Interview Q&A

Answer: Feature Abstract Class Normal Class Instantiation Cannot instantiate Can instantiate Methods Can have abstract methods All methods must have implementation Purpose Serve as base for inheritance General purpose use

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Feature Abstract Class Interface

Implementation Can have full/partial

implementation

Cannot have full implementation (except

default methods in C# 8+)

Fields Can have fields Cannot have fields

Inheritance Single class inheritance Multiple interface inheritance allowed

Constructors Allowed Not allowed

ccess

Modifiers

Can have public,

protected, private

Members are public by default

• High-level modules should not depend on low-level modules. Both should depend

on abstractions.

• Interfaces allow decoupling and easier testing.

interface ILogger { void Log(string message); }
class FileLogger : ILogger { public void Log(string message) =>

Console.WriteLine("File: " + message); }

class UserService
{
private readonly ILogger _logger;
public UserService(ILogger logger) { _logger = logger; }
}

Answer: ffect it? Derived classes should be replaceable by base class without affecting correctness. Inheritance violating this principle can cause unexpected behavior.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: Derived classes should be replaceable by base class without affecting correctness. Inheritance violating this principle can cause unexpected behavior.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# OOP architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• Duck typing: "If it looks like a duck and quacks like a duck, it is a duck."
• Behavior is based on method/property availability, not type inheritance.
• C# does not support full dynamic duck typing, but interfaces enable a similar

concept by relying on contract-based behavior.

• Dynamic types in C# (dynamic) can also simulate duck typing.

interface IFlyable { void Fly(); }
void MakeItFly(IFlyable obj) => obj.Fly(); // Any object

implementing IFlyable works

• Allows interfaces to provide default method implementations.
• Reason: Enables adding new methods to interfaces without breaking existing

implementations.

interface IPrinter
{

void Print(string msg);

void PrintInfo(string msg) => Console.WriteLine("Info: " + msg);

// Default

}