Object-Oriented Programming (OOPs) is a fundamental concept in C# that every developer must master for technical interviews. Whether you’re a beginner or an experienced programmer, having a strong grasp of OOPs principles like encapsulation, inheritance, polymorphism, and abstraction is essential to crack C# interviews.
In this blog, we have compiled 50+ commonly asked OOPs interview questions with detailed answers to help you prepare effectively. These questions cover basic to advanced OOPs concepts, ensuring you are well-prepared for both freshers’ and experienced developer interviews.
What You’ll Learn in This Blog:
- Fundamental OOPs concepts in C# (Encapsulation, Inheritance, Polymorphism, Abstraction)
- Real-world examples and practical coding scenarios
- Common interview questions from top tech companies
- Best practices for writing object-oriented code in C#
Prepare confidently with these expert-curated C# OOPs interview questions and answers and boost your chances of success!
1.What are the main concepts of OOPS? What are classes and objects?
Main Concepts of OOPS (Object-Oriented Programming System)
Object-Oriented Programming (OOP) is a programming paradigm based on the concept of objects and classes. It helps in organizing code efficiently and makes it reusable, scalable, and maintainable. The four main concepts of OOP are:
1.Encapsulation
Encapsulation is the process of wrapping data (variables) and code (methods) together in a single unit, called a class. It restricts direct access to some components, which improves security and reduces complexity.
2.Abstraction
Abstraction is the concept of hiding unnecessary details and exposing only the relevant information. It simplifies the implementation and enhances code readability.
3.Inheritance
Inheritance allows one class (child class) to acquire the properties and behaviors of another class (parent class). It promotes code reusability and reduces redundancy.
4.Polymorphism
Polymorphism means “many forms.” It allows a single function, method, or operator to work in different ways. It can be achieved through method overloading (same method name with different parameters) and method overriding (redefining a method in a derived class).
What Are Classes and Objects?
Class
A class is a blueprint or template for creating objects. It defines the structure and behavior of an object, including its attributes (variables) and methods (functions).
Example of a Class in C#:
public class Car
{
public string Brand;
public string Model;
public void DisplayInfo()
{
Console.WriteLine($"Car: {Brand} {Model}");
}
}Object
An object is an instance of a class. It represents a real-world entity with specific values assigned to its attributes.
Example of Creating an Object in C#:
Car myCar = new Car();
myCar.Brand = "Toyota";
myCar.Model = "Camry";
myCar.DisplayInfo(); 2.What is Inheritance? Why Inheritance is important?
What is Inheritance in OOP?
Inheritance is a core concept of Object-Oriented Programming (OOP) that allows a child class to inherit properties and behaviors from a parent class. It promotes code reusability and establishes a relationship between classes.
Example of Inheritance in C#:
public class Animal
{
public void MakeSound()
{
Console.WriteLine("Animal makes a sound");
}
}
public class Dog : Animal
{
}
Dog myDog = new Dog();
myDog.MakeSound(); // Output: Animal makes a soundWhy is Inheritance Important?
- Code Reusability – Avoids code duplication by reusing parent class features.
- Improved Maintainability – Changes in the parent class reflect in child classes.
- Better Organization – Creates a logical hierarchy in the program.
- Extensibility – Easily add new features without modifying existing code.
3.What are the different types of Inheritance?
Inheritance in Object-Oriented Programming (OOP) allows a class to derive properties and methods from another class. The main types of inheritance are:
1.Single Inheritance
A child class inherits from a single parent class.
public class Animal { }
public class Dog : Animal { }2.Multiple Inheritance (Not Supported in C#)
A child class inherits from multiple parent classes. (C# supports multiple inheritance using interfaces.)
public interface IWalk { }
public interface IRun { }
public class Dog : IWalk, IRun { }3.Multilevel Inheritance
A class inherits from another derived class.
public class Animal { }
public class Mammal : Animal { }
public class Dog : Mammal { }4.Hierarchical Inheritance
Multiple child classes inherit from a single parent class.
public class Animal { }
public class Dog : Animal { }
public class Cat : Animal { }5. Hybrid Inheritance (Combination of different types, achieved via interfaces in C#)
4.How to prevent a class from being Inherited?
