.NET Full Stack Interview Questions in Pune

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Each request needs a thread from the thread pool. If a controller blocks while waiting for SQL Server or an external API, that thread sits idle and cannot serve other users, so under load the pool runs out and requests queue. With async I/O, the thread is released during the wait and reused, so the same server handles many more concurrent requests. It does not make a single request faster.

How are events different from plain delegates?

OOP in C#Medium

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An event is a delegate wrapped with restricted access. Outside the class, subscribers can only add or remove handlers with += and -=; they cannot invoke the event or overwrite the whole list with =. This protects the publisher. The standard pattern uses EventHandler<TEventArgs> with sender and args. Remember to unsubscribe long-lived subscriptions, otherwise the publisher keeps subscribers alive and causes memory leaks.

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

Async Programming and MultithreadingHard

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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.

Profiling shows a hot cache-lookup method returning Task<T> allocates millions of Task objects. Would ValueTask help?

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ValueTask<T> is a struct that avoids allocating a Task when a method usually completes synchronously, for example when a value is served from an in-memory cache most of the time. In very hot paths this reduces GC pressure. The rules are stricter: await a ValueTask only once, do not await it concurrently, and do not call .Result before it completes. For normal application code, Task is simpler and recommended.

What is the virtual DOM in React?

Angular and React Front EndEasy

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The virtual DOM is a lightweight JavaScript copy of the UI tree. When state changes, React renders a new virtual tree, compares it with the previous one in a process called reconciliation, and applies only the minimal changes to the real DOM, which is slow to update. Keys on list items help React match elements correctly. This lets developers write UI declaratively without manually updating DOM nodes.

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

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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.

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.

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.

Sort a list of students by marks descending, then by name ascending.

LINQ, Collections and GenericsEasy

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var ranked = students .OrderByDescending(s => s.Marks) .ThenBy(s => s.Name) .ToList(); // in-place alternative with a comparison students.Sort((a, b) => b.Marks != a.Marks ? b.Marks.CompareTo(a.Marks) : string.Compare(a.Name, b.Name, StringComparison.Ordinal)); OrderBy is stable and returns a new sequence; List.Sort sorts in place and is not stable.

Write an extension method that checks whether a string is a valid Indian mobile number.

C# FundamentalsEasy

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public static class StringExtensions { public static bool IsIndianMobile(this string? value) => value is not null && System.Text.RegularExpressions.Regex.IsMatch(value, @"^[6-9]\d{9}$"); } // usage bool ok = "9876543210".IsIndianMobile(); Extension methods are static methods in a static class whose first parameter uses this, so they appear like instance methods.

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

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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.

Show an example of the Dependency Inversion Principle in C#.

OOP in C#Medium

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public interface INotifier { Task SendAsync(string to, string msg); } public class SmsNotifier : INotifier { public Task SendAsync(string to, string msg) => Task.CompletedTask; // call SMS API } public class OrderService(INotifier notifier) { public Task ConfirmAsync(Order o) => notifier.SendAsync(o.Phone, "Order placed"); } // Program.cs builder.Services.AddScoped<INotifier, SmsNotifier>(); OrderService depends on the abstraction, so you can swap to email or a mock without changing it.

Implement paging for a product list in EF Core.

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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.

Where should you store secrets like connection strings and API keys in an ASP.NET Core app?

ASP.NET Core Web APIMedium

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Never commit them in appsettings.json. During development use the Secret Manager (dotnet user-secrets), which stores them outside the project folder. In production use environment variables or, better, a vault such as Azure Key Vault or AWS Secrets Manager, loaded through a configuration provider and accessed with managed identity so no credential sits in code. Rotate secrets regularly and restrict who can read them.

Tell me about a disagreement with the front-end team over an API contract and how you resolved it.

ASP.NET Core Web APIMedium

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On a dashboard feature (Situation) the React team wanted one endpoint returning everything, while I worried about payload size. My task was to agree a design quickly. I set up a short call, we listed exact screen needs, and I proposed one aggregated endpoint with paging and optional fields. We wrote the OpenAPI spec together. Result: payload dropped by half, the UI got one call, and the spec became our template for later features.

An API endpoint calls _db.Orders.ToList().Where(o => o.CustomerId == id) and is slow as the table grows. Explain and fix it.

LINQ, Collections and GenericsHard

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ToList executes immediately, so the entire Orders table is loaded into memory and then filtered in C#. As data grows, memory and response time grow with it. Move the filter before materialisation: await _db.Orders.Where(o => o.CustomerId == id).AsNoTracking().ToListAsync(). Now EF Core sends a WHERE clause, the database uses an index on CustomerId, and only matching rows travel over the network.

