Interview Q&A

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: dd()? Insert(index, item) adds an item at a specific index. Add(item) adds to the end of the list. list.Insert(0, 99); // Add at beginning list.Add(100); // Add at end dd()? Insert(index, item) adds an item at a specific index. Add(item) adds to the end of the list.

Example code

list.Insert(0, 99); // Add at beginning… list.Add(100); // Add at end dd()? Insert(index, item) adds an item at a specific index. Add(item) adds to the end of the list. Example: list.Insert(0, 99); // Add at beginning list.Add(100); // Add at end dd()? Insert(index, item) adds an item at a specific index. Add(item) adds to the end of the list. Example: list.Insert(0, 99); // Add at beginning… list.Add(100); // Add at end

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: 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: Use the Count property. if (stack.Count == 0)

Example code

{ Console.WriteLine("Stack is empty"); } Unlike some languages, C# stacks do not provide an IsEmpty property.

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: Use a foreach loop. Iteration does not modify the queue. Example: foreach (var order in orders)

Example code

{ Console.WriteLine(order); } You can also use .ToArray() if needed: string[] items = orders.ToArray();

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: dictionary.Keys – returns a collection of all keys dictionary.Values – returns a collection of all values Example: foreach (var key in dictionary.Keys) Console.WriteLine(key); foreach (var value in dictionary.Values) Console.WriteLine(value);

Example code

dictionary.Keys – returns a collection of all keys dictionary.Values – returns a collection of all values Example: foreach (var key in dictionary.Keys) Console.WriteLine(key); foreach (var value in dictionary.Values) Console.WriteLine(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: Insert(index, item) adds an item at a specific index. Add(item) adds to the end of the list. Example: list.Insert(0, 99); // Add at beginning list.Add(100); // Add at end

Example code

Insert(index, item) adds an item at a specific index. Add(item) adds to the end of the list. Example: list.Insert(0, 99); // Add at beginning
list.Add(100); // Add at end

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: Feature Non-Generic Generic Type Safety No Yes Performanc Slower (boxing/unboxing) Faster Casting Required Not required Syntax Less readable Clean and type-specific Examples: // Non-generic ArrayList arr = new ArrayList(); arr.Add(1); arr.Add("text"); // Allowed, but risky // Generic List<int> list = new List<int>(); dict["id"] = 101; Real-world use case: Generic collections are recommended for new development due…

Explain a bit more

to safety and performance. Non-generic collections are often found in older legacy systems.

Example code

list.Add(1); // list.Add("text"); // Compile-time error // Hashtable vs Dictionary Hashtable ht = new Hashtable();
ht["id"] = 101;
Dictionary<string, int> dict = new Dictionary<string, int>();

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: Use a foreach loop; the iteration order is not guaranteed. foreach (var item in uniqueNumbers) { Console.WriteLine(item); } 📘 C# LinkedList<T> – Interview Questions & Answers

Example code

Use a foreach loop; the iteration order is not guaranteed. foreach (var item in uniqueNumbers)
{ Console.WriteLine(item); } 📘 C# LinkedList<T> – 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: Use the Clear() method to remove all elements. stack.Clear(); After this, Count becomes 0, and the internal array is reset.

Example code

Use the Clear() method to remove all elements. stack.Clear(); After this, Count becomes 0, and the internal array is reset.

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: Again, use the Peek() method: var front = queue.Peek(); Difference from Dequeue(): Peek() returns the front element without removing it. Dequeue() returns and removes the front element. 📘 C# Stack<T> – Interview Questions & Answers

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: Using Add() will throw a System.ArgumentException Using the indexer (dictionary[key] = value) will overwrite the existing value Example: dictionary.Add("John", 25);

Example code

dictionary.Add("John", 30); // Exception
dictionary["John"] = 30; // Overwrites the value safely

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: Use the Sort() method or provide a custom comparer. Example: list.Sort(); // Default sort (ascending) list.Sort((a, b) => b.CompareTo(a)); // Descending

Example code

Use the Sort() method or provide a custom comparer. Example: list.Sort(); // Default sort (ascending)
list.Sort((a, b) => b.CompareTo(a)); // Descending

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: 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: Use a foreach loop, which iterates from top to bottom (LIFO order). Example: foreach (var item in stack)

Example code

{ Console.WriteLine(item); } This does not modify the stack — it's read-only iteration. 📘 C# HashSet<T> – Interview Questions & Answers

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: Use a foreach loop with KeyValuePair<TKey, TValue>: foreach (KeyValuePair<string, int> pair in dictionary)

Example code

{ Console.WriteLine($"Key: {pair.Key}, Value: {pair.Value}"); } Or use deconstruction (C# 7+): foreach (var (key, value) in dictionary)
{
Console.WriteLine($"{key} = {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: Use the Reverse() method. Example: list.Reverse(); Use case: Reversing order of recent messages or results.

Example code

Use the Reverse() method. Example: list.Reverse(); Use case: Reversing order of recent messages or results.

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: Why or why not? For reference types, null keys are not allowed — adding one throws an ArgumentNullException. For value types, keys must be non-nullable (like int, Guid). This restriction ensures the integrity of hashing, which is used internally by the dictionary.

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: Use the Clear() method to remove all elements. list.Clear(); Use the Clear() method to remove all elements.

Example code

list.Clear(); Use the Clear() method to remove all elements. Example: list.Clear(); Use the Clear() method to remove all elements. Example: list.Clear();

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: You can iterate using:

Real-world example (ShopNest)

In ShopNest checkout, choose the collection by need: List for ordered cart lines, Dictionary for fast SKU lookup, HashSet for unique tags.

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: Use the Clear() method to remove all key-value pairs. dictionary.Clear(); This resets the dictionary to an empty state. 📘 C# Queue<T> – Interview Questions & Answers

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.