Easy .NET Full Stack Interview Questions

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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 is the CLR and what does it do?

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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.

Explain value types and reference types in C#.

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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 are boxing and unboxing?

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Boxing converts a value type to object or an interface type by wrapping it in a heap object, for example object o = 42. Unboxing extracts the value back with an explicit cast, such as int n = (int)o. Both cost allocations and CPU time, and repeated boxing in loops increases garbage collection pressure. Generic collections like List<int> avoid boxing, which is one reason ArrayList is no longer used.

When should you use StringBuilder instead of string?

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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.

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

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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.

What is the difference between var, dynamic and object?

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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.

A report builder concatenates 50,000 rows into one string with += and takes several seconds. What do you change?

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Each += creates a new string and copies everything built so far, so the cost grows quadratically. Replace it with a StringBuilder, ideally created with an estimated capacity, and call Append or AppendLine in the loop, then ToString once at the end. For very large output, write directly to a StreamWriter or the response stream instead of building the whole string in memory.

Write a C# method to reverse a string without using Array.Reverse or LINQ.

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static string ReverseText(string input) { char[] chars = input.ToCharArray(); int left = 0, right = chars.Length - 1; while (left < right) { (chars[left], chars[right]) = (chars[right], chars[left]); left++; right--; } return new string(chars); } Two pointers swap from both ends, giving O(n) time and O(n) space for the char array.

Tell me about a time you had to learn a new C# or .NET feature quickly.

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In my final-year project (Situation) our team needed async file uploads, but I only knew synchronous code. My task was to build the upload API within a week. I read the official async/await docs, built a small console demo, then refactored our controller to use async methods and IFormFile streaming. The result was uploads that no longer blocked requests, and I shared a short note with my team.

Why do you want to work as a .NET full stack developer?

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I enjoy building complete features, from database to screen, and .NET lets me do that with one strong, typed language on the back end. It is widely used by IT services firms, banks and product companies in India, so there are steady opportunities. I have built projects with ASP.NET Core and React, and I like that the platform is open source, cross-platform and keeps improving every year.

What are the four pillars of object-oriented programming?

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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.

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

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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.

What is the difference between method overloading and method overriding?

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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 does the sealed keyword do?

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A sealed class cannot be inherited, and a sealed override method cannot be overridden further in derived classes. Use it when a class is not designed for extension, such as a security-sensitive or immutable type; string is sealed for this reason. Sealing also lets the JIT devirtualise calls, giving small performance gains. Many teams seal classes by default and unseal only when inheritance is genuinely needed.

Show constructor chaining in C#.

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public class Customer { public string Name { get; } public string City { get; } public Customer(string name) : this(name, "Unknown") { } public Customer(string name, string city) { Name = name; City = city; } } public class PremiumCustomer : Customer { public PremiumCustomer(string name) : base(name, "Delhi") { } } this() calls another constructor of the same class and base() calls the parent constructor, avoiding duplicated setup code.

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

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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.

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

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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.

What are partial classes and partial methods?

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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.

What is LINQ and why is it useful?

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LINQ, Language Integrated Query, lets you query collections, databases, XML and other sources with the same C# syntax using methods like Where, Select, OrderBy and GroupBy. Queries are type-checked at compile time and read close to the intent, for example orders.Where(o => o.Total > 500).OrderBy(o => o.Date). With EF Core, the same LINQ expression is translated into SQL, so you rarely write raw SQL strings.

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

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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.

When would you use a HashSet<T>?

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HashSet<T> stores unique values with O(1) average lookup, add and remove. Use it when you need fast membership checks, such as checking if an email already exists in a batch of 1 lakh records, or to remove duplicates. It also supports set operations like UnionWith, IntersectWith and ExceptWith. It does not keep insertion order or allow index access; use SortedSet<T> if you need sorted unique values.

Explain First, FirstOrDefault, Single and SingleOrDefault.

