C# interview questions for web devs

iamprovidence

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Aug 17, 2024

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Just a list of subjects, topics, and questions that I believe developers at junior, middle, and senior levels should know. You can use it as a roadmap, to prepare for an interview, or to conduct one 🙃.

Enjoy 😉

Code conventions

junior

• knows programming language’s conventions

middle

• can introduce changes to the codebase without breaking existing conventions

senior

• can set up linting to automate code conventions (Roslyn Analysers, StyleCop, FxCop, SonarLint)

Refactoring

junior

• what is Refactoring?
• what is Clean Code?
• can simplify if/else statement

middle

• refactoring techniques (extract method, rename method, etc)
• code smells (long methods, long class, primitive obsession, etc)

senior

• knows code metrics (depth of inheritance, coupling, cyclomatic complexity)

Debugging

junior

• debugging in Visual Studio
• settings breakpoints
• conditional breakpoints
• changing runtime values

middle

• Stopwatch
• know and can use DevTools

senior

• remote debugging
• knows how to debug multi-thread app

Git

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junior

• GitHub
• basic operations (init, clone, fetch, pull, commit, push)
• branching
• .gitignore

middle

• stash
• rebase vs merge
• branching models (GitFlow, GitHub Flow, etc)
• git blame

senior

• .gitattributes
• checkout on a commit
• cherry-pick
• squash

Source Code Review

junior

• knows what a Pull Request is

middle

• knows how to conduct code review and provide useful and understandable feedback

senior

• can set up, drive, and control code review practices within a team
• has a check-list for PR review
• writes architecture tests to sustain conventions
• automatization of code review

Data Structures

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junior

• array
• matrix
• linked list
• LIFO vs FIFO
• stack
• queue
• binary tree

middle

• hash table. how does a hash table behave when there is a collision?
• hash function. what happens when mutable fields are used in the hash function? what is a collision in hash tables?

senior

• graph

Algorithms

junior

• O notation, space/time complexity
• swap values in an array
• min, max, average in an array
• recursion
• sorting (bubble sort, insertion sort, selection sort)
• searching (linear search, binary search, knows precondition for binary search)

middle

• quick sort

senior

• tree traversal (with queue, with stack, NLR, LRN, RLN)
• divide-and-conquer algorithms

Object-Oriented Programming

junior

• Abstraction, Encapsulation, Inheritance, Polymorphism
• can explain OOD Principles in examples
  - DRY
  - KISS
  - YAGNI
• primitive obsession issue

middle

• Association, Aggregation, Composition
• OOD
• knows OOD concepts
  - low coupling
  - high cohesion
• SOLID

senior

• aspect-oriented programming. how to implement cross-cutting concerns?
• rich vs anemic model
• strong and weak encapsulation

Functional Programming

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junior

• what is a pure function?

middle

• how to achieve determinisity?
• ways to get rid of side effects?

senior

• immutability
• exceptions and Result class

AI

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junior

• can use AI tools (e.g., Copilot, ChatGPT) for code snippets, explanations, and documentation
• can generate unit tests or boilerplate with AI

middle

• can evaluate AI-generated output critically, refine prompts, and integrate AI into the development workflow
• uses Claude for reasoning-heavy tasks (e.g., refactoring, architecture explanations)

senior

• ensures compliance, privacy, and security in AI-assisted development
• sets up automation with AI (code review, documentation generation, test generation)
• can integrate AI APIs (OpenAI, Anthropic, local LLMs) into internal tools and workflows
• evaluates new AI tools, models, and frameworks, guiding adoption strategically

.NET

junior

• what .NET is
• can create a custom library
• stack vs heap

middle

• .Net CLI
• difference between compile time and runtime

senior

• global.json
• dotnet-tool.json
• can create custom NuGet
• CLR, CIL, CTS, CLS, GAC
• DLL, EXE

Core C#

junior

• basic types, enums, value types vs reference types
• struct vs class
• string, StringBuilder, interpolation, string.Format()
• boxing, unboxing
• type casting (is, as, type conversion operator)
• method overload vs method optional parameters, named parameters
• extension methods
• in, ref, out, params
• interface vs abstract class, inheritance
• access modifiers (public, private, protected)
• virtual vs new vs override
• static class and static class members
• indexers and properties
• generics, generic constraints
• exceptions, finally
• linq. First() vs FirstOrDefault() vs Single() vs SingleOrDefault()
• garbage collection, IDisposable vs Finalizer, using statement