In C#, you can prevent a class from being inherited using the sealed keyword. A sealed class cannot be extended by any other class.
public sealed class FinalClass
{
public void ShowMessage()
{
Console.WriteLine("This class cannot be inherited!");
}
}
// This will cause an error
// public class DerivedClass : FinalClass { }Other Ways to Restrict Inheritance:
1.Sealing Methods – Use sealed with override to prevent further overriding.
public class BaseClass
{
public virtual void Show() {}
}
public class DerivedClass : BaseClass
{
public sealed override void Show() {}
} 2.Private Constructors – Prevents object instantiation and inheritance.
public class NoInheritance
{
private NoInheritance() {}
} 5.What is Abstraction?
Abstraction is an Object-Oriented Programming (OOP) concept that hides unnecessary details and shows only the essential features of an object. It improves code readability and security.
How Abstraction Works?
- Implemented using abstract classes and interfaces in C#.
- Focuses on what an object does, rather than how it does it.
Example of Abstraction in C#:
public abstract class Vehicle
{
public abstract void Start(); // Abstract method (no implementation)
}
public class Car : Vehicle
{
public override void Start()
{
Console.WriteLine("Car is starting...");
}
} Benefits of Abstraction:
- Hides Complexity – Users only interact with necessary parts.
- Enhances Security – Prevents direct access to implementation details.
- Improves Maintainability – Changes in implementation don’t affect users.
6.What is Encapsulation?
Encapsulation is an Object-Oriented Programming (OOP) principle that bundles data (variables) and methods (functions) together within a class while restricting direct access to the data. It enhances security and data protection.
How Encapsulation Works?
- Uses private and protected access modifiers to hide data.
- Provides public methods (getters & setters) to access and update data safely.
Example of Encapsulation in C#:
public class Person
{
private string name; // Private field
public string GetName() // Public method to access private field
{
return name;
}
public void SetName(string newName)
{
name = newName;
}
}Benefits of Encapsulation:
- Data Protection – Prevents unauthorized access to variables.
- Code Maintainability – Changes in implementation don’t affect other parts of the code.
- Modularity – Keeps code structured and organized.
7.What is Polymorphism and what are its types?
Polymorphism is an Object-Oriented Programming (OOP) concept that allows a single method, function, or operator to have multiple forms. It enhances code flexibility and reusability.
Types of Polymorphism in C#:
1.Compile-Time Polymorphism (Method Overloading)
- Multiple methods with the same name but different parameters.
- Resolved at compile-time.
public class MathOperations
{
public int Add(int a, int b) { return a + b; }
public double Add(double a, double b) { return a + b; }
}2.Runtime Polymorphism (Method Overriding)
- A child class redefines a method from the parent class.
- Uses
virtualandoverridekeywords. - Resolved at runtime.
public class Animal
{
public virtual void MakeSound()
{
Console.WriteLine("Animal makes a sound");
}
}
public class Dog : Animal
{
public override void MakeSound()
{
Console.WriteLine("Dog barks");
}
}Benefits of Polymorphism:
- Increases Flexibility – The same method works differently for different objects.
- Improves Code Reusability – Avoids duplicate code.
- Enhances Maintainability – Easy to modify and extend functionality.
8.What is Method Overloading? In how many ways a method can be overloaded?
Method Overloading is a feature in C# that allows multiple methods to have the same name but with different parameters. It is a type of compile-time polymorphism.
Ways to Overload a Method in C#:
- By Changing the Number of Parameters
public void Show(int a) { }
public void Show(int a, int b) { } - By Changing the Data Type of Parameters
public void Display(int a) { }
public void Display(double a) { } - By Changing the Order of Parameters
public void Print(int a, string b) { }
public void Print(string b, int a) { } Benefits of Method Overloading:
- Increases Code Readability – Same method name for related functionalities.
- Enhances Code Reusability – No need to create multiple method names.
- Supports Compile-Time Polymorphism – Improves performance.
9.What is the difference between Overloading and Overriding?
Difference Between Method Overloading and Method Overriding
| Feature | Method Overloading | Method Overriding |
| Definition | Multiple methods with the same name but different parameters. | Redefining a method in a child class that exists in a parent class. |
| Type | Compile-time polymorphism | Runtime polymorphism |
| Method Signature | Must be different (number, type, or order of parameters). | Must be the same as the parent method. |
| Keywords Used | No special keywords required. | Uses virtual (parent) and override (child). |
| Example in C# | void Show(int a) { }void Show(double a) { } | public virtual void Display() { }public override void Display() { } |
Method Overloading:
public class MathOperations
{
public int Add(int a, int b) { return a + b; }
public double Add(double a, double b) { return a + b; }
}Method Overriding:
public class Parent
{
public virtual void Show() { Console.WriteLine("Parent method"); }
}
public class Child : Parent
{
public override void Show() { Console.WriteLine("Child method"); }
}10.What is the difference between Method Overriding and Method Hiding?