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First returns the first match and throws if none exists. FirstOrDefault returns the default value, null for classes, if none exists. Single expects exactly one match and throws if there are zero or more than one. SingleOrDefault returns default for zero but still throws for more than one. Use Single when duplicates would indicate a data bug, such as lookup by unique PAN number; otherwise FirstOrDefault is cheaper.

Find the second highest salary from a list using LINQ.

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int? second = employees .Select(e => e.Salary) .Distinct() .OrderByDescending(s => s) .Skip(1) .Cast<int?>() .FirstOrDefault(); Distinct handles ties, so if two people earn the top salary the next lower value is returned. Casting to int? makes the result null instead of 0 when fewer than two distinct salaries exist.

What are generics and what benefits do they give?

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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.

Why is Any() usually better than Count() > 0?

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Any() stops as soon as it finds one element, so it is O(1) for most sources, and in EF Core it translates to an efficient SQL EXISTS. Count() may need to walk the entire sequence or run COUNT(*) on the table, which is wasteful just to check for existence. For List<T> or arrays, the Count or Length property is also fine because it is stored, but Any() expresses the intent clearly.

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

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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.

You need to remove inactive users from a List<User> while looping through it with foreach, and you get an InvalidOperationException. How do you fix it?

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foreach does not allow modifying the collection it is enumerating, so the enumerator throws. The simplest fix is users.RemoveAll(u => !u.IsActive), which is efficient and clear. Alternatives are looping backwards with a for loop and RemoveAt, or building a new filtered list with Where and ToList. For a dictionary, collect keys to remove first, then remove them after the loop.

What do async and await do in C#?

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async marks a method that can contain await, and the compiler turns it into a state machine. await pauses the method until a Task finishes without blocking the thread; the thread returns to the pool to do other work and the method resumes later, often on another thread. It is ideal for I/O such as database calls, HTTP requests and file reads. An async method returns Task, Task<T> or ValueTask.

A dashboard API calls three independent services one after another, each taking about 1 second. How can you make it faster?

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Because the calls do not depend on each other, start all three tasks first without awaiting, then await Task.WhenAll. Total time becomes roughly the slowest call, about 1 second instead of 3. Add a timeout and handle failure of one service gracefully, for example showing partial data. If the same data is requested often, also add caching so most requests skip the calls entirely.

What is the difference between concurrency and parallelism?

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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 CPU-bound and I/O-bound work, and how do you handle each in .NET?

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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.

What is ASP.NET Core and what are its main features?

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ASP.NET Core is the open-source, cross-platform framework for building web APIs, MVC sites, Razor Pages, Blazor apps and real-time apps with SignalR on modern .NET. Key features are built-in dependency injection, a configurable middleware pipeline, the fast Kestrel web server, unified configuration and logging, and support for Docker and cloud hosting. It runs on Windows, Linux and macOS and consistently ranks among the fastest web frameworks.

What is the middleware pipeline in ASP.NET Core?

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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.

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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.

Show attribute routing for a products controller with route constraints.

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[ApiController] [Route("api/[controller]")] public class ProductsController(IProductService service) : ControllerBase { [HttpGet] public async Task<IActionResult> GetAll() => Ok(await service.GetAllAsync()); [HttpGet("{id:int}")] public async Task<IActionResult> Get(int id) { var p = await service.GetAsync(id); return p is null ? NotFound() : Ok(p); } [HttpPost] public async Task<IActionResult> Create(ProductDto dto) { var created = await service.CreateAsync(dto); return CreatedAtAction(nameof(Get), new { id = created.Id }, created); } }

What is the difference between authentication and authorization in ASP.NET Core?

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Authentication answers who you are: middleware reads a cookie or JWT, validates it and builds a ClaimsPrincipal for HttpContext.User. Authorization answers what you may do: it checks that user against rules such as [Authorize(Roles = "Admin")] or a policy like requiring an Employee claim with a certain department. A missing or invalid identity gives 401 Unauthorized, while a valid user without permission gets 403 Forbidden.

