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.NET Full Stack Technical Round Questions & Answers

40 questions for the technical round stage — 1 easy, 33 medium, 6 hard.

About the technical round round

The technical round is the core of a .NET Full Stack interview: concept questions with follow-ups, a live hands-on task and a deep dive into one of your projects. Interviewers check depth, not just definitions.

Tips for the technical round round

  • Say your approach out loud before you start writing.
  • After a correct answer, expect 'why?' and 'what if?' follow-ups; go one level deeper than the question.
  • Pick one project you can explain end to end, including what you would do differently.
  • If you get stuck, explain what you would try next instead of going silent.
Solved — technical roundStart practicing
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01

What is the difference between a shallow copy and a deep copy?

OOP in C#Medium
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A shallow copy duplicates the object but its reference fields still point to the same child objects; MemberwiseClone and record with-expressions do this. A deep copy duplicates the whole object graph, so changes to the copy never affect the original. Deep copies are usually written by hand or by serialising and deserialising. ICloneable is avoided because it does not say which kind of copy it returns.
02

How do you use CancellationToken in an ASP.NET Core API?

Async Programming and MultithreadingMedium
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[HttpGet("report")] public async Task<IActionResult> Report(CancellationToken ct) { var rows = await _db.Sales .Where(s => s.Year == 2026) .ToListAsync(ct); await _http.PostAsJsonAsync("/audit", rows.Count, ct); return Ok(rows); } ASP.NET Core binds the token to HttpContext.RequestAborted, so if the user closes the browser the token is cancelled and EF Core and HttpClient stop work, saving database and CPU time.
03

What is the difference between a clustered and a non-clustered index?

SQL Server and T-SQLMedium
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A clustered index defines the physical order of the table's data, so the leaf level is the data itself, and a table can have only one. A non-clustered index is a separate structure with key values and a pointer back to the row, and a table can have many. Range queries on the clustered key are fast. Non-clustered indexes speed up searches on other columns but add cost to inserts and updates.
04

How do you use an explicit transaction in EF Core across multiple SaveChanges calls?

Entity Framework CoreMedium
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await using var tx = await db.Database.BeginTransactionAsync(); try { db.Orders.Add(order); await db.SaveChangesAsync(); var stock = await db.Stocks.SingleAsync(s => s.ProductId == order.ProductId); stock.Quantity -= order.Quantity; await db.SaveChangesAsync(); await tx.CommitAsync(); } catch { await tx.RollbackAsync(); throw; } A single SaveChanges is already transactional; explicit transactions are needed when you combine several saves or raw SQL into one unit.
05

What are generic constraints? Give examples.

LINQ, Collections and GenericsMedium
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Constraints restrict which types can be used for a type parameter so the generic code can rely on certain members. Examples: where T : class (reference type), where T : struct, where T : new() (has a parameterless constructor), where T : BaseEntity (inherits a class), where T : IComparable<T>, and where T : notnull. A repository might use where T : class, IEntity so it can read the Id property safely.
06

Write an Angular HTTP interceptor that adds a JWT to every API request.

Angular and React Front EndMedium
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import { HttpInterceptorFn } from '@angular/common/http'; import { inject } from '@angular/core'; export const authInterceptor: HttpInterceptorFn = (req, next) => { const token = inject(AuthService).token(); if (token) { req = req.clone({ setHeaders: { Authorization: `Bearer ${token}` } }); } return next(req); }; // app.config.ts provideHttpClient(withInterceptors([authInterceptor])); Requests are immutable, so clone() creates a modified copy. The ASP.NET Core JWT middleware then reads the header.
07

What is a delegate in C#?

OOP in C#Medium
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A delegate is a type-safe reference to a method with a specific signature, similar to a function pointer but checked by the compiler. You can pass methods as parameters, store them and call them later. Delegates can be multicast, holding several methods that run in order. Lambdas, LINQ, events and callbacks are all built on delegates, and the built-in Func and Action types cover most needs.
08

You need to add a new method to a widely implemented interface without breaking existing classes. How can you do it?

OOP in C#Medium
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Since C# 8, an interface can provide a method body. Classes implementing the interface get that behaviour without writing it, though they can override it. The main use is evolving a published interface: you can add a new member without breaking every existing implementation. The default method is only callable through the interface type, not the class, and overusing it blurs the line with abstract classes.
09

Implement paging for a product list in EF Core.