| Feature | Method Overriding | Method Hiding |
| Definition | Redefining a parent class method in a child class using override. | Hiding a parent class method in a child class using new. |
| Type | Runtime polymorphism | Compile-time mechanism |
| Method Signature | Must be the same as the parent method. | Can have the same or different signature. |
| Keywords Used | virtual (parent) and override (child). | new keyword in the child class. |
| Example in C# | public override void Show() { } | public new void Show() { } |
Method Overriding (Uses override)
public class Parent
{
public virtual void Display() { Console.WriteLine("Parent method"); }
}
public class Child : Parent
{
public override void Display() { Console.WriteLine("Child method"); }
}Method Hiding (Uses new)
public class Parent
{
public void Display() { Console.WriteLine("Parent method"); }
}
public class Child : Parent
{
public new void Display() { Console.WriteLine("Child method"); }
}11.What are the advantages and limitations of OOPS?
Advantages of Object-Oriented Programming (OOP)
- Code Reusability – Use inheritance to reuse existing code, reducing duplication.
- Scalability & Maintainability – Easy to modify and extend applications.
- Encapsulation – Protects data by restricting direct access to class members.
- Abstraction – Hides complex implementation details from the user.
- Polymorphism – Allows flexibility by enabling a single method to work in multiple ways.
12.What is the difference between an Abstract class and an Interface (atleast 4)?
Abstract classes and interfaces both define a blueprint for classes, but they serve different purposes.
| Feature | Abstract Class | Interface |
| Definition | A class that can have both abstract (unimplemented) and concrete (implemented) methods. | A contract that defines only method signatures without implementation. |
| Method Implementation | Can have method implementations. | Cannot have method implementations (except default methods in C# 8+). |
| Access Modifiers | Supports public, protected, and private members. | All members are public by default. |
| Multiple Inheritance | A class can inherit only one abstract class. | A class can implement multiple interfaces. |
Abstract Class:
public abstract class Animal
{
public abstract void MakeSound();
public void Sleep() { Console.WriteLine("Sleeping..."); }
} Interface:
public interface IAnimal
{
void MakeSound(); // No implementation
} 13.When to use Interface and when Abstract class?
Use an Interface When:
- Multiple Inheritance is Required – A class can implement multiple interfaces.
- Only Method Signatures are Needed – No implementation, just a contract.
- Loose Coupling is Preferred – Helps in dependency injection and unit testing.
public interface IAnimal
{
void MakeSound();
}Use an Abstract Class When:
- Shared Code Needs to be Reused – Allows method implementation.
- Partial Implementation is Needed – Some methods can be defined, others can be abstract.
- Inheritance is Required – A base class for similar types.
public abstract class Animal
{
public abstract void MakeSound();
public void Sleep() { Console.WriteLine("Sleeping..."); }
}14.Why to even create Interfaces?
Interfaces play a crucial role in flexible, maintainable, and scalable software design. Here’s why:
- Supports Multiple Inheritance : A class can implement multiple interfaces, overcoming the single inheritance limitation of classes.
- Ensures Loose Coupling : Interfaces allow dependency injection, making code more flexible and testable.
- Defines a Contract : Ensures that all implementing classes follow a common structure without enforcing implementation details.
- Improves Code Maintainability : Encourages modular design, making it easier to modify and extend functionalities.
public interface IAnimal
{
void MakeSound();
}
public class Dog : IAnimal
{
public void MakeSound() { Console.WriteLine("Bark!"); }
}15.Do Interface can have a Constructor?
No, an interface cannot have a constructor in C# because:
- Interfaces Cannot Have Instance Members – They only define method signatures, not implementations.
- No Object Creation – You cannot create an instance of an interface directly.
- No Fields or State – Unlike classes, interfaces cannot store data.
No Fields or State – Unlike classes, interfaces cannot store data.
To initialize values, you can use a constructor in an abstract or base class instead.
public interface IAnimal
{
void MakeSound();
}
public class Dog : IAnimal
{
public Dog() { Console.WriteLine("Dog created"); }
public void MakeSound() { Console.WriteLine("Bark!"); }
}16.Can you create an instance of an Abstract class or an Interface?
