As a fresher stepping into the world of Object-Oriented Programming (OOP) with C#, it’s essential to be well-prepared for interviews. Understanding OOP concepts in C# can significantly boost your confidence and competence. Here are the top 20 OOP interview questions along with their answers to help you ace your next interview.
OOPs interview questions answers
1. What is Object-Oriented Programming?
Object-Oriented Programming (OOP) is a programming paradigm that uses “objects” to design applications and programs. Objects are instances of classes, which encapsulate data and methods. The four main principles of OOP are encapsulation, abstraction, inheritance, and polymorphism.
2. What is a Class in C#?
A class in C# is a blueprint for creating objects. It defines the data and behaviour of the objects. For example:
public class Car
{
public string Brand { get; set; }
public string Model { get; set; }
public void DisplayInfo()
{
Console.WriteLine($"Car: {Brand} {Model}");
}
}
3. What is an Object in C#?
An object is an instance of a class. It is a concrete entity based on a class that can have unique values for its attributes. For example:
Car myCar = new Car();
myCar.Brand = "Toyota";
myCar.Model = "Corolla";
myCar.DisplayInfo(); // Output: Car: Toyota Corolla
4. Explain Encapsulation with an Example.
Encapsulation is the concept of bundling data and methods that operate on that data within one unit (class) and restricting access to some of the object’s components. This is done using access modifiers.
public class BankAccount
{
private decimal _balance;
public BankAccount(decimal initialBalance)
{
_balance = initialBalance;
}
public void Deposit(decimal amount)
{
_balance += amount;
}
public decimal GetBalance()
{
return _balance;
}
}
BankAccount account = new BankAccount(1000);
account.Deposit(500);
Console.WriteLine(account.GetBalance()); // Output: 1500
5. What is Inheritance in C#?
Inheritance is a mechanism where one class (derived class) can inherit the attributes and methods of another class (base class). This allows for code reuse and the creation of a hierarchical relationship between classes.
public class Vehicle
{
public string Brand { get; set; }
public void DisplayBrand()
{
Console.WriteLine($"Brand: {Brand}");
}
}
public class Car : Vehicle
{
public string Model { get; set; }
public void DisplayInfo()
{
Console.WriteLine($"Car: {Brand} {Model}");
}
}
Car myCar = new Car();
myCar.Brand = "Toyota";
myCar.Model = "Corolla";
myCar.DisplayInfo(); // Output: Car: Toyota Corolla
6. What is Polymorphism in C#?
Polymorphism allows methods to do different things based on the object it is acting upon. It can be achieved through method overriding and method overloading.
public class Animal
{
public virtual void Speak()
{
Console.WriteLine("Animal speaks");
}
}
public class Dog : Animal
{
public override void Speak()
{
Console.WriteLine("Dog barks");
}
}
public class Cat : Animal
{
public override void Speak()
{
Console.WriteLine("Cat meows");
}
}
Animal[] animals = { new Dog(), new Cat() };
foreach (Animal animal in animals)
{
animal.Speak();
}
// Output:
// Dog barks
// Cat meows
7. What is Abstraction in C#?
Abstraction is the concept of hiding the complex implementation details and showing only the essential features of the object. It can be achieved using abstract classes and interfaces.
public abstract class Shape
{
public abstract double Area();
}
public class Circle : Shape
{
private double _radius;
public Circle(double radius)
{
_radius = radius;
}
public override double Area()
{
return 3.14 * _radius * _radius;
}
}
Circle circle = new Circle(5);
Console.WriteLine(circle.Area()); // Output: 78.5
8. What is the difference between an Abstract Class and an Interface in C#?
- An abstract class can have method implementations and member variables, but an interface can only have abstract methods (without implementation) and constants.
- A class can inherit from multiple interfaces but only from one abstract class.
