.NET Full Stack Rapid-Fire Questions &amp; Answers

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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.

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.

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

Entity Framework CoreEasy

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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 is the difference between WHERE and HAVING?

Async Programming and MultithreadingEasy

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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 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.

What types of filters exist in ASP.NET Core MVC?

ASP.NET Core Web APIMedium

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Authorization filters run first and can block a request. Resource filters run before model binding, useful for caching. Action filters run before and after an action, for logging or validation. Exception filters handle errors thrown by actions. Result filters run around the result execution, for example to add headers. Filters apply globally, per controller or per action. Unlike middleware, they can access MVC details like action arguments.

What is the difference between concurrency and parallelism?

Async Programming and MultithreadingEasy

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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 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 the difference between compile-time and runtime polymorphism?

OOP in C#Easy

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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.

Explain the SOLID principles briefly.

OOP in C#Medium

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Single Responsibility: a class has one reason to change. Open/Closed: extend behaviour without editing existing code. Liskov Substitution: derived types must work wherever the base type is expected. Interface Segregation: prefer small, focused interfaces over one fat interface. Dependency Inversion: depend on abstractions, not concrete classes. In ASP.NET Core, injecting IEmailSender into a controller instead of creating SmtpClient directly applies dependency inversion.

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.

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.

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

ASP.NET Core Web APIMedium

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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 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.

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.

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

Async Programming and MultithreadingMedium

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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 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.

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 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.

What is deferred execution in LINQ?

LINQ, Collections and GenericsMedium

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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.

What does the sealed keyword do?

OOP in C#Easy

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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.

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.

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.

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.

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.

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 is the difference between method overloading and method overriding?

OOP in C#Easy

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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.

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.

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.

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.

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.

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.

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.

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.

What is Entity Framework Core?

Entity Framework CoreEasy

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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 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.

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.

What are useMemo and useCallback used for in React?

Angular and React Front EndMedium

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useMemo caches the result of an expensive calculation and recalculates only when its dependencies change, such as filtering 10,000 rows. useCallback caches a function reference so that a child wrapped in React.memo does not re-render just because a new function was created. Both are optimisations and add complexity, so use them where profiling shows a problem. The React Compiler, stable since late 2025, can apply much of this memoisation automatically.

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.

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.

What is the difference between == and Equals() in C#?

C# FundamentalsMedium

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For reference types, == by default checks reference equality, meaning both variables point to the same object, while Equals can be overridden to compare content. String overrides both, so == compares text. For value types, Equals compares field values. Records generate value-based equality automatically. When you override Equals, also override GetHashCode so objects behave correctly in dictionaries and hash sets.

Explain value types and reference types in C#.

Async Programming and MultithreadingEasy

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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 the four pillars of object-oriented programming?

OOP in C#Easy

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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.

Explain the virtual, override and new keywords.

OOP in C#Medium

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virtual marks a base method as replaceable. override in a derived class replaces it, so calls through a base reference run the derived version. new hides the base method instead: a call through a base reference still runs the base version, and only a derived-type reference sees the new one. Method hiding is usually a sign of a design problem, and the compiler warns when you hide without writing new.

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

OOP in C#Easy

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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 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 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.

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 is the difference between reactive forms and template-driven forms in Angular?

Angular and React Front EndMedium

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Template-driven forms are defined mainly in HTML with ngModel and directives; they are quick for simple forms but harder to test. Reactive forms are defined in TypeScript with FormGroup, FormControl and Validators, giving explicit, strongly typed models, easy dynamic fields, custom and async validators, and straightforward unit testing. For enterprise apps such as loan application or KYC forms, reactive forms are the usual choice.

When should you use StringBuilder instead of string?