No, you cannot create an instance of an abstract class or an interface because:
- Abstract Classes are Incomplete – They may contain abstract methods without implementations.
- Interfaces Have No Implementation – They only define method signatures, not behavior.
- Designed for Inheritance – Both are meant to be extended by concrete classes.
How to Use Them?
You can create an instance of a derived class that implements the interface or extends the abstract class.
public abstract class Animal
{
public abstract void MakeSound();
}
public class Dog : Animal
{
public override void MakeSound() { Console.WriteLine("Bark!"); }
}
// Creating an instance of the derived class
Animal myDog = new Dog(); 17.What are Access Specifiers? What is the default access modifier in a class?
What Are Access Specifiers?
Access specifiers in C# control the visibility of classes, methods, and variables. They define how members of a class can be accessed within and outside the class.
Types of Access Specifiers in C#:
- public – Accessible from anywhere.
- private – Accessible only within the same class.
- protected – Accessible within the same class and derived classes.
- internal – Accessible within the same assembly.
- protected internal – Accessible within the same assembly and derived classes.
- private protected – Accessible only within the same class and its derived classes in the same assembly.
What Is the Default Access Modifier in a Class?
- For a Class (Top-Level):
internal(Accessible within the same assembly). - For Class Members:
private(Accessible only within the class).
class Example // Default access modifier is 'internal'
{
int number; // Default access modifier is 'private'
}18.What is Boxing and Unboxing?
Boxing
- Converting a value type (e.g., int, float) into a reference type (object).
- Stores value type in the heap memory.
- Implicit conversion (happens automatically).
int num = 10;
object obj = num; // Boxing (int → object)Unboxing
- Converting a boxed object back to a value type.
- Explicit conversion (requires casting).
- Extracts value from the heap back into the stack.
object obj = 10;
int num = (int)obj; // Unboxing (object → int)Key Points:
- Boxing is implicit, unboxing is explicit.
- Boxing stores in heap, unboxing retrieves from stack.
- Unboxing must match the original value type.
19.What is the difference between “String” and “StringBuilder”? When to use what?
Difference Between String and StringBuilder in C#
| Feature | String | StringBuilder |
| Mutability | Immutable (creates a new object on modification). | Mutable (modifies the same object). |
| Performance | Slower for frequent changes (due to object creation). | Faster for frequent modifications. |
| Memory Usage | Higher memory consumption. | Efficient memory usage. |
| Thread Safety | Thread-safe (since it’s immutable). | Not thread-safe unless explicitly handled. |
When to Use What?
Use String when:
- The value is not changing frequently.
- Performance is not a concern.
- Working with small data manipulations.
Using String (Inefficient for large loops)
string text = "Hello";
text += " World"; // Creates a new object Use StringBuilder when:
- Frequent string modifications (e.g., loops, concatenations).
- Better performance and lower memory consumption are needed.
- Working with large text processing.
Using StringBuilder (Efficient for modifications)
StringBuilder sb = new StringBuilder("Hello");
sb.Append(" World"); // Modifies the same object 20.What are the basic string operations in C#?
C# provides various string operations for manipulation and formatting.
1.Concatenation (Joining Strings)
Using + operator or String.Concat().
string first = "Hello";
string second = "World";
string result = first + " " + second; // "Hello World"2.String Interpolation (Efficient Concatenation)
Uses $ for cleaner syntax.
string name = "Alice";
string message = $"Hello, {name}!"; // "Hello, Alice!"3.Substring (Extracting Part of a String)
Substring(startIndex, length) extracts a portion.
string text = "Hello World";
string sub = text.Substring(0, 5); // "Hello"4.String Comparison
Equals(), Compare(), or ==.
string str1 = "hello";
string str2 = "Hello";
bool isEqual = str1.Equals(str2, StringComparison.OrdinalIgnoreCase); // true5.Changing Case (Uppercase/Lowercase)
ToUpper(), ToLower()
string text = "hello";
string upper = text.ToUpper(); // "HELLO"
string lower = text.ToLower(); // "hello"6.Trimming Spaces
Trim(), TrimStart(), TrimEnd().
string text = " hello ";
string trimmed = text.Trim(); // "hello"7.String Splitting
Split() separates a string into an array.
string text = "apple,banana,grape";
string[] fruits = text.Split(','); // ["apple", "banana", "grape"]8.String Replacement
Replace() changes parts of a string.
string text = "Hello World";
string newText = text.Replace("World", "C#"); // "Hello C#"
