Interview Q&A

Unit testing involves testing the smallest parts of an application (units) independently to

ensure they work correctly. It is important because it helps detect bugs early, improves code

quality, supports refactoring, and provides documentation for expected behavior.

Answer: Unit Testing: Tests individual units in isolation. Integration Testing: Tests interaction between multiple components or systems. Functional Testing: Tests end-to-end functionality from the user's perspective.

What interviewers expect

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

Real-world example

In a production Unit Testing 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 Unit Testing 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: Assertions verify that the actual outcome of a test matches the expected result, determining if a test passes or fails.

What interviewers expect

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

Real-world example

In a production Unit Testing 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 Unit Testing 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: Code coverage measures the percentage of code executed by tests. It helps identify untested parts but doesn’t guarantee test quality. High coverage with meaningful tests improves confidence in code correctness. xUnit Framework

What interviewers expect

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

Real-world example

In a production Unit Testing 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 Unit Testing 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.

Another term for runtime polymorphism, achieved via method overriding.

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: An interface is a contract that defines method signatures, properties, events, or indexers without providing implementation. Classes or structs that implement the interface must provide the implementation.

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: Use the interface keyword. interface IDriveable { void Drive(); int Speed { get; set; } }

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: Use the colon (:) symbol and implement all members. class Car : IDriveable { public int Speed { get; set; } public void Drive() => Console.WriteLine("Car is driving"); }

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 Interface Class

Implementatio

No implementation (except default

methods)

Can have full

implementation

Fields Cannot have fields Can have fields

Instantiation Cannot instantiate Can instantiate

Inheritance Can inherit multiple interfaces Single class inheritance only

Answer: Implementing an interface member explicitly so it can only be called via interface reference, not class object. IDriveable car = new Car(); car.Drive(); // Works // Car c = new Car(); c.Drive(); // Won't compile

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: Define contracts for classes. Achieve abstraction, polymorphism, and loose coupling.

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: Provides a standard method to compare objects for sorting. class Employee : IComparable<Employee> { public int Id { get; set; } public int CompareTo(Employee other) => this.Id.CompareTo(other.Id); }

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: Provides Dispose() method for releasing unmanaged resources. class FileHandler : IDisposable { public void Dispose() => Console.WriteLine("Resources released"); }

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: IEnumerable → Provides collection traversal capability (GetEnumerator() method). IEnumerator → Used to iterate over a collection (MoveNext(), Current, Reset()).

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: An abstract class is a class that cannot be instantiated directly. Can contain abstract methods (without implementation) and concrete methods (with implementation). Used to define a common base for other classes.

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.

• A method declared with abstract without implementation.
• Must be overridden in a derived class.

bstract class Vehicle

{
public abstract void Start();
}
class Car : Vehicle
{
public override void Start() => Console.WriteLine("Car

started");

}

llow derived class to optionally override

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 Method Virtual Method

Implementatio

No implementation Has implementation

Must override? Must be overridden Optional to override

Class type Must be in abstract class Can be in any class

Purpose Force derived classes to

implement

Allow derived class to optionally

override

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

}