Interview Q&A

Short 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.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: IList<T> extends ICollection<T> and allows: Indexed access (like arrays) Inserting and removing at specific positions

Example code

IList<string> fruits = new List<string>(); fruits.Add("Apple"); fruits.Insert(0, "Banana"); // Insert at index 0 Console.WriteLine(fruits[1]); // Access by index Real-world use case: Use IList<T> when order matters and you need to access, insert, or remove elements at specific positions, like reordering tasks in a to-do list.

Real-world example (ShopNest)

In ShopNest, an order page loads products with List<Product> so you can Add items to the cart and access them by index. Use IEnumerable<T> when you only need to loop (for example, printing invoice lines).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: A blueprint or template for creating objects. Defines properties (data) and methods (behavior) that the objects will have. public class Car

Example code

{
public string Model { get; set; }
public void Start() { Console.WriteLine("Car started"); }
}

Real-world example (ShopNest)

Think of ShopNest’s Product, Cart, and Order classes: each object holds data + behavior, so pricing rules stay next to the data they use.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Dictionary? Feature TryGetValue() Indexer (dictionary[key]) Safe? Yes – avoids exception No – throws if key doesn't exist Returns Boolean (and output value) Direct value Use case When unsure if key exists When key is guaranteed to exist if (dictionary.TryGetValue("Bob", out int age)) { Console.WriteLine(age); } //… dictionary["Unknown"]; // throws…… Dictionary? Feature TryGetValue() Indexer (dictionary[key]) Safe?…

Explain a bit more

Yes – avoids exception No – throws if key doesn't exist Returns Boolean (and output value) Direct value Use case When unsure if key exists When key is guaranteed to exist

Example code

if (dictionary.TryGetValue("Bob", out int age)) { Console.WriteLine(age); } // dictionary["Unknown"]; // throws KeyNotFoundException if missing Dictionary? Feature TryGetValue() Indexer (dictionary[key]) Safe? Yes – avoids exception No – throws if key doesn't exist Returns Boolean (and output value) Direct value Use case When unsure if key exists When key is guaranteed to exist Example: if (dictionary.TryGetValue("Bob", out int age)) { Console.WriteLine(age); } //… dictionary["Unknown"]; // throws… Dictionary? Feature TryGetValue() Indexer (dictionary[key]) Safe? Yes – avoids exception No – throws if key doesn't exist Returns Boolean (and output value) Direct value Use case When unsure if key exists When key is guaranteed to exist Example: if (dictionary.TryGetValue("Bob", out int age)) { Console.WriteLine(age); } // dictionary["Unknown"]; // throws KeyNotFoundException if missing

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Due to the underlying balanced tree structure, these operations have O(log n) time complexity. 📘 C# Collection Initializers & LINQ – Interview Questions & Answers

Real-world example (ShopNest)

When applying a coupon, ShopNest keeps used coupon codes in a HashSet<string> so “already used?” checks stay fast and unique.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Peek() returns the top element without removing it. It’s helpful for: Conditional checks Previewing what's next Preventing accidental removal Example: if (stack.Count > 0)

Example code

{
var current = stack.Peek();
} Throws InvalidOperationException if the stack is empty.

Real-world example (ShopNest)

ShopNest uses a Queue<Order> for “orders waiting for payment confirmation,” and a Stack<Uri> for back-navigation in the admin UI.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Feature TryGetValue() Indexer (dictionary[key]) Safe? Yes – avoids exception No – throws if key doesn't exist Returns Boolean (and output value) Direct value Use case When unsure if key exists When key is guaranteed to exist

Example code

if (dictionary.TryGetValue("Bob", out int age)) { Console.WriteLine(age); } // dictionary["Unknown"]; // throws KeyNotFoundException if missing

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Method Purpose Add() Adds one item AddRange () Adds multiple items (collection) Example: list.Add(1); // One item list.AddRange(new[] { 2, 3, 4 }); // Multiple

Example code

Method Purpose Add() Adds one item AddRange () Adds multiple items (collection) Example: list.Add(1); // One item
list.AddRange(new[] { 2, 3, 4 }); // Multiple

Real-world example (ShopNest)

In ShopNest, an order page loads products with List<Product> so you can Add items to the cart and access them by index. Use IEnumerable<T> when you only need to loop (for example, printing invoice lines).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: These interfaces provide read-only access to collections: IReadOnlyCollection<T> provides Count and enumeration. IReadOnlyList<T> provides index-based access without modification.

Example code

IReadOnlyList<int> ids = new List<int> { 1, 2, 3 }; Console.WriteLine(ids[0]); // Access element // ids[0] = 10; // Compile-time error – read-only Real-world use case: Useful in APIs where you want to expose collection data but prevent consumers from modifying it—e.g., returning a list of supported currencies from a service.