What is CORS and how do you enable it in ASP.NET Core?

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Browsers block JavaScript from calling an API on a different origin (scheme, host or port) unless the server allows it via CORS headers. If an Angular app on localhost:4200 calls an API on localhost:5001, you add a policy with builder.Services.AddCors, allowing that origin with WithOrigins, plus required methods and headers, and call app.UseCors before authorization. Avoid AllowAnyOrigin in production, especially with credentials.

Which HTTP status codes and methods should a REST API use, and what is the difference between PUT and PATCH?

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GET reads, POST creates, PUT replaces, PATCH partially updates and DELETE removes. Return 200 OK for success with a body, 201 Created with a Location header after POST, 204 No Content after delete, 400 for invalid input, 401 unauthenticated, 403 forbidden, 404 not found, 409 conflict and 500 for server errors. PUT sends the full resource and is idempotent; PATCH sends only changed fields.

What is Entity Framework Core?

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EF Core is Microsoft's object-relational mapper for .NET. It maps C# classes to database tables, translates LINQ queries into SQL, tracks changes to loaded objects and saves them with SaveChanges. It supports SQL Server, PostgreSQL, MySQL, SQLite, Cosmos DB and more through providers. It removes most hand-written data access code, while still allowing raw SQL when needed. EF Core 10 ships with .NET 10.

What is the difference between code-first and database-first approaches?

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In code-first, you write entity classes and a DbContext, and EF Core migrations create and evolve the database schema from code; it suits new projects and keeps schema in source control. In database-first, the database already exists and you scaffold entities with dotnet ef dbcontext scaffold. It suits legacy databases managed by a DBA team. Many teams scaffold once and then continue code-first.

What are DbContext and DbSet, and what lifetime should DbContext have?

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DbContext represents a session with the database: it holds configuration, the change tracker and connection handling. Each DbSet<T> property represents a table you can query and add to. DbContext is lightweight and not thread-safe, so it should be short-lived; in ASP.NET Core it is registered as scoped, one per request, via AddDbContext. For background services or Blazor, use IDbContextFactory to create contexts on demand.

Which commands do you use to create and apply EF Core migrations?

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dotnet tool install --global dotnet-ef # create a migration from model changes dotnet ef migrations add AddOrderStatus # apply pending migrations to the database dotnet ef database update # undo the last unapplied migration dotnet ef migrations remove # generate an idempotent SQL script for production dotnet ef migrations script --idempotent -o deploy.sql In Visual Studio the Package Manager Console equivalents are Add-Migration and Update-Database.

How do you keep up with new .NET and EF Core releases?

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Microsoft ships a major .NET version every November, so I plan for it. I read the official What's New pages and the .NET blog, watch .NET Conf sessions, and try new features in a small side project before suggesting them at work. For example, when ExecuteUpdate arrived, I tested it on a sample and then proposed replacing a slow loop in our cleanup job, which cut its runtime significantly.

What is the difference between a primary key and a unique key in SQL Server?

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Both enforce uniqueness. A table can have only one primary key, it cannot contain NULLs, and by default SQL Server creates a clustered index for it. A table can have many unique constraints, each allowing a single NULL in SQL Server, and they create non-clustered indexes by default. Use the primary key for row identity, such as EmployeeId, and unique keys for business rules like a unique email or PAN.

Explain the different types of SQL joins.

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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.

Write a SQL Server query to find the second highest salary.

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-- using DENSE_RANK (handles ties) SELECT Salary FROM ( SELECT Salary, DENSE_RANK() OVER (ORDER BY Salary DESC) AS rnk FROM Employees ) t WHERE rnk = 2; -- using OFFSET SELECT DISTINCT Salary FROM Employees ORDER BY Salary DESC OFFSET 1 ROW FETCH NEXT 1 ROW ONLY; Change 2 to N for the Nth highest salary.

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

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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.

