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Top 50 Most Asked .NET Full Stack Interview Questions

The questions that appear most often across company-wise sets — master these first.

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What is the difference between concurrency and parallelism?

Async Programming and MultithreadingIn 10 company setsEasy

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Concurrency means dealing with many tasks at once by interleaving them, for example a web server handling thousands of requests that mostly wait on I/O. Parallelism means doing many computations at the same instant on multiple cores, such as processing an image in four chunks. async/await gives concurrency; Parallel.For and PLINQ give parallelism. A program can be concurrent on a single core but parallel only with multiple cores.

What is the difference between DELETE, TRUNCATE and DROP?

SQL Server and T-SQLIn 9 company setsEasy

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DELETE removes rows one by one, supports WHERE, fires triggers and is fully logged, so it is slower on big tables. TRUNCATE removes all rows by deallocating pages, is minimally logged, resets identity values and cannot have WHERE; it fails if foreign keys reference the table. DROP removes the whole table including structure, indexes and permissions. In SQL Server, both DELETE and TRUNCATE can be rolled back inside a transaction.

What is the CLR and what does it do?

C# FundamentalsIn 8 company setsEasy

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The Common Language Runtime is the execution engine of .NET. It loads compiled IL code, converts it to machine code with the JIT compiler, manages memory through the garbage collector, enforces type safety, handles exceptions and manages threads. Because every .NET language compiles to the same IL, C#, F# and VB.NET code can run on the same CLR and call each other.

What is the difference between compile-time and runtime polymorphism?

OOP in C#In 8 company setsEasy

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Compile-time, or static, polymorphism is resolved by the compiler, mainly through method overloading and operator overloading, and through generics. Runtime, or dynamic, polymorphism is resolved when the program runs, through virtual methods and interfaces: a Shape reference may call Circle.Area or Square.Area depending on the actual object. Runtime polymorphism is what makes plugin designs and dependency injection possible.

What is the difference between Select and SelectMany?

LINQ, Collections and GenericsIn 8 company setsMedium

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Select projects each element to one result, so a list of customers mapped to their orders gives a list of lists. SelectMany projects each element to a sequence and flattens all sequences into one, so customers.SelectMany(c => c.Orders) gives a single list of all orders. In EF Core, SelectMany over a navigation collection translates to a SQL join, which is handy for reports.

How do model binding and validation work in ASP.NET Core Web API?

ASP.NET Core Web APIIn 8 company setsMedium

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Model binding maps request data into action parameters from the route, query string, headers or body, using [FromRoute], [FromQuery] and [FromBody]. Validation uses data annotations like [Required], [StringLength] and [Range] on DTOs. With [ApiController], invalid models automatically return 400 Bad Request with ProblemDetails listing the errors, so you do not need to check ModelState manually. FluentValidation is popular for complex rules.

What is the difference between method overloading and method overriding?

OOP in C#In 7 company setsEasy

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Overloading means several methods with the same name but different parameter lists in the same class, resolved at compile time, for example Add(int, int) and Add(double, double). Overriding means a derived class provides a new implementation of a virtual or abstract base method with the same signature, resolved at runtime based on the actual object type. Overloading is compile-time polymorphism; overriding is runtime polymorphism.

What is deferred execution in LINQ?

LINQ, Collections and GenericsIn 7 company setsMedium

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Most LINQ operators such as Where and Select do not run when you write them; they build a query that executes only when you enumerate it with foreach, ToList, Count or First. This means the query sees the latest data at enumeration time and can be composed step by step. The catch is that enumerating the same query twice runs it twice, which can mean two database calls. Call ToList to materialise once.

Explain value types and reference types in C#.

C# FundamentalsIn 6 company setsEasy

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Value types such as int, double, bool, enums and structs hold their data directly, and assigning one copies the value. Reference types such as classes, arrays, strings and delegates hold a reference to an object on the managed heap, so assigning copies only the reference. Changing a copied struct does not affect the original, but changing an object through a copied reference changes what both variables see.

