Interview Q&A

Remove duplicates without using LINQ Distinct()

What interviewers test

• Hashing fundamentals
• Time vs memory trade-offs
• Whether you understand why Distinct() works

Real-world scenario

You are processing millions of user IDs coming from logs or Kafka messages.

You must remove duplicates fast and predictably.

Optimized approach (HashSet)

public static List<int> RemoveDuplicates(int[] input)
{
var seen = new HashSet<int>();
var result = new List<int>();
foreach (var item in input)
{
if (seen.Add(item)) // Add returns false if already exists
{

result.Add(item);

}
}
return result;
}

Complexity

Aspect Value

Time O(n)

Space O(n) (hash storage)

Why this is better than naive loops

• Nested loops = O(n²) ❌
• HashSet = constant-time lookup ✅

Interview Tip:

Say “I prefer HashSet because it gives O(1) average lookup and preserves intent clearly.”

functionality.

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: OOP is a programming paradigm that organizes software around objects, which contain data (fields/properties) and behavior (methods/functions). Helps model real-world entities and their interactions.

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.

Explain async / await with a real production

scenario

What interviewers test

• Thread utilization
• Scalability thinking
• Non-blocking I/O knowledge

Real-world scenario

API calls:

• Database
• Payment gateway
• Email service

Blocking threads = server collapse under load

Bad (Blocking)

public string GetUser()
{
var response = httpClient.GetAsync(url).Result; // Blocks thread

return response.Content.ReadAsStringAsync().Result;
}

Good (Async, scalable)

public async Task<string> GetUserAsync()
{
var response = await httpClient.GetAsync(url);
return await response.Content.ReadAsStringAsync();
}

Why async/await matters

• Thread is released during I/O wait
• ASP.NET can serve more concurrent requests
• No thread starvation

Key interview line

“Async doesn’t make code faster; it makes servers scalable.”

IEnumerable vs ICollection vs IList

What interviewers test

• Execution pipeline understanding
• Abstraction knowledge

Hierarchy

IEnumerable

└── ICollection

└── IList

IEnumerable<T>

• Read-only iteration
• Lazy execution

IEnumerable<int> numbers = GetNumbers();
foreach (var n in numbers) { }

✅ Best for streaming, read-only, deferred execution

ICollection<T>

• Adds count + add/remove

ICollection<int> list = new List<int>();
list.Add(1);

✅ When modifying collection without index access

IList<T>

• Index-based access

IList<int> list = new List<int>();
int first = list[0];

✅ When order & index matter

Interview rule of thumb

“Expose the least powerful interface required.”

Answer: class Vehicle {} // Base class Car : Vehicle {} // Single/Multilevel class Bike : Vehicle {} // Hierarchical

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: Promotes code reusability through classes and objects. Easier to maintain and extend large applications. Models real-world problems better. Supports modularity, abstraction, and encapsulation, which procedural programming lacks.

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.

Logic

• Count character frequency.
• Traverse string again to find the first with count = 1.

string input = "swiss";
Dictionary<char, int> map = new Dictionary<char, int>();
foreach (char c in input)
map[c] = map.ContainsKey(c) ? map[c] + 1 : 1;
foreach (char c in input)
{
if (map[c] == 1)
{

Console.WriteLine(c);

break;

}
}

• Encourages modular code → easier to maintain and test.
• Reduces code duplication through inheritance and composition.
• Improves scalability and flexibility in large projects.
• Enhances team collaboration as objects represent real-world entities.

overloading/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.

Handle millions of records efficiently

What interviewers test

• Memory pressure awareness
• Streaming & batching

❌ Bad (Loads everything)

var users = db.Users.ToList(); // Memory explosion

✅ Streaming (Best)

await foreach (var user in db.Users.AsAsyncEnumerable())
{

Process(user);

}

✅ Batching

const int batchSize = 1000;
for (int i = 0; i < total; i += batchSize)
{
var batch = GetBatch(i, batchSize);

ProcessBatch(batch);

}

Key principles

• Never load everything
• Prefer streams
• Control memory explicitly

Answer: A blueprint or template for creating objects. Defines properties (data) and methods (behavior) that the objects will have. public class Car { public string Model { get; set; } public void Start() { Console.WriteLine("Car started"); } }

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.