What is the difference between WHERE and HAVING?

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WHERE filters individual rows before grouping and cannot use aggregate functions. HAVING filters groups after GROUP BY and can use aggregates like COUNT or SUM. For example, WHERE City = 'Pune' keeps only Pune rows, then GROUP BY DepartmentId HAVING COUNT(*) > 10 keeps departments with more than 10 such employees. Put non-aggregate conditions in WHERE so fewer rows are grouped, which is faster.

What is the difference between DELETE, TRUNCATE and DROP?

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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.

Explain normalisation and the first three normal forms.

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Normalisation organises tables to reduce redundancy and update anomalies. 1NF: each column holds atomic values and there are no repeating groups. 2NF: in 1NF and every non-key column depends on the whole primary key, not part of a composite key. 3NF: in 2NF and non-key columns do not depend on other non-key columns, for example City should not be stored with an order if it depends on the customer. Reporting databases may denormalise for speed.

What is SQL injection and how do you prevent it in a .NET application?

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SQL injection happens when user input is concatenated into SQL text, letting attackers change the query, for example entering ' OR 1=1 -- in a login box. Prevent it by always using parameters: SqlCommand with SqlParameter, EF Core LINQ, FromSql with interpolation, or Dapper parameters. Also use least-privilege database accounts, validate input and never show raw database errors to users. Stored procedures help only if they also avoid dynamic SQL concatenation.

What are your strengths and weaknesses as a developer?

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My strength is that I go deep when debugging; I like reading execution plans and logs until I find the real cause, which has helped my team fix slow queries. A weakness was that I spent too long polishing code before asking for feedback. I now open small draft pull requests early and ask for review, which has made me faster and helped me learn from seniors sooner.

Where do you see yourself in five years?

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In five years I want to be a senior full stack engineer who can design a complete .NET system, from database and APIs to front end and cloud deployment, and guide newer developers. In the near term I want to master ASP.NET Core, EF Core and SQL Server performance, then grow into Azure and system design. I would like to grow inside one company where I can take on bigger modules over time.

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

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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 components and standalone components in Angular?

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A component is a TypeScript class with the @Component decorator, a template and styles, representing one piece of UI such as a product card. Earlier, every component had to be declared in an NgModule. Standalone components import their own dependencies directly through the imports array and need no module; since Angular 19 they are the default. This simplifies apps, enables easier lazy loading and is the recommended approach in 2026.

Explain the types of data binding in Angular.

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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.

Show the useState and useEffect hooks in a React component.

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import { useState, useEffect } from 'react'; export function Timer() { const [seconds, setSeconds] = useState(0); useEffect(() => { const id = setInterval(() => setSeconds(s => s + 1), 1000); return () => clearInterval(id); // cleanup on unmount }, []); // empty array: run once return <p>Elapsed: {seconds}s</p>; } useState stores state that triggers re-render; useEffect runs side effects after render, and its dependency array controls when it re-runs.

What is the virtual DOM in React?

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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.

What is the difference between props and state in React?

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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 on localhost:4200 gets a CORS error when calling the ASP.NET Core API on localhost:5001. How do you fix it?

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The browser blocks the call because the origins differ and the API sends no CORS headers. In the API, register a policy with AddCors allowing http://localhost:4200, the needed methods and headers, and call app.UseCors before UseAuthorization. Alternatively, use the Angular dev server proxy (proxy.conf.json) to forward /api calls, so the browser sees the same origin. In production, configure the real front-end domain, not AllowAnyOrigin.

Walk me through a full stack project you built.

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I built an online bus-pass system (Situation) as my final project with two classmates. My task was the API and database. I designed SQL Server tables, built an ASP.NET Core Web API with EF Core and JWT login, and connected an Angular front end with reactive forms and route guards. I deployed it on Azure App Service. Result: it handled pass applications, approvals and PDF passes, and our college used it for a semester demo.

Why should we hire you as a .NET full stack developer?