Entity Framework CoreMedium
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public async Task<PagedResult<ProductDto>> GetPageAsync(int page, int size) { var query = db.Products.AsNoTracking().OrderBy(p => p.Id); int total = await query.CountAsync(); var items = await query .Skip((page - 1) * size) .Take(size) .Select(p => new ProductDto(p.Id, p.Name, p.Price)) .ToListAsync(); return new PagedResult<ProductDto>(items, total, page, size); } Always order before Skip and Take. For very deep pages, keyset paging with Where(p => p.Id > lastId) is faster than large offsets.
10

Configure a one-to-many relationship between Customer and Order using the Fluent API.

Entity Framework CoreMedium
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public class Customer { public int Id { get; set; } public List<Order> Orders { get; set; } = []; } public class Order { public int Id { get; set; } public int CustomerId { get; set; } public Customer Customer { get; set; } = null!; } protected override void OnModelCreating(ModelBuilder mb) { mb.Entity<Order>() .HasOne(o => o.Customer) .WithMany(c => c.Orders) .HasForeignKey(o => o.CustomerId) .OnDelete(DeleteBehavior.Restrict); mb.Entity<Order>().Property(o => o.Total).HasPrecision(18, 2); } Restrict prevents deleting a customer who still has orders.
11

A query that used to run in 1 second now takes 30 seconds. How do you troubleshoot it?

SQL Server and T-SQLMedium
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Capture the actual execution plan and look for table or index scans on large tables, key lookups, big differences between estimated and actual rows, and warnings like implicit conversions or spills to tempdb. Check missing or fragmented indexes and outdated statistics, and update them. Use Query Store to compare the old and new plans; if a plan regressed, you can force the good plan while fixing the root cause.
12

How does change tracking work and what are entity states?

Entity Framework CoreHard
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The change tracker keeps a snapshot of each tracked entity and compares it on SaveChanges. Each entity has a state: Added (insert), Modified (update), Deleted (delete), Unchanged (no action) and Detached (not tracked). Calling Add, Update or Remove sets states, and changing a property of a tracked entity marks it Modified. SaveChanges then generates SQL only for changed columns, all inside one transaction.
13

Which design patterns do you commonly see in ASP.NET Core projects?

OOP in C#Medium
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Dependency Injection is built in. The Repository and Unit of Work patterns wrap data access, although DbContext already implements them. Factory creates objects by type, for example IHttpClientFactory. Strategy swaps algorithms, such as different tax or discount rules, behind one interface. Options pattern binds configuration to classes. Middleware is a Chain of Responsibility, and Mediator libraries implement CQRS by sending commands to handlers.
14

How does logging work in ASP.NET Core?

ASP.NET Core Web APIMedium
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ASP.NET Core has a built-in ILogger<T> abstraction injected into any class. You log with levels Trace, Debug, Information, Warning, Error and Critical, and configure minimum levels per category in appsettings.json. Use structured logging with message templates, like logger.LogInformation("Order {OrderId} placed", id), so values become searchable fields. Providers such as Serilog or OpenTelemetry send logs to files, Seq, Elasticsearch or Application Insights.
15

Users trigger a large Excel report that takes 3 minutes, and the HTTP request times out. How would you redesign it?

Async Programming and MultithreadingMedium
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Make it asynchronous at the workflow level. The API validates the request, saves a job record and returns 202 Accepted with a job ID. A background worker, using a queue such as a Channel, Hangfire or a message broker, generates the file and stores it in blob storage. The UI polls a status endpoint or receives a SignalR notification, then downloads the file. This avoids timeouts and survives retries.
16

Write a thread-safe singleton in C#.

OOP in C#Hard
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public sealed class AppConfig { private static readonly Lazy<AppConfig> _instance = new(() => new AppConfig()); public static AppConfig Instance => _instance.Value; private AppConfig() { // load settings once } } Lazy<T> is thread-safe by default and creates the object only on first access. The private constructor and sealed class prevent other instances. In ASP.NET Core, prefer AddSingleton.
17

What are record types and when would you use them?

C# FundamentalsMedium
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Records are reference types (or record struct for value types) designed for immutable data. The compiler generates value-based equality, a readable ToString, a deconstructor and support for with-expressions that create a modified copy. A positional record is one line: public record Order(int Id, decimal Total). They suit DTOs, API responses, events and value objects, but not EF Core entities that need change tracking by identity.
18

What are signals in Angular and why were they introduced?

