Interview Q&A

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.

Short answer: Use the Clear() method to remove all key-value pairs.

Explain a bit more

dictionary.Clear(); This resets the dictionary to an empty state. 📘 C# Queue<T> – Interview Questions &amp; 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 &amp; 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 &amp; 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 &amp;

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: Contains(item) checks whether the item exists in the list. It uses Equals() internally.

Example code

bool exists = list.Contains(10); Note: For custom objects, override Equals() and GetHashCode().

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: 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: 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: Use GroupBy, Distinct, or nested loops. Example: bool hasDuplicates = list.Count != list.Distinct().Count();

Example code

Use GroupBy, Distinct, or nested loops. Example: bool hasDuplicates = list.Count != list.Distinct().Count();

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 ToArray() method. Example: int[] array = list.ToArray(); Useful when interfacing with APIs that require arrays.

Example code

Use ToArray() method. Example: int[] array = list.ToArray(); Useful when interfacing with APIs that require arrays.

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: list = list.Distinct().ToList(); For custom objects, override Equals() and GetHashCode().

Example code

list = list.Distinct().ToList();
For custom objects, override Equals() and GetHashCode().

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 IndexOf(item) or FindIndex(predicate). Example: int index = list.IndexOf(10); int indexByCondition = list.FindIndex(x => x > 100); 📘 C# Dictionary<TKey, TValue> – Interview Questions & Answers

Example code

Use IndexOf(item) or FindIndex(predicate). Example: int index = list.IndexOf(10);
int indexByCondition = list.FindIndex(x => x > 100); 📘 C# Dictionary<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.