.
class Car
{
public void DriveType()
{
Console.WriteLine("Right Hand Drive");
}
}
class Ford : Car
{
public void DriveType()
{
Console.WriteLine("Right Hand ");
}
}
18) Define Abstract Class in C#?
If we don't want a class object to be created define the class as abstract. An abstract class can have abstract and non abstract classes. If a method in abstract id defined as abstract , it must be implemented in derived class. For example , in the classes given below , method DriveType is defined as abstract.
abstract class Car
{
public Car()
{
Console.WriteLine("Base Class Car");
}
public abstract void DriveType();
}
class Ford : Car
{
public void DriveType()
{
Console.WriteLine("Right Hand ");
}
}
Method DriveType get implemented in derived class.
19) Define Sealed Classes in c# ?
If a class is defined as Sealed it cannot be inherited in derived class. Example is given below.
public sealed class Car
{
public Car()
{
Console.WriteLine("Base Class Car");
}
public void DriveType()
{
Console.WriteLine("Right Hand ");
}
}
20) Define Interfaces in C# ?
An interface is similar to a class with method signatures. There wont be any implementation of the methods in Interface. Classes which implements interface should have implementation of methods defined in abstract class.
interface Breaks
{
void BreakType();
}
interface Wheels
{
void WheelType();
}
class Ford : Breaks, Wheels
{
public Ford()
{
Console.WriteLine("Derived Class Ford");
}
public void Price()
{
Console.WriteLine("Ford Price : 100K $");
}
public void BreakType()
{
Console.WriteLine("Power Break");
}
public void WheelType()
{
Console.WriteLine("Bridgestone");
}
}
21) Define Constructor in C# ?
Constructor is a special method that get invoked / called automatically whenever an object of a given class gets instantiated.
For example in our class car constructor is defined as shown below
public Car()
{
Console.WriteLine("Base Class Car");
}
When ever an instance of class car is created from the same class or its derived class(Except Few Scenarios) Constructor get called and sequence of code written in the constructor get executed.
22) Define Destructor in C# ?
Destructor is a special method that get invoked / called automatically whenever an object of a given class gets destroyed. Main idea behind using destructor is to free the memory used by the object.
To overide a base class method which is defined as virtual , Overide keyword is used. In the above example , method DriveType is overrided in derived class.
17) Define Method Hiding in C# ?
If the derived class doesn't want to use methods in base class , derived class can implement the same method in derived class with same signature. For example in the classes given below, DriveType() is implemented in the derived class with same signature. This is called Method Hiding.
class Car
{
public void DriveType()
{
Console.WriteLine("Right Hand Drive");
}
}
class Ford : Car
{
public void DriveType()
{
Console.WriteLine("Right Hand ");
}
}
18) Define Abstract Class in C#?
If we don't want a class object to be created define the class as abstract. An abstract class can have abstract and non abstract classes. If a method in abstract id defined as abstract , it must be implemented in derived class. For example , in the classes given below , method DriveType is defined as abstract.
abstract class Car
{
public Car()
{
Console.WriteLine("Base Class Car");
}
public abstract void DriveType();
}
class Ford : Car
{
public void DriveType()
{
Console.WriteLine("Right Hand ");
}
}
Method DriveType get implemented in derived class.
19) Define Sealed Classes in c# ?
If a class is defined as Sealed it cannot be inherited in derived class. Example is given below.
public sealed class Car
{
public Car()
{
Console.WriteLine("Base Class Car");
}
public void DriveType()
{
Console.WriteLine("Right Hand ");
}
}
20) Define Interfaces in C# ?
An interface is similar to a class with method signatures. There wont be any implementation of the methods in Interface. Classes which implements interface should have implementation of methods defined in abstract class.
interface Breaks
{
void BreakType();
}
interface Wheels
{
void WheelType();
}
class Ford : Breaks, Wheels
{
public Ford()
{
Console.WriteLine("Derived Class Ford");
}
public void Price()
{
Console.WriteLine("Ford Price : 100K $");
}
public void BreakType()
{
Console.WriteLine("Power Break");
}
public void WheelType()
{
Console.WriteLine("Bridgestone");
}
}
21) Define Constructor in C# ?
Constructor is a special method that get invoked / called automatically whenever an object of a given class gets instantiated.
For example in our class car constructor is defined as shown below
public Car()
{
Console.WriteLine("Base Class Car");
}
When ever an instance of class car is created from the same class or its derived class(Except Few Scenarios) Constructor get called and sequence of code written in the constructor get executed.
22) Define Destructor in C# ?
Destructor is a special method that get invoked / called automatically whenever an object of a given class gets destroyed. Main idea behind using destructor is to free the memory used by the object.
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