Interview Q&A

• Define your custom object class.
• Create a collection class that holds objects of that type using generics or directly.

Example:

public class Employee
{
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.

Use methods like ToList(), ToArray(), or ToDictionary() to convert LINQ query

results to different collection types.

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

Answer: Use a foreach loop which iterates over the elements in sorted ascending order: foreach (var item in sortedSet) { Console.WriteLine(item); }

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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.

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) { /* ... */ }
foreach (var value in sortedList.Values) { /* ... */ }

📘 C# SortedSet<T> – Interview

Questions & Answers

Answer: No, HashSet&lt;T&gt; does not allow duplicates. Attempting to add a duplicate value will return false and not change the set. bool added = uniqueNumbers.Add(2); // returns false because 2 lready exists

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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: 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.

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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.

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

Example:

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.

• Contains(item)
• IndexOf(item)
• Find(predicate)
• FindAll(predicate)
• Exists(predicate)
• BinarySearch(item) (for sorted lists)

Example:

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

• 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:

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.

Operation LinkedList<T> List<T>

Indexed access O(n) (no indexing) O(1) (direct access)

dd/Remove at

start/end

O(1) O(n) (start), O(1) (end)

dd/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. Use List<T> for fast random access and better memory efficiency.

📘 C# SortedList<TKey, TValue> –

Interview Questions & Answers

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 };
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}

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

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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: 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

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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: Use the Count property. if (stack.Count == 0) { Console.WriteLine("Stack is empty"); } Unlike some languages, C# stacks do not provide an IsEmpty property.

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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: Use a foreach loop. Iteration does not modify the queue. Example: foreach (var order in orders) { Console.WriteLine(order); } You can also use .ToArray() if needed: string[] items = orders.ToArray();

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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.

• 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);

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

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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.

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

rrayList arr = new ArrayList();

rr.Add(1);

rr.Add("text"); // Allowed, but risky

// Generic

List<int> list = new List<int>();
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>();
dict["id"] = 101;

Real-world use case:

Generic collections are recommended for new development due to safety and performance.

Non-generic collections are often found in older legacy systems.

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

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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: Use the Clear() method to remove all elements. stack.Clear(); fter this, Count becomes 0, and the internal array is reset.

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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: 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&lt;T&gt; – Interview Questions &amp;

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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.

• Using Add() will throw a System.ArgumentException
• Using the indexer (dictionary[key] = value) will overwrite the existing value

Example:

dictionary.Add("John", 25);
dictionary.Add("John", 30); // Exception
dictionary["John"] = 30; // Overwrites the value safely

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

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections 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# Collections 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.

Use a foreach loop, which iterates from top to bottom (LIFO order).

Example:

foreach (var item in stack)
{

Console.WriteLine(item);

}

This does not modify the stack — it's read-only iteration.

📘 C# HashSet<T> – Interview Questions

& Answers

Use a foreach loop with KeyValuePair<TKey, TValue>:

foreach (KeyValuePair<string, int> pair in dictionary)
{

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

}

Or use deconstruction (C# 7+):

foreach (var (key, value) in dictionary)
{
Console.WriteLine($"{key} = {value}");
}