Angular and React Front EndMedium
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Signals are reactive values: count = signal(0) holds state, computed() derives values and effect() runs side effects when dependencies change. Angular knows exactly which parts of the view read which signal, so it updates only those parts instead of checking the whole component tree. This enables zoneless change detection, which became the default for new apps in Angular 21, and gives simpler state handling than RxJS for local UI state.
19

Your team debates wrapping EF Core in a generic repository. What is your view?

Entity Framework CoreHard
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DbContext already implements Repository and Unit of Work, so a thin generic repository with GetAll and Add often adds code while hiding useful features like Include, projections and AsNoTracking. I prefer either using DbContext directly in services, or specific repositories with meaningful methods like GetOverdueInvoicesAsync that encapsulate queries. For tests, use the SQLite in-memory provider or Testcontainers rather than mocking DbSet.
20

Show how to stream data with IAsyncEnumerable.

Async Programming and MultithreadingMedium
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public async IAsyncEnumerable<Order> GetOrdersAsync( [EnumeratorCancellation] CancellationToken ct = default) { await foreach (var o in _db.Orders.AsNoTracking().AsAsyncEnumerable().WithCancellation(ct)) yield return o; } // consumer await foreach (var order in service.GetOrdersAsync()) Console.WriteLine(order.Id); Items are produced and consumed one at a time asynchronously. ASP.NET Core can stream an IAsyncEnumerable result as JSON without buffering the whole list.
21

Write a query to find duplicate email addresses and delete the extra rows, keeping the lowest Id.

SQL Server and T-SQLEasy
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-- find duplicates SELECT Email, COUNT(*) AS Cnt FROM Users GROUP BY Email HAVING COUNT(*) > 1; -- delete duplicates, keep lowest Id WITH d AS ( SELECT Id, ROW_NUMBER() OVER (PARTITION BY Email ORDER BY Id) AS rn FROM Users ) DELETE FROM d WHERE rn > 1; After cleanup, add a unique index on Email to stop duplicates returning.
22

Write custom middleware that logs how long each request takes.

ASP.NET Core Web APIMedium
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public class TimingMiddleware(RequestDelegate next, ILogger<TimingMiddleware> logger) { public async Task InvokeAsync(HttpContext context) { var sw = Stopwatch.StartNew(); await next(context); sw.Stop(); logger.LogInformation("{Method} {Path} -> {Status} in {Ms} ms", context.Request.Method, context.Request.Path, context.Response.StatusCode, sw.ElapsedMilliseconds); } } // Program.cs app.UseMiddleware<TimingMiddleware>(); Code before next runs on the request, code after it runs on the response.
23

Show how to implement IDisposable and use the using statement.

C# FundamentalsMedium
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public class FileLogger : IDisposable { private readonly StreamWriter _writer = new("log.txt"); public void Log(string msg) => _writer.WriteLine(msg); public void Dispose() => _writer.Dispose(); } // using declaration disposes at end of scope using var logger = new FileLogger(); logger.Log("Started"); Dispose releases unmanaged resources such as file handles and connections deterministically instead of waiting for the GC.
24

An API creates a new HttpClient inside a using block for every outgoing call, and under load you see socket exhaustion errors. How do you fix it?

ASP.NET Core Web APIMedium
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Disposing HttpClient does not immediately free the underlying socket; connections stay in TIME_WAIT, so thousands of short-lived clients exhaust ports. A single static client fixes that but misses DNS changes. The recommended fix is IHttpClientFactory: register builder.Services.AddHttpClient<PaymentClient>() and inject the typed client. The factory pools and rotates handlers, and you can add resilience with Microsoft.Extensions.Http.Resilience for retries and timeouts.
25

How do you run raw SQL safely in EF Core?

Entity Framework CoreMedium
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// entity query with parameters (interpolation becomes SqlParameter) var orders = await db.Orders .FromSql($"SELECT * FROM Orders WHERE City = {city}") .Where(o => o.Total > 1000) .ToListAsync(); // non-entity results var totals = await db.Database .SqlQuery<decimal>($"SELECT SUM(Total) AS Value FROM Orders") .ToListAsync(); // stored procedure await db.Database.ExecuteSqlAsync($"EXEC usp_CloseMonth {month}"); Never build SQL by string concatenation; FromSqlRaw with concatenated input invites SQL injection.
26

A page listing 100 orders with customer names runs 101 SQL queries. What is the problem and how do you fix it?