Real-world example (ShopNest)

In ShopNest, an order page loads products with List<Product> so you can Add items to the cart and access them by index. Use IEnumerable<T> when you only need to loop (for example, printing invoice lines).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

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

Example code

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

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Method Purpose Return Value Add(item) Adds item if not already present true if added, false if duplicate Contains(it em) Checks if item exists in the set true if found, false otherwise

Real-world example (ShopNest)

When applying a coupon, ShopNest keeps used coupon codes in a HashSet<string> so “already used?” checks stay fast and unique.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short 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();

Real-world example (ShopNest)

Think of ShopNest’s Product, Cart, and Order classes: each object holds data + behavior, so pricing rules stay next to the data they use.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Feature IEnumerable <T> ICollection<T> Read-only Yes No Modification Not supported Supports Add, Remove, Clear Count property No Yes (Count)

Example code

IEnumerable<string> names = new List<string> { "A", "B" }; // names.Add("C"); // Error ICollection<string> nameCollection = new List<string> { "A", "B" }; nameCollection.Add("C"); // Allowed Real-world scenario: Use IEnumerable<T> for read-only, query-focused tasks like LINQ. Use ICollection<T> when managing and modifying the collection.

Real-world example (ShopNest)

In ShopNest, an order page loads products with List<Product> so you can Add items to the cart and access them by index. Use IEnumerable<T> when you only need to loop (for example, printing invoice lines).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short 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

Example code

{
public string Model;
public Car(string model) { Model = model; }
}
Car car = new Car("Tesla");

Real-world example (ShopNest)

Think of ShopNest’s Product, Cart, and Order classes: each object holds data + behavior, so pricing rules stay next to the data they use.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Feature List<T> ArrayList Generic Yes No Type Safety Compile-time Runtime (casting required) Performanc Better (no boxing) Slower for value types (boxing) Example: List<int> list = new List<int>(); ArrayList arrayList = new ArrayList(); arrayList.Add(10); int num = (int)arrayList[0]; // Casting required Best Practice: Always prefer List<T> in new code. Use ArrayList only when working with legacy systems.

Example code

list.Add(10); // Type-safe

Real-world example (ShopNest)

In ShopNest, an order page loads products with List<Product> so you can Add items to the cart and access them by index. Use IEnumerable<T> when you only need to loop (for example, printing invoice lines).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: 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"); }

Real-world example (ShopNest)

Think of ShopNest’s Product, Cart, and Order classes: each object holds data + behavior, so pricing rules stay next to the data they use.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: KeyValuePair<TKey, TValue> represents a single item in a dictionary — a key-value pair. Used in: Iteration LINQ queries Return values from dictionary enumerators Example: foreach (KeyValuePair<string, int> entry in dictionary)

Example code

{ Console.WriteLine($"Key: {entry.Key}, Value: {entry.Value}"); }

Real-world example (ShopNest)

ShopNest caches product prices in a Dictionary<string, decimal> keyed by SKU so checkout can look up a price in O(1) instead of scanning a list.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Method Returns Find() First match FindAll () All matches in a new list Example: var firstEven = list.Find(x => x % 2 == 0); var allEvens = list.FindAll(x => x % 2 == 0);

Example code

Method Returns Find() First match FindAll () All matches in a new list Example: var firstEven = list.Find(x => x % 2 == 0);
var allEvens = list.FindAll(x => x % 2 == 0);

Real-world example (ShopNest)

In ShopNest, an order page loads products with List<Product> so you can Add items to the cart and access them by index. Use IEnumerable<T> when you only need to loop (for example, printing invoice lines).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: The practice of hiding internal details of a class and exposing only necessary functionality through access modifiers and properties. private int speed;

Example code

public int Speed
{ get { return speed; } set { speed = value; }
}

Real-world example (ShopNest)

ShopNest’s Order keeps _items private and exposes AddItem() so totals stay correct—callers cannot put a negative quantity directly into the list.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Hiding implementation details and showing only the essential features of an object. Achieved using abstract classes and interfaces. abstract class Vehicle {

Example code

public abstract void Start();
}

Real-world example (ShopNest)

Checkout calls IPaymentGateway.Charge(). Razorpay and Stripe adapters implement the same interface, so ShopNest can switch gateways without rewriting the order service.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Capacity is the number of elements the list can hold before resizing. It is greater than or equal to Count.

Example code

list.Capacity = 100; // Optional performance tuning

Real-world example (ShopNest)

In ShopNest, an order page loads products with List<Product> so you can Add items to the cart and access them by index. Use IEnumerable<T> when you only need to loop (for example, printing invoice lines).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Mechanism where a class (derived/child) inherits properties and methods from another class (base/parent). Promotes code reusability. class Car : Vehicle

Example code

{
public override void Start() { Console.WriteLine("Car started");
}
}

Real-world example (ShopNest)

ShopNest has a base PaymentMethod with virtual decimal Fee(). UpiPayment and CardPayment override the fee logic without changing the checkout caller.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Property Meaning Count Number of elements currently in the list Capacit Total allocated slots (memory reserved)

Example code

Console.WriteLine($"Count: {list.Count}, Capacity: {list.Capacity}");

Real-world example (ShopNest)

In ShopNest, an order page loads products with List<Product> so you can Add items to the cart and access them by index. Use IEnumerable<T> when you only need to loop (for example, printing invoice lines).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short 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

Real-world example (ShopNest)

Checkout calls IPaymentGateway.Charge(). Razorpay and Stripe adapters implement the same interface, so ShopNest can switch gateways without rewriting the order service.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.

Short answer: Car object: Class → Car Objects → myCar, yourCar Properties → Color, Model, Speed Methods → Start(), Stop(), Accelerate() Shows encapsulation, inheritance (e.g., ElectricCar : Car), and polymorphism in action.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.