Dependency Injection without any framework

What interviewers test

• SOLID
• Architecture fundamentals

Step 1: Abstraction

public interface IMessageService
{

void Send(string message);

}

Step 2: Implementation

public class EmailService : IMessageService
{
public void Send(string message)
{

Console.WriteLine("Email: " + message);

}
}

Step 3: Injection

public class Notification
{
private readonly IMessageService _service;
public Notification(IMessageService service)
{
_service = service;
}
public void Notify(string msg)
{

_service.Send(msg);

}
}

Why this matters

• Loose coupling
• Testable code
• Swappable implementations

Answer: An instance of a class with actual values. Represents a real-world entity in memory. Car myCar = new Car(); // Object of Car class

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.

struct vs class (real-world)

What interviewers test

• Memory & performance awareness

Use struct when:

• Small
• Immutable
• Value-type behavior

public readonly struct Point
{
public int X { get; }
public int Y { get; }
}

Use class when:

• Large
• Mutable
• Shared references

public class User
{
public string Name { get; set; }
}
Interview statement

“Structs live on stack or inline, classes live on heap with GC cost.”

Answer: Feature Class Object Definition Blueprint/Template Instance of a class Memory Does not occupy memory Occupies memory Example class Car { } Car myCar = new Car();

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.

Design a rate limiter

What interviewers test

• Concurrency
• Thread safety
• System design

Token bucket (simple)

public class RateLimiter
{
private readonly int _limit;
private int _count;
private DateTime _windowStart = DateTime.UtcNow;
private readonly object _lock = new();
public RateLimiter(int limit)
{
_limit = limit;
}
public bool Allow()
{

lock (_lock)

{
if ((DateTime.UtcNow - _windowStart).TotalSeconds >= 1)
{
_count = 0;
_windowStart = DateTime.UtcNow;
}
if (_count < _limit)
{

_count++;

return true;
}
return false;
}
}
}

Used in

• APIs
• Login attempts
• OTP systems

Answer: A special method used to initialize objects when they are created. Has the same name as the class and no return type. public class Car { public string Model; public Car(string model) { Model = model; } } Car car = new Car("Tesla");

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.

LINQ vs IQueryable

What interviewers test

• Deferred execution
• DB performance

LINQ (IEnumerable)

var data = users.Where(x => x.Age > 30).ToList();

• Executes in memory

IQueryable

var data = db.Users.Where(x => x.Age > 30);

• Translates to SQL
• Executes in DB

Interview line

“IQueryable builds expressions; IEnumerable executes them.”

🔟 Write clean, production-ready C# code

What interviewers test

• Professional maturity

Principles

• Small methods
• Clear naming
• No magic values
• Proper exceptions

public class OrderService
{
public void PlaceOrder(Order order)
{
if (order == null)

throw new ArgumentNullException(nameof(order));

Validate(order);

Save(order);

}
private void Validate(Order order)
{
if (order.Total <= 0)

throw new InvalidOperationException("Invalid total");

}
private void Save(Order order)
{

// persistence logic

}
}

• A method called automatically when an object is destroyed.
• Used to release resources before the object is removed from memory.
• In C#, destructors are rarely needed due to garbage collection.

~Car() { Console.WriteLine("Car object destroyed"); }

Answer: Instance members → Belong to each object, require object to access. Static members → Belong to the class itself, shared by all objects. public class Car { public string Model; // Instance public static int Count; // Static }

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: The practice of hiding internal details of a class and exposing only necessary functionality through access modifiers and properties. private int speed; public int Speed { get { return speed; } set { speed = value; } }

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: Hiding implementation details and showing only the essential features of an object. Achieved using abstract classes and interfaces. bstract class Vehicle { public abstract void Start(); }

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.

• Inheritance is an OOP mechanism where a class (derived/child) inherits

properties and methods from another class (base/parent).

• Promotes code reusability and hierarchical relationships.

class Vehicle { public void Start() => Console.WriteLine("Vehicle

started"); }

class Car : Vehicle { } // Car inherits from Vehicle

Answer: Ability of an object to take multiple forms. Types: Compile-time (method overloading) Run-time (method overriding) Vehicle v = new Car(); v.Start(); // Run-time polymorphism

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.