Entity Framework CoreMedium
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This is the N+1 problem: one query loads the orders, then lazy loading fires a separate query for each order's Customer. Fix it by loading everything in one query, either with Include(o => o.Customer) or better with a projection: Select(o => new OrderListDto(o.Id, o.Total, o.Customer.Name)), which fetches only needed columns via a join. Turn on EF Core logging to spot such patterns early.
27

An admin page renders 20,000 rows in a table and the browser freezes. How do you fix it?

Angular and React Front EndMedium
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Do not send or render everything at once. Add server-side paging, sorting and filtering in the ASP.NET Core API so the page loads perhaps 50 rows at a time. If users truly need to scroll a long list, use virtual scrolling, such as Angular CDK virtual scroll or TanStack Virtual in React, so only visible rows exist in the DOM. Also use trackBy or keys, and debounce search inputs.
28

Show how Task.WhenAll and Task.WhenAny are used.

Async Programming and MultithreadingMedium
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// run three independent calls in parallel var userTask = userApi.GetAsync(id); var ordersTask = orderApi.GetAsync(id); var walletTask = walletApi.GetAsync(id); await Task.WhenAll(userTask, ordersTask, walletTask); var dashboard = new Dashboard(userTask.Result, ordersTask.Result, walletTask.Result); // timeout using WhenAny var work = SlowCallAsync(); if (await Task.WhenAny(work, Task.Delay(2000)) != work) throw new TimeoutException(); WhenAll completes when all finish; WhenAny returns the first task to finish.
29

Show how Func, Action and Predicate are used.

OOP in C#Medium
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Func<decimal, decimal> addGst = price => price * 1.18m; Action<string> log = msg => Console.WriteLine($"[LOG] {msg}"); Predicate<int> isAdult = age => age >= 18; decimal total = addGst(1000m); // 1180 log($"Total {total}"); bool ok = isAdult(21); // true Func returns a value (last type argument is the return type), Action returns void, and Predicate<T> returns bool. They save you from declaring custom delegate types.
30

Write a LINQ query that joins orders with customers and returns customer name and order total.

LINQ, Collections and GenericsMedium
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var result = orders .Join(customers, o => o.CustomerId, c => c.Id, (o, c) => new { c.Name, o.Total }) .ToList(); // query syntax var result2 = (from o in orders join c in customers on o.CustomerId equals c.Id select new { c.Name, o.Total }).ToList(); Join is an inner join. For a left join use GroupJoin with DefaultIfEmpty, or the LeftJoin operator added in .NET 10.
31

Why should you avoid async void methods?

Async Programming and MultithreadingMedium
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An async void method cannot be awaited, so the caller does not know when it finishes. Exceptions thrown inside it cannot be caught by the caller; they go to the synchronization context and can crash the process. It is also hard to unit test. Use async Task instead. The only accepted use of async void is for event handlers, such as button click handlers in desktop apps, where the signature is fixed.
32

Write a stored procedure that transfers money between accounts with TRY-CATCH and a transaction.

SQL Server and T-SQLMedium
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CREATE OR ALTER PROCEDURE usp_Transfer @From INT, @To INT, @Amt DECIMAL(18,2) AS BEGIN SET NOCOUNT ON; SET XACT_ABORT ON; BEGIN TRY BEGIN TRAN; UPDATE Accounts SET Balance = Balance - @Amt WHERE Id = @From AND Balance >= @Amt; IF @@ROWCOUNT = 0 THROW 50001, 'Insufficient balance', 1; UPDATE Accounts SET Balance = Balance + @Amt WHERE Id = @To; COMMIT; END TRY BEGIN CATCH IF @@TRANCOUNT > 0 ROLLBACK; THROW; END CATCH END
33

Fetch products from an ASP.NET Core API in React with loading and error states.

Angular and React Front EndMedium
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function Products() { const [items, setItems] = useState([]); const [loading, setLoading] = useState(true); const [error, setError] = useState(null); useEffect(() => { const ctrl = new AbortController(); fetch('/api/products', { signal: ctrl.signal }) .then(r => { if (!r.ok) throw new Error(r.status); return r.json(); }) .then(setItems) .catch(e => { if (e.name !== 'AbortError') setError(e.message); }) .finally(() => setLoading(false)); return () => ctrl.abort(); }, []); if (loading) return <p>Loading...</p>; if (error) return <p>Error: {error}</p>; return <ul>{items.map(p => <li key={p.id}>{p.name}</li>)}</ul>; }
34

Demonstrate a race condition on a counter and fix it.