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

C# FundamentalsIn 6 company setsMedium

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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.

Explain the ref, out and in parameter modifiers.

C# FundamentalsIn 6 company setsMedium

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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.

Show a class using a primary constructor with dependency injection.

C# FundamentalsIn 6 company setsMedium

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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.

What are the four pillars of object-oriented programming?

OOP in C#In 6 company setsEasy

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Encapsulation hides internal state behind methods and properties. Abstraction exposes only what the caller needs, using abstract classes and interfaces. Inheritance lets a class reuse and extend another class. Polymorphism lets one interface have many implementations, so code calls Pay() without knowing whether it is a card or UPI payment. Together they make code easier to change, test and reuse.

Why does using async in ASP.NET Core controllers improve scalability?

Async Programming and MultithreadingIn 6 company setsMedium

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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.

What is the difference between CPU-bound and I/O-bound work, and how do you handle each in .NET?

Async Programming and MultithreadingIn 6 company setsEasy

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CPU-bound work keeps the processor busy, like encryption, image processing or heavy calculations; speed it up with parallelism using Parallel.For, PLINQ or Task.Run in desktop apps. I/O-bound work spends most time waiting on databases, files or networks; handle it with async/await so threads are released while waiting. Choosing wrongly wastes resources, such as using Parallel.ForEach for HTTP calls or Task.Run around database queries.

Explain eager, lazy and explicit loading in EF Core.

Entity Framework CoreIn 6 company setsMedium

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Eager loading fetches related data in the same query using Include and ThenInclude, for example orders with their items. Lazy loading loads a navigation property automatically the first time you access it, requiring proxies and virtual properties; it is convenient but easily causes N+1 queries. Explicit loading loads a navigation on demand with Entry(order).Collection(o => o.Items).LoadAsync(). Projection with Select is often better than all three.

Implement paging for a product list in EF Core.

Entity Framework CoreIn 6 company setsMedium

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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.

Explain the types of data binding in Angular.

Angular and React Front EndIn 6 company setsEasy

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Interpolation {{ value }} shows component data in the template. Property binding [src]="imageUrl" sets an element or child property. Event binding (click)="save()" calls a component method on a DOM event. Two-way binding [(ngModel)]="name" combines property and event binding so the input and the field stay in sync. The first two flow from component to view, event binding flows from view to component.

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

C# FundamentalsIn 5 company setsEasy

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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 nullable value types and nullable reference types?

C# FundamentalsIn 5 company setsMedium

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Nullable value types like int? wrap a struct so it can hold null, with HasValue and Value properties. Nullable reference types, enabled by <Nullable>enable</Nullable>, are a compile-time feature: string means non-null and string? means may be null, and the compiler warns when you dereference a possible null. They do not change runtime behaviour but catch many NullReferenceException bugs before deployment.

You are designing a type to hold a 2D point used millions of times in a calculation. Struct or class?

C# FundamentalsIn 5 company setsMedium

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A struct is a good fit because the point is small, logically a single value and created in huge numbers. Structs avoid heap allocation and GC pressure when stored in arrays or locals. Make it a readonly struct so it is immutable and avoids defensive copies. Choose a class instead if the type is large, needs inheritance, or is frequently boxed through interfaces, since copying and boxing remove the benefit.

What is the difference between throw and throw ex inside a catch block?

C# FundamentalsIn 5 company setsMedium

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throw rethrows the current exception and preserves the original stack trace, so logs show where the error really started. throw ex resets the stack trace to the catch block, hiding the real source. Always use throw when rethrowing. If you need more context, wrap it: throw new OrderException("Payment failed", ex), which keeps the original as InnerException. Also avoid catching Exception unless you log and rethrow.

When would you choose an array, a List<T> or a LinkedList<T>?