9. What is Method Overloading in C#?
Method overloading is the ability to define multiple methods with the same name but with different parameters within the same class.
public class Math
{
public int Add(int a, int b)
{
return a + b;
}
public int Add(int a, int b, int c)
{
return a + b + c;
}
}
Math math = new Math();
Console.WriteLine(math.Add(2, 3)); // Output: 5
Console.WriteLine(math.Add(2, 3, 4)); // Output: 9
10. What is Method Overriding in C#?
Method overriding allows a derived class to provide a specific implementation of a method that is already defined in its base class.
public class Animal
{
public virtual void Speak()
{
Console.WriteLine("Animal speaks");
}
}
public class Dog : Animal
{
public override void Speak()
{
Console.WriteLine("Dog barks");
}
}
Dog dog = new Dog();
dog.Speak(); // Output: Dog barks
11. What is a Constructor in C#?
A constructor is a special method that is automatically called when an object of a class is created. It initializes the object’s attributes.
public class Car
{
public string Brand { get; set; }
public string Model { get; set; }
public Car(string brand, string model)
{
Brand = brand;
Model = model;
}
public void DisplayInfo()
{
Console.WriteLine($"Car: {Brand} {Model}");
}
}
Car myCar = new Car("Toyota", "Corolla");
myCar.DisplayInfo(); // Output: Car: Toyota Corolla
12. What is the Difference Between ‘==’ and ‘Equals()’ in C#?
In C#, == checks if two object references are the same (reference equality), while Equals() checks if the two objects are equal based on their values (value equality).
string s1 = new string("hello");
string s2 = new string("hello");
Console.WriteLine(s1 == s2); // Output: True
Console.WriteLine(s1.Equals(s2)); // Output: True
13. What are Access Modifiers in C#?
Access modifiers define the visibility and accessibility of classes, methods, and variables. Common access modifiers are:
- private: Accessible only within the same class.
- protected: Accessible within the same class and its derived classes.
- public: Accessible from any other class.
- internal: Accessible within the same assembly.
14. What is a Static Method in C#?
A static method belongs to the class rather than any object instance and can be called without creating an instance of the class. It can access static data members and change their values.
public class Math
{
public static int Add(int a, int b)
{
return a + b;
}
}
Console.WriteLine(Math.Add(5, 3)); // Output: 8
15. What is Multiple Inheritance and Does C# Support it?
Multiple inheritance is when a class can inherit from more than one class. C# does not support multiple inheritance directly to avoid complexity and ambiguity (Diamond Problem). However, it can be achieved using interfaces.
16. Explain the Diamond Problem in Multiple Inheritance.
The Diamond Problem occurs when a class inherits from two classes that have a common ancestor, leading to ambiguity. For example, if both classes have a method with the same name, the subclass doesn’t know which method to inherit.
17. What is an Interface in C#?
An interface in C# is a reference type, similar to a class, that can contain only abstract methods (without implementation) and constants. It is used to specify a contract that classes must implement.
public interface IAnimal
{
void Speak();
}
public class Dog : IAnimal
{
public void Speak()
{
Console.WriteLine("Dog barks");
}
}
18. What is a Final Class and Final Method in C#?
C# does not have a keyword called final. However, to achieve similar functionality, the sealed keyword is used to prevent a class from being inherited and sealed methods to prevent method overriding.
public sealed class MathUtils
{
// Methods here
}
public class ExtendedMathUtils : MathUtils
{
// Compilation error
}
19. What is an Object and How is it Different from a Class?
A class is a blueprint or template for creating objects. An object is an instance of a class that contains actual values and states defined by the class.
20. What is the Role of the ‘base’ Keyword in Inheritance?
The base keyword in C# is used to refer to the immediate parent class object. It is used to call the parent class’s methods and constructors.
public class Animal
{
public virtual void Eat()
{
Console.WriteLine("Animal eats");
}
}
public class Dog : Animal
{
public override void Eat()
{
base.Eat();
Console.WriteLine("Dog eats");
}
}
Dog dog = new Dog();
dog.Eat();
// Output:
// Animal eats
// Dog eats
Conclusion
Understanding these fundamental OOP concepts in C# and articulating them clearly will greatly enhance your interview performance. OOP is a cornerstone of modern software development, and mastering it is essential for a successful career in programming. Good luck with your interviews! If you want to prepare for C# Interview or asp.net interview, click the proper link.
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