⭐ Introduction
At the intermediate level, interviewers expect more than definitions.
They test your ability to:
- Explain why a concept is used
- Compare similar OOPS concepts
- Apply OOPS in real-world scenarios
- Understand design decisions
This article covers commonly asked intermediate-level OOPS interview questions with practical, interview-ready answers.
🔹 1. Difference between Encapsulation and Abstraction
| Encapsulation | Abstraction |
|---|---|
| Data hiding | Implementation hiding |
| Focuses on how data is accessed | Focuses on what functionality is exposed |
| Uses private fields + properties | Uses abstract classes / interfaces |
👉 Short answer:
Encapsulation protects data, abstraction hides complexity.
🔹 2. Can we achieve abstraction without encapsulation?
❌ No
Abstraction depends on encapsulation.
If data is not protected (encapsulation), abstraction cannot be enforced safely.
🔹 3. Why does C# not support multiple inheritance using classes?
Because multiple inheritance causes ambiguity (Diamond Problem).
Example:
Class A → Method X()
Class B → Method X()
Class C : A, B → Which X()?
✔ C# solves this using interfaces.
🔹 4. What is the Diamond Problem?
When a class inherits from two classes that have a common base class, method ambiguity occurs.
C# avoids this by:
✔ Allowing multiple inheritance via interfaces only
🔹 5. Difference between Method Overloading and Method Overriding
| Overloading | Overriding |
|---|---|
| Compile-time polymorphism | Runtime polymorphism |
| Same class | Parent–child classes |
| Different parameters | Same parameters |
| Faster | Slightly slower |
🔹 6. Why are fields usually private in OOPS?
Because:
✔ It enforces encapsulation
✔ Prevents invalid data
✔ Allows validation
✔ Improves maintainability
👉 Best practice:
Fields → private
Access → public properties
🔹 7. Can constructors be overridden?
❌ No
Constructors are not inherited, so they cannot be overridden.
✔ They can be overloaded.
🔹 8. Can static methods be overridden?
❌ No
Static methods belong to the class, not the object.
They can only be hidden, not overridden.
🔹 9. Difference between new and override keywords
| new | override |
|---|---|
| Hides base method | Replaces base method |
| No polymorphism | Supports polymorphism |
| Compile-time decision | Runtime decision |
🔹 10. What is Polymorphism and why is it important?
Polymorphism allows:
Same method call → different behavior
It enables:
✔ Flexible design
✔ Extensible systems
✔ Framework development
✔ Clean APIs
Example:Pay() → UPI / Card / NetBanking
🔹 11. What is the difference between abstract class and interface?
| Abstract Class | Interface |
|---|---|
| Can have method bodies | No method bodies (pre-C# 8) |
| Single inheritance | Multiple inheritance |
| Can have fields | No fields |
| Strong relationship | Loose coupling |
🔹 12. When should you use an interface instead of an abstract class?
Use interface when:
✔ Multiple inheritance is required
✔ Classes are unrelated
✔ You want loose coupling
✔ You are designing APIs
🔹 13. What is a virtual method?
A method marked with virtual allows derived classes to override it.
public virtual void Print() { }
🔹 14. What happens if we don’t use virtual but use override?
❌ Compilation error
The base method must be marked virtual.
🔹 15. What is method hiding?
Method hiding occurs when:
- Child method uses
new - Parent method is hidden, not overridden
It does not support runtime polymorphism.
🔹 16. What is a sealed class and why is it used?
A sealed class:
✔ Cannot be inherited
✔ Protects logic
✔ Improves performance
✔ Prevents misuse
Used when a class design is final.
🔹 17. Can a sealed class implement interfaces?
✔ Yes
Sealed blocks inheritance, not interface implementation.
🔹 18. What is the difference between static and instance members?
| Static | Instance |
|---|---|
| Belongs to class | Belongs to object |
| Single copy | Multiple copies |
| Accessed via class | Accessed via object |
🔹 19. Why can static methods not access instance members?
Because static methods:
❌ Do not belong to any object
❌ Cannot access object-specific data
🔹 20. What is constructor chaining?
Calling one constructor from another using:
this()→ same classbase()→ parent class
It reduces duplication.
🔹 21. What is tight coupling vs loose coupling?
| Tight Coupling | Loose Coupling |
|---|---|
| Classes depend directly | Depend via interfaces |
| Hard to modify | Easy to extend |
| Poor testability | Better testing |
🔹 22. How does OOPS help in real projects?
✔ Modularity
✔ Reusability
✔ Maintainability
✔ Scalability
✔ Team collaboration
🔹 23. Can we create an object of an abstract class?
❌ No
But we can create a reference of abstract class pointing to a derived object.
🔹 24. What is the default access modifier in C#?
- Class →
internal - Class members →
private
🔹 25. Difference between IS-A and HAS-A relationship
- IS-A → Inheritance
- Car IS-A Vehicle
- HAS-A → Composition
- Car HAS-A Engine
🧠 Interview Tip
For intermediate interviews:
✔ Explain with example
✔ Use real-world analogy
✔ Compare similar concepts
✔ Mention why & when, not just what
🎉 Conclusion
These intermediate OOPS questions test your depth of understanding, not memorization.
If you can confidently answer these, you are ready for senior-level and system design questions.