LINQ, Collections and GenericsIn 5 company setsEasy

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An array has a fixed size and fastest index access, good when the count is known. List<T> wraps a resizable array, with O(1) index access and amortised O(1) add at the end, and is the default choice for most code. LinkedList<T> gives O(1) insert and remove at a known node but O(n) access by index and poor cache locality, so it is rarely the better option in practice.

What are generics and what benefits do they give?

LINQ, Collections and GenericsIn 5 company setsEasy

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Generics let you write classes and methods with a type parameter, such as List<T> or Repository<T>, that work with any type while staying type-safe. Benefits are compile-time checking instead of casts, no boxing for value types, and code reuse. For example, one generic Result<T> type can wrap any API response. The CLR creates specialised code for value types and shares code for reference types.

Show the newer LINQ methods Chunk, DistinctBy and MaxBy.

LINQ, Collections and GenericsIn 5 company setsMedium

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// split 1,000 ids into batches of 100 for bulk API calls foreach (int[] batch in ids.Chunk(100)) await SendBatchAsync(batch); // unique customers by email var unique = customers.DistinctBy(c => c.Email).ToList(); // highest-paid employee object, not just the salary Employee? top = employees.MaxBy(e => e.Salary); These were added in .NET 6 and remove the need for custom helpers or third-party libraries like MoreLINQ.

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

LINQ, Collections and GenericsIn 5 company setsEasy

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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.

Why should you avoid async void methods?

Async Programming and MultithreadingIn 5 company setsMedium

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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.

Show how to stream data with IAsyncEnumerable.

Async Programming and MultithreadingIn 5 company setsMedium

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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.

Write a BackgroundService that cleans expired OTPs every 5 minutes.

Async Programming and MultithreadingIn 5 company setsMedium

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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 APIIn 5 company setsEasy

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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.

Explain the transient, scoped and singleton service lifetimes.

ASP.NET Core Web APIIn 5 company setsEasy

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Transient services are created every time they are requested, suitable for lightweight stateless helpers. Scoped services are created once per HTTP request and shared within it; DbContext is registered as scoped. Singleton services are created once for the whole application lifetime, suitable for caches and configuration. Choose based on state: anything holding per-request data must not be singleton, and singletons must be thread-safe.

Implement global exception handling that returns ProblemDetails.

ASP.NET Core Web APIIn 5 company setsMedium

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public class GlobalExceptionHandler(ILogger<GlobalExceptionHandler> logger) : IExceptionHandler { public async ValueTask<bool> TryHandleAsync(HttpContext ctx, Exception ex, CancellationToken ct) { logger.LogError(ex, "Unhandled error"); var status = ex is KeyNotFoundException ? 404 : 500; ctx.Response.StatusCode = status; await ctx.Response.WriteAsJsonAsync(new ProblemDetails { Status = status, Title = status == 404 ? "Not found" : "Server error" }, ct); return true; } } // Program.cs builder.Services.AddExceptionHandler<GlobalExceptionHandler>(); builder.Services.AddProblemDetails(); app.UseExceptionHandler();

Show how to bind configuration from appsettings.json using the Options pattern.

ASP.NET Core Web APIIn 5 company setsMedium

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// appsettings.json // "Smtp": { "Host": "smtp.example.com", "Port": 587 } public class SmtpOptions { public string Host { get; set; } = ""; public int Port { get; set; } } builder.Services.Configure<SmtpOptions>(builder.Configuration.GetSection("Smtp")); public class EmailService(IOptions<SmtpOptions> options) { private readonly SmtpOptions _smtp = options.Value; } appsettings.{Environment}.json and environment variables override base values. Use IOptionsMonitor to pick up changes without restart.

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

Entity Framework CoreIn 5 company setsMedium

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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.

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

SQL Server and T-SQLIn 5 company setsMedium

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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.

Explain the different types of SQL joins.