Async Programming and MultithreadingMedium
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int count = 0; Parallel.For(0, 100_000, _ => count++); Console.WriteLine(count); // often less than 100000 int safe = 0; Parallel.For(0, 100_000, _ => Interlocked.Increment(ref safe)); Console.WriteLine(safe); // always 100000 count++ is a read, add and write, so two threads can read the same value and lose an update. Interlocked performs the increment atomically without a full lock.
35

What is the difference between temporary tables and table variables?

SQL Server and T-SQLMedium
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Temporary tables (#Temp) live in tempdb, support indexes and statistics, and are visible to nested procedures, so they suit larger intermediate results. Table variables (@T) also use tempdb but have limited statistics, which made old optimisers estimate one row; SQL Server 2019 improved this with deferred compilation. Table variables are not affected by transaction rollback. Use table variables for small sets and temp tables for large ones.
36

What is a view and what is an indexed view?

SQL Server and T-SQLMedium
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A view is a saved SELECT query that acts like a virtual table; it simplifies complex joins and can restrict which columns users see, but by default it stores no data. An indexed view has a unique clustered index, so SQL Server physically stores and maintains its results, which speeds up heavy aggregations. The cost is slower writes on base tables, and it requires SCHEMABINDING and deterministic expressions.
37

Your Angular app's first load is slow on mobile because the main bundle is very large. How does lazy loading of routes help?

Angular and React Front EndMedium
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Lazy loading splits the app into chunks that download only when the user navigates to that route, so the initial bundle is smaller and the first page loads faster, especially on mobile data. In Angular use loadComponent or loadChildren with a dynamic import in the route config; in React use React.lazy with Suspense or the router's lazy option. Angular's @defer blocks can also delay heavy parts of a page.
38

How are exceptions handled in async code, and what is AggregateException?

Async Programming and MultithreadingHard
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When you await a faulted task, the original exception is rethrown, so normal try-catch works. If you block with .Wait() or .Result, the exception is wrapped in AggregateException. With Task.WhenAll, await throws only the first exception, but the returned task's Exception property holds all of them in an AggregateException. Unobserved exceptions from fire-and-forget tasks are silently lost, so always await or log them.
39

Write a recursive CTE that lists an employee hierarchy with levels.

SQL Server and T-SQLHard
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WITH Org AS ( SELECT Id, Name, ManagerId, 0 AS Lvl FROM Employees WHERE ManagerId IS NULL -- anchor: CEO UNION ALL SELECT e.Id, e.Name, e.ManagerId, o.Lvl + 1 FROM Employees e JOIN Org o ON e.ManagerId = o.Id -- recursive part ) SELECT REPLICATE(' ', Lvl) + Name AS Hierarchy, Lvl FROM Org OPTION (MAXRECURSION 100); The anchor query runs once and the recursive part repeats until no new rows are found.
40

Implement soft delete with a global query filter.

Entity Framework CoreHard
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public class Product { public int Id { get; set; } public bool IsDeleted { get; set; } } protected override void OnModelCreating(ModelBuilder mb) { mb.Entity<Product>().HasQueryFilter(p => !p.IsDeleted); } public override Task<int> SaveChangesAsync(CancellationToken ct = default) { foreach (var e in ChangeTracker.Entries<Product>().Where(e => e.State == EntityState.Deleted)) { e.State = EntityState.Modified; e.Entity.IsDeleted = true; } return base.SaveChangesAsync(ct); } Every query now hides deleted rows automatically; use IgnoreQueryFilters() for admin screens.

Frequently asked questions

What happens in the .NET Full Stack technical round round?

The technical round is the core of a .NET Full Stack interview: concept questions with follow-ups, a live hands-on task and a deep dive into one of your projects. Interviewers check depth, not just definitions.

How do I clear the technical round round?

Say your approach out loud before you start writing. After a correct answer, expect 'why?' and 'what if?' follow-ups; go one level deeper than the question. Pick one project you can explain end to end, including what you would do differently.

Which round comes next?

A typical order is online assessment, telephonic screening, one or two technical rounds, a managerial round and HR. Use the other round pages to practise each stage.

Practise every interview round

Online Assessment questionsTelephonic Interview questionsManagerial Round questionsRapid-Fire questionsHR & behavioural questionsFull mock interview

More ways to practise

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