Interview Q&A

Short answer: Define your custom object class. Create a collection class that holds objects of that type using generics or directly. Example: public class Employee

Example code

{
public int Id { get; set; }
public string Name { get; set; }
}
public class EmployeeCollection : Collection<Employee>
{ // You can add custom methods specific to Employee collection here } Or simply use List<Employee> directly for flexibility.

Real-world example (ShopNest)

Prefer List<Order> over ArrayList in ShopNest. Generics catch type mistakes at compile time and avoid boxing when you store ints or decimals.

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 methods like ToList(), ToArray(), or ToDictionary() to convert LINQ query results to different collection types.

Explain a bit more

Examples: var numbers = new int[] { 1, 2, 3, 4, 5 }; // Convert to List<int> List<int> numberList = numbers.ToList(); // Convert to array int[] numberArray = numberList.ToArray(); // Convert to dictionary (key = number, value = square) Dictionary<int, int> numberDict = numbers.ToDictionary(n => n, n => n * n); 📘 C# Thread-Safe Collections – Interview Questions & Answers

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: Use a foreach loop which iterates over the elements in sorted ascending order: foreach (var item in sortedSet)

Example code

{ Console.WriteLine(item); }

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: You can use a foreach loop over KeyValuePair<TKey, TValue> elements, which iterates in sorted key order: foreach (var kvp in sortedList) { Console.WriteLine($"Key: {kvp.Key}, Value: {kvp.Value}"); } You can also iterate over keys or values separately: foreach (var key in sortedList.Keys) { /* ... */ }

Example code

foreach (var value in sortedList.Values) { /* ... */ } 📘 C# SortedSet<T> – Interview Questions & Answers

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: No, HashSet<T> does not allow duplicates. Attempting to add a duplicate value will return false and not change the set.

Example code

bool added = uniqueNumbers.Add(2); // returns false because 2 already exists

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 Peek() method. Example: int top = stack.Peek(); This is useful when you just want to inspect the top element without altering the stack.

Example code

Use the Peek() method. Example: int top = stack.Peek(); This is useful when you just want to inspect the top element without altering the stack.

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 the Peek() method to view the front element without removing it.

Example code

Queue<string> tasks = new Queue<string>(); tasks.Enqueue("Task1"); string nextTask = tasks.Peek(); // Returns "Task1", does not remove it Useful when you want to see what’s next without modifying the queue.

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: Contains(item) IndexOf(item) Find(predicate) FindAll(predicate) Exists(predicate) BinarySearch(item) (for sorted lists) Example: bool hasItem = numbers.Contains(10);

Example code

int index = numbers.IndexOf(10);
var result = numbers.Find(x => x > 50);

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: Actually, SortedList<TKey, TValue> stores key-value pairs sorted by keys. To store items in a specific order, use the key to represent the sorting criteria. Keys must be unique and implement IComparable or provide a custom IComparer.

Example code

SortedList<int, string> sortedList = new SortedList<int, string>(); sortedList.Add(10, "Ten"); sortedList.Add(5, "Five"); sortedList.Add(20, "Twenty"); // Items automatically sorted by keys: 5, 10, 20 If you want to sort by custom criteria, implement an IComparer and pass it to the SortedList constructor.

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: Operation LinkedList<T> List<T> Indexed access O(n) (no indexing) O(1) (direct access) Add/Remove at start/end O(1) O(n) (start), O(1) (end) Add/Remove in middle O(1) (with node ref) O(n) (shifts elements) Memory overhead Higher (extra pointers) Lower (array storage) Summary: Use LinkedList<T> when you need fast insertions/deletions anywhere and don’t require indexed access.

Explain a bit more

Use List<T> for fast random access and better memory efficiency. 📘 C# SortedList<TKey, TValue> – Interview Questions & Answers

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: Union: Combines all unique elements from both sets set1.UnionWith(set2); Intersection: Keeps only elements present in both sets set1.IntersectWith(set2); Example: HashSet<int> set1 = new HashSet<int> { 1, 2, 3 };

Example code

HashSet<int> set2 = new HashSet<int> { 3, 4, 5 };
set1.UnionWith(set2); // set1 = {1, 2, 3, 4, 5}
set1.IntersectWith(set2); // set1 = {3, 4, 5}

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. Example: tasks.Clear(); After calling Clear(), the queue is empty (Count == 0).

Example code

Use the Clear() method to remove all elements. Example: tasks.Clear(); After calling Clear(), the queue is empty (Count == 0).

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: 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: 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: 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.