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Start practice examC# MNC Coding Interview C# Programming Tutorial · MNC Coding
Remove duplicates without using LINQ Distinct()
What interviewers test
Real-world scenario
You are processing millions of user IDs coming from logs or Kafka messages.
You must remove duplicates fast and predictably.
Optimized approach (HashSet)
public static List<int> RemoveDuplicates(int[] input)
{
var seen = new HashSet<int>();
var result = new List<int>();
foreach (var item in input)
{
if (seen.Add(item)) // Add returns false if already exists
{result.Add(item);
}
}
return result;
}Complexity
Aspect Value
Time O(n)
Space O(n) (hash storage)
Why this is better than naive loops
Interview Tip:Say “I prefer HashSet because it gives O(1) average lookup and preserves intent clearly.”
C# MNC Coding Interview C# Programming Tutorial · MNC Coding
Explain async / await with a real production
scenario
What interviewers test
Real-world scenario
API calls:
Blocking threads = server collapse under load
Bad (Blocking)
public string GetUser()
{
var response = httpClient.GetAsync(url).Result; // Blocks threadreturn response.Content.ReadAsStringAsync().Result;
}Good (Async, scalable)
public async Task<string> GetUserAsync()
{
var response = await httpClient.GetAsync(url);
return await response.Content.ReadAsStringAsync();
}Why async/await matters
Key interview line
“Async doesn’t make code faster; it makes servers scalable.”
C# MNC Coding Interview C# Programming Tutorial · MNC Coding
IEnumerable vs ICollection vs IList
What interviewers test
Hierarchy
IEnumerable
└── ICollection
└── IList
IEnumerable<T>
IEnumerable<int> numbers = GetNumbers();
foreach (var n in numbers) { }✅ Best for streaming, read-only, deferred execution
ICollection<T>
ICollection<int> list = new List<int>();
list.Add(1);✅ When modifying collection without index access
IList<T>
IList<int> list = new List<int>();
int first = list[0];✅ When order & index matter
Interview rule of thumb“Expose the least powerful interface required.”
C# MNC Coding Interview C# Programming Tutorial · MNC Coding
Logic
string input = "swiss";
Dictionary<char, int> map = new Dictionary<char, int>();
foreach (char c in input)
map[c] = map.ContainsKey(c) ? map[c] + 1 : 1;
foreach (char c in input)
{
if (map[c] == 1)
{Console.WriteLine(c);
break;
}
}C# MNC Coding Interview C# Programming Tutorial · MNC Coding
Handle millions of records efficiently
What interviewers test
❌ Bad (Loads everything)
var users = db.Users.ToList(); // Memory explosion✅ Streaming (Best)
await foreach (var user in db.Users.AsAsyncEnumerable())
{Process(user);
}✅ Batching
const int batchSize = 1000;
for (int i = 0; i < total; i += batchSize)
{
var batch = GetBatch(i, batchSize);ProcessBatch(batch);
}Key principles
C# MNC Coding Interview C# Programming Tutorial · MNC Coding
Dependency Injection without any framework
What interviewers test
Step 1: Abstraction
public interface IMessageService
{void Send(string message);
}Step 2: Implementation
public class EmailService : IMessageService
{
public void Send(string message)
{Console.WriteLine("Email: " + message);
}
}Step 3: Injection
public class Notification
{
private readonly IMessageService _service;
public Notification(IMessageService service)
{
_service = service;
}
public void Notify(string msg)
{_service.Send(msg);
}
}Why this matters
C# MNC Coding Interview C# Programming Tutorial · MNC Coding
struct vs class (real-world)
What interviewers test
Use struct when:
public readonly struct Point
{
public int X { get; }
public int Y { get; }
}Use class when:
public class User
{
public string Name { get; set; }
}
Interview statement“Structs live on stack or inline, classes live on heap with GC cost.”
C# MNC Coding Interview C# Programming Tutorial · MNC Coding
Design a rate limiter
What interviewers test
Token bucket (simple)
public class RateLimiter
{
private readonly int _limit;
private int _count;
private DateTime _windowStart = DateTime.UtcNow;
private readonly object _lock = new();
public RateLimiter(int limit)
{
_limit = limit;
}
public bool Allow()
{lock (_lock)
{
if ((DateTime.UtcNow - _windowStart).TotalSeconds >= 1)
{
_count = 0;
_windowStart = DateTime.UtcNow;
}
if (_count < _limit)
{_count++;
return true;
}
return false;
}
}
}Used in
C# MNC Coding Interview C# Programming Tutorial · MNC Coding
LINQ vs IQueryable
What interviewers test
LINQ (IEnumerable)
var data = users.Where(x => x.Age > 30).ToList();IQueryable
var data = db.Users.Where(x => x.Age > 30);Interview line“IQueryable builds expressions; IEnumerable executes them.”
🔟 Write clean, production-ready C# code
What interviewers test
Principles
public class OrderService
{
public void PlaceOrder(Order order)
{
if (order == null)throw new ArgumentNullException(nameof(order));
Validate(order);
Save(order);
}
private void Validate(Order order)
{
if (order.Total <= 0)throw new InvalidOperationException("Invalid total");
}
private void Save(Order order)
{// persistence logic
}
}