Explain the ref, out and in parameter modifiers.

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ref passes a variable by reference; it must be initialised before the call and the method can read and change it. out also passes by reference but need not be initialised, and the method must assign it before returning, as in int.TryParse. in passes by reference as read-only, which avoids copying large structs while preventing changes. Overusing ref and out makes code harder to read, so tuples are often cleaner.

What is the difference between const, readonly and static readonly?

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const is a compile-time constant, implicitly static, and its value is copied into calling assemblies, so changing it requires recompiling them. readonly fields are set at declaration or in the constructor and can differ per instance. static readonly is set once per type at runtime, so it can hold values like DateTime.Now or a new object. Use const only for values that will never change, such as Math.PI.

Demonstrate a race condition on a counter and fix it.

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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.

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.

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.

What is the difference between .NET Framework, .NET Core and modern .NET?

C# FundamentalsEasy

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.NET Framework is the original Windows-only runtime, now in maintenance mode at version 4.8.1. .NET Core was the cross-platform, open-source rewrite. From .NET 5 onwards the name became just .NET, and it runs on Windows, Linux and macOS. In 2026, .NET 10 is the current LTS release. New projects should use modern .NET; Framework is kept only for legacy apps such as old Web Forms or WCF services.

What are partial classes and partial methods?

OOP in C#Easy

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A partial class splits one class definition across multiple files, and the compiler merges them. It is used heavily with generated code: designers, EF Core scaffolding and source generators write one part while you write the other, so regeneration does not erase your code. Partial methods declare a signature in one part and an optional implementation in another; since C# 9 they can have return types and access modifiers if implemented.

How does EF Core handle many-to-many relationships?

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Since EF Core 5, you can define collection navigations on both sides, such as Student.Courses and Course.Students, and EF Core creates a hidden join table automatically. If the join needs extra data, like EnrolledOn or Grade, create an explicit join entity Enrollment with two foreign keys and configure it with UsingEntity or two one-to-many relationships. Querying works naturally with Include or projections through the navigations.

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.

Show encapsulation using properties, including init-only and validation.

OOP in C#Easy

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public class BankAccount { public string AccountNo { get; init; } = ""; public decimal Balance { get; private set; } public void Deposit(decimal amount) { if (amount <= 0) throw new ArgumentOutOfRangeException(nameof(amount)); Balance += amount; } } var acc = new BankAccount { AccountNo = "SB1001" }; Balance can only change through Deposit, and AccountNo can be set only during object creation.

Show a class using a primary constructor with dependency injection.

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public class OrderService(IOrderRepository repo, ILogger<OrderService> logger) { public async Task<Order?> GetAsync(int id) { logger.LogInformation("Fetching order {Id}", id); return await repo.FindAsync(id); } } Primary constructors on classes arrived in C# 12. Parameters are captured and usable in all members, removing boilerplate fields. They are not readonly, so assign to a readonly field if immutability matters.

Why should you avoid async void methods?

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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.

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

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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.

Write a BackgroundService that cleans expired OTPs every 5 minutes.

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public class OtpCleanupService(IServiceScopeFactory scopes) : BackgroundService { protected override async Task ExecuteAsync(CancellationToken ct) { using var timer = new PeriodicTimer(TimeSpan.FromMinutes(5)); while (await timer.WaitForNextTickAsync(ct)) { using var scope = scopes.CreateScope(); var db = scope.ServiceProvider.GetRequiredService<AppDbContext>(); await db.Otps.Where(o => o.ExpiresAt < DateTime.UtcNow).ExecuteDeleteAsync(ct); } } } // builder.Services.AddHostedService<OtpCleanupService>(); A scope is created because DbContext is scoped while hosted services are singletons.

What is the middleware pipeline in ASP.NET Core?

ASP.NET Core Web APIEasy

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Every HTTP request passes through a chain of middleware components configured in Program.cs. Each component can inspect or modify the request, call the next one, and then work on the response on the way back. Order matters: exception handling first, then HTTPS redirection, static files, routing, CORS, authentication, authorization and finally endpoints. For example, putting UseAuthorization before UseAuthentication means users never appear logged in.

Show how yield return works with an example.

LINQ, Collections and GenericsMedium

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static IEnumerable<int> EvenNumbers(int max) { for (int i = 0; i <= max; i += 2) { yield return i; // pause here, resume on next MoveNext } } foreach (var n in EvenNumbers(10).Take(3)) Console.WriteLine(n); // 0 2 4 The compiler builds a state machine, so values are produced lazily one at a time. Only three values are generated here, which saves memory for large or infinite sequences.

What are record types and when would you use them?