SQL Server and T-SQLIn 5 company setsEasy

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INNER JOIN returns rows that match in both tables. LEFT JOIN returns all rows from the left table plus matches from the right, with NULLs where there is no match. RIGHT JOIN is the mirror. FULL OUTER JOIN returns all rows from both sides. CROSS JOIN returns every combination. A self join joins a table to itself, such as employees to their managers. LEFT JOIN with WHERE right.Id IS NULL finds unmatched rows.

Angular or React with ASP.NET Core: how do you choose?

Angular and React Front EndIn 5 company setsEasy

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Angular is a complete framework with routing, forms, HTTP, dependency injection and TypeScript built in, which suits large enterprise apps and teams that want strong conventions. React is a UI library with a large ecosystem, giving more freedom to pick routing and state tools, and is popular in product startups. Both work equally well with an ASP.NET Core API. Choose based on team skills, project size and the company's existing stack.

What are signals in Angular and why were they introduced?

Angular and React Front EndIn 5 company setsMedium

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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.

What is the difference between props and state in React?

Angular and React Front EndIn 5 company setsEasy

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Props are inputs passed from a parent component to a child, like function arguments, and the child must not modify them. State is data owned and managed inside a component with useState or useReducer, and changing it re-renders the component. For example, a ProductCard receives product details as props, while a Cart component keeps the quantity in state. Shared state is lifted up to a common parent or moved into a store.

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 EndIn 5 company setsMedium

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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.

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

Angular and React Front EndIn 5 company setsMedium

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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.

When should you use StringBuilder instead of string?

C# FundamentalsIn 4 company setsEasy

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Strings in C# are immutable, so every concatenation creates a new string object and copies the old content. In a loop that appends thousands of times, this creates heavy allocations. StringBuilder keeps a resizable internal buffer and appends in place, so it is much faster for repeated changes. For two or three concatenations or interpolation in a single statement, a normal string is simpler and fine.

What are record types and when would you use them?

C# FundamentalsIn 4 company setsMedium

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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.

Write a switch expression using pattern matching to calculate shipping cost.

C# FundamentalsIn 4 company setsMedium

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decimal Shipping(Order o) => o switch { { Total: >= 1000 } => 0m, { Country: "IN", Weight: < 2 } => 49m, { Country: "IN" } => 99m, { Weight: > 20 } => throw new ArgumentException("Too heavy"), _ => 499m }; Property and relational patterns replace long if-else chains, and the compiler warns if cases are not exhaustive.

What is the difference between var, dynamic and object?

C# FundamentalsIn 4 company setsEasy

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var is implicitly typed: the compiler infers the real type at compile time, so it is fully type-safe. object is the base type of everything; you need casts to use specific members, and value types get boxed. dynamic skips compile-time checking and resolves members at runtime, so typos fail only when the code runs. Use var for readability, object rarely, and dynamic only for COM interop or truly dynamic data.

What is the difference between an abstract class and an interface in C#?

OOP in C#In 4 company setsEasy

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An abstract class can have fields, constructors, implemented and abstract members, and a class can inherit only one. An interface defines a contract; a class can implement many. Since C# 8, interfaces can have default method implementations but still no instance fields. Use an abstract class for shared state and base behaviour among closely related types, and interfaces for capabilities like IComparable or for dependency injection.

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

OOP in C#In 4 company setsMedium

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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.

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

OOP in C#In 4 company setsMedium

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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.

A teammate created a deep inheritance chain Employee > Manager > SeniorManager > RegionalHead to add features. What would you suggest?

OOP in C#In 4 company setsMedium

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Deep hierarchies are fragile: a change in a base class ripples into every child, and behaviours that do not fit the tree get duplicated. I would suggest composition over inheritance. Keep a single Employee entity and give it composed parts such as a Role, an ApprovalPolicy and a list of Permissions, injected as interfaces. New behaviour then means a new policy class, not a new subclass, which is easier to test.

What is a delegate in C#?

OOP in C#In 4 company setsMedium

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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.

Frequently asked questions

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We count how often each question appears across the 20 company-wise .NET Full Stack practice sets on this site and show the 50 most common. They are the safest questions to master first.

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