middle

• what’s new in C#?
• sealed class, partial class
• access modifiers (internal)
• tuple, named tuple, deconstruction
• const vs static readonly
• understand regex and its usage
• Equals(), inherit EqualityComparer<T>, Comparer<T>.Default
• GetHashCode(), HashCode.Combine()
• rethrowing exception (throw ex vs throw), when in a catch clause
• deferred linq execution
• basic of working with files. File, FileInfo, Streams (Stream, FileStream, etc)
• HttpClient

senior

• dynamic
• can work efficiently with time and timezones (DateTime, TimeSpan, DateTimeOffset, DateOnly, TimeOnly, TimeZoneInfo)
• string interning, string culture, StringComparer
• invariance, covariance, contravariance
• record, readonly struct
• access modifiers (protected internal, private protected, file), nested classes access
• streams in detail (backing storage, decorators, adapters, working efficiently with MemoryStream)
• garbage collection in details (stack, heap, large object heap)
• linq. DefaultIfEmpty(), Zip(), Aggregate()
• HttpClient
- how to create one? (port exhaustion and DNS change problems)

Collections

junior

• array
• generic collections (List<T>, LinkedList<T>, Dictionary<K, V>, HashSet<T>, Stack<T>, Queue<T>, PriorityQueue<T, P>)

middle

• interfaces (ICollection<T>, IList<T>, IDictionary<K, V>)
• readonly interfaces (IEnumerable<T>, IReadOnlyList<T>, IReadOnlyDictionary<K, V>)
• ObservableCollection<T>
• iterators, IEnumerator (MoveNext(), Current, Reset())

senior

• ILookup<K, V>
• Enumerable.SequenceEqual()
• base classes for custom collections (Collection<T>, KeyedCollection<K, V>)
• readonly collections (ReadOnlyCollection<T>, ReadOnlyDictionary<K,V>), Array.AsReadOnly()
• concurrent collections (BlockingCollection<T>, ConcurrentBag<T>, ConcurrentDictionary<K, V>, ConcurrentQueue<T>, ConcurrentStack<T>)
• immutable collections
• frozen collections
• create an empty collection (Array.Empty<int>(), Enumerable.Empty<int>(), ImmutableArray<T>.Empty, ImmutableDictionary<TKey,TValue>.Empty, FrozenDictionary<TKey,TValue>.Empty, etc)
• yield return in details
• IAsyncEnumerable

Delegates

junior

• delegates, lambda expressions
• understand events
• Action, Func, Predicate

middle

• create custom delegate
• local functions

senior

• lambda closures
• combine delegates
• create event, EventHandler, EventArgs
• event forwarding (add, remove keyword)

Reflection

junior

• basic understanding of reflection
• attributes
• .GetType() vs typeof()

middle

• create a custom attribute, AttributeUsage

senior

• capable of applying reflection
• expression trees
• Activator.CreateInstance<T>(), Activator.CreateInstance(Type t)
• invoking instance’s methods, invoking static methods

Multithreading

junior

• being able to use Task

middle

• difference between parallel and asynchronous execution
• knows a variety of timers and the differences between them
• understand synchronization issues (deadlock and race condition)
• using lock
• Task.WhenAll()
• CancellationToken. Knows how to cancel Task. Cancel HTTP request in ASP. Write your own methods that support cancellation.
• async void issues

senior

• synchronization primitives (Mutex, Monitor, Semaphore, SemaphoreSlim)
• difference between Thread and Task
• TaskCompletionSource
• exception handling, exceptions in Task.WhenAll()
• .ConfigureAwait(false), synchronization context
• distributed lock
• optimistic locking, pessimistic locking
• optimize IO/CPU bound operations

Libraries

junior

• what is NuGet?
• Moq
• Swagger
• Newtonsoft.Json or System.Text.Json
• AutoMapper

middle

• FluentValidation
• Autofac, Scrutor (DI registration with assembly scanning)
• FluentAssertion
• Bogus
• Semantic Versioning
• Local SMTP server (Papercut-SMTP / Mailpit / etc)
• Temp Mail

senior

• Polly for fault handling (Retry, Circuit-Breaker)
• BenchmarkDotNet
• SignalR
• Refit
• FusionCache
• Ocelot
• Quartz vs Hangfire vs BackgroundService
• NBomber
• ngrok
• T4 Templates

Networking

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junior

• JSON and XML
• HTTP methods (GET, POST, PUT, DELETE, etc)
• GET vs POST
• HTTP response groups (1XX, 2XX, 3XX, 4XX, 5XX)
• HTTP headers
• REST
• knows the basic difference between HTTP and HTTPS

middle

• capable of issuing HTTP requests using Postman
• understand URL structure

senior

• can explain the basics of the HTTP request flow
• knows Postman in advance (creating the environment, using variables, etc)
• API Versioning
• Rate Limiting
• Polling vs WebHooks
• WebSockets
• gRPC
• GraphQL

ASP .NET API

junior

• MVC pattern
• routing
• model validation with attributes
• dependency injection
• middleware
• swagger
• environment configurations (appsettings.json)
• configuration binding, IOptions

middle

• filters. filters vs middlewares
• exception handling
• FluentValidation
• environment configurations (appsettings.dev.json, user secrets, environment variables, command line arguments)
• IHostedService/BackgroundService
  - what lifecycle does BackgroundService have?
  - how do you inject a service into the BackgroundService?

senior

• run web service in the integration test
• configuration binding, IOptionsSnapshot, IOptionsMonitor
• can service with any lifecycle be injected any other? building dependency graph? when error visible (runtime vs on start)?
• advanced injection technique
  - assembly scanning
  - IServiceScopeFactory
  - inject services regardless of lifecycle
  - register service with multiple interfaces
  - inject multiple services
  - TryRegister
  - resolve service with multiple implementations
• minimal API

ASP.NET MVC

junior

• MPA
• understand ASP.Net MVC conventions
• creating a view, passing data between views and controllers
• razor syntax
• configuration binding, IOptions

middle

• partial views
• exception handling
• FluentValidation
• environment configurations (appsettings.dev.json, user secrets, environment variables, command line arguments)
• IHostedService/BackgroundService
• run web service in the integration test

senior

• configuration binding, IOptionsSnapshot, IOptionsMonitor
• IServiceScopeFactory

Web Security

junior

• authorization vs authentication
• knows how to store passwords in DB (encryption, hashing, salt)
• knows what hash function is
• hashing vs encryption vs encoding vs serializing

middle

• knows basic auth schemes (Basic, Cookie, JWT, API key)
• role-based, policy, claims authorization
• JWT
  - what claims are?
  - registered claims vs custom claims
  - issuer claim? audience claim?
• can implement JWT authentication

senior

• two-factor authentication
• symmetric encryption vs asymmetric encryption
• types of tokens and their usage:
  - access token
  - refresh token
  - ID token
• OAuth 2.0
  - client types
  - authorization server endpoints
  - authorization flows (code flow, implicit flow, client credentials). in which cases which should be used?
• SSO
• OpenID
• SAML
• Identity Server
• Active Directory
• OTP (one-time password)
• self-contained vs reference token
• cross-origin resource sharing (CORS) policies
• knows and can prevent password attacks
  - brute force
  - dictionary attack
  - rainbow table attack
• knows and can prevent vulnerabilities
  - DDoS attack
  - SQL injection attacks
  - open redirect URL attacks
  - cross-site scripting attack (XSS)
  - cross-site request forgery (CSRF)
  - clickjacking
  - replay attack. The purpose of nonce

SQL Databases

junior

• data types
• relational model design (one-to-one, one-to-many, many-to-many)
• CREATE, ALTER, DROP
• INSERT, UPDATE, DELETE
• SELECT, WHERE, LIKE, ORDER BY, GROUP BY, HAVING, DISTINCT
• JOINS
• AVG, SUM, COUNT, MAX, MIN
• UNION vs UNION ALL

middle

• ACID
• CAP
• normalization (1NF, 2NF, 3NF)
• clustered and non-clustered indexes. should we create an index for a bool column? what about IsDeleted? Index sequential number vs GUID?
• unique and foreign keys
• check constraints
• transactions
• views, materialized views
• cursors
• soft delete vs hard delete
• Cartesian explosion problem
• Entity–Relationship model
• SQLite

senior

• i̶s̶o̶l̶a̶t̶i̶o̶n̶ l̶e̶v̶e̶l̶s̶ (probably the worst question that could be asked)
• SQL optimization, execution plan
• Common Table Expressions (CTE)
• temporary table
• SQL RANK functions (ROW_NUMBER(), RANK())
• stored procedures
• triggers
• local transactions vs distributed transactions
• indexes in details
  - composite index
  - covering index
  - filtered index
  - why would you use a covering index instead of a composite index?
• denormalization. when should be done?
• char vs nchar
• char vs varchar
• DB partition vs sharding

NoSQL Databases

junior

• NoSQL concepts

middle

• SQL vs NoSQL. Advantages/disadvantages of NoSQL Databases over Relational Databases

senior

• BASE
• can review and use NoSQL databases based on a project needs
• has experience with MongoDb/Elasticsearch/Redis etc

Entity Framework

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junior

• what is DbContext
• configuring the DB model. data annotations vs FluentAPI
• EF approaches (Code First, Database First, Model First)
• implement CRUD
• one-to-one, one-to-many, many-to-many relations conventions
• configure relations with FluentAPI
• load data: lazy vs explicit vs eager
• IEnumerable vs IQueryable
• create and apply a migration

middle

• change tracker, AsNoTracking()
• update/delete data in disconnected scenarios
• owned entities
• shadow property
• backing fields
• value conversions
• global filters. how to implement soft delete in EF?

senior

• inheritance strategies (TPH, TPT, TPC)
• raw SQL queries
• working with stored procedures
• working with views
• transactions, transactions in SaveChanges(), TransactionScope
• interceptors
• unit test code with DbContext
• view generated SQL (.ToString(), log, profiler)
• Hi/Lo
• linqpad

Monitoring

junior

• benefits of logging
• can use ILogger<T>

middle

• different log levels (Trace, Debug, Info, Warning, Error, Fatal)
• health checks

senior

• tools how to track events in the cloud (Grafana, Prometheus, etc). when to use?
• diagnostic/health check
• trace identifier/correlation id
• structure log
• audit log
• monitoring on-premises systems (Grafana, ELK stack, Seq, etc)
• tracing requests in a distributed system

Performance

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junior

• can find performance issues in the app?

middle

• can optimize algorithms
• can optimize memory usage

senior

• can detect and optimize IO/CPU bound operation
• benchmarking. understand statistics (mean, error, etc)
• SQL optimization, execution plan
• latency vs throughput. How can those be measured?

Cache

junior

• understand what cache is
• can explain when we need caching and when we don’t

middle

• IMemoryCache vs IDistibutedCache vs HybridCache
• AbsoluteExpiration vs SlidingExpiration
• Redis
• memoization

senior

• FusionCache
• two-level cache
• Backplane
• сaching strategies
  - Cache-Aside (Lazy loading)
  - Read-Through Cache
  - Write-Through Cache
  - Write-Back
  - Write-Around
• cache invalidation methods
  - manual invalidation
  - time-based invalidation (TTL)
  - event-based invalidation
• cache metrics
  - latency
  - throughput
  - cache hit ratio
  - cache miss rate
  - cache size

Tests

junior

• alpha testing vs beta testing
• unit tests
• unit test characteristics (quick, consistent, isolated, atomic, independent)
• Mock objects
• Arrange-Act-Assert (AAA) pattern
• Assert statement
• TDD. knows when to apply
• unit test naming conventions

middle

• integration tests
• run web service in the integration test
• BDD
• Given-When-Then (GWT) pattern
• FluentAssertion
• Bogus
• Assert vs Expect statement
• Stub objects

senior

• testing pyramid
• e2e tests. playwright vs selenium
• PageObject pattern
• unit test code with DbContext
• performance testing. NBomber
• snapshot tests
• mutation testing. Stryker.NET
• builder pattern in unit tests
• architecture tests
• contract testing. PactNet

Design patterns

junior

• OOD Principles with examples
  - DRY
  - KISS
  - YAGNI
• basic patterns
  - singelton
  - strategy
  - prototype
  - etc
• specialized patterns
  - lazy loading

middle

• antipatterns
  - service locator
  - singelton
  - etc
• SOLID
• specialized patterns
  - unit of work and repository
  - marker interface

senior

• design patterns inside the .NET?
• knows how to use and apply patterns in your work?
• advanced patterns
  - visitor
  - builder for unit tests
  - state
  - bridge
  - etc
• specialized patterns
  - specification pattern
  - null object (special case pattern)
  - object pool
• architecture patterns
  - MVC vs MVP vs MVVM
  - reporting database pattern

Architecture

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junior

• breaking a problem into multiple functions

middle

• separation of concern
• N-layered

senior

• dependency inversion
• low coupling and high cohesion
• onion architecture
• MVC vs MVP vs MVVM
• screaming architecture
• vertical slice architecture
• multitenant architecture
• modular monolith
• microservices
• Domain Driven Design
• event sourcing
  - snapshotting events
  - how can an entity be deleted?
  - can we delete the event?
  - what if the event’s structure changed?
• CQRS
  - when should it be applied?
  - what are the options for the read model?
  - why is returning the result from the command considered a bad idea?
• state machine
• Architecture Decision Record
• architecture tests

Domain Driven Design

junior

• entity
• dto
• factory pattern

middle

• unit of work and repository

senior

• ubiquitous language
• sub-domain / bounded context
• business invariant
• aggregate root
• value object
• specification pattern
• domain events and integration events
• rich vs anemic model
• validation (always-valid model / not-always-valid-model)
• context map
• relationships between bounded contexts
  - Shared Kernel
  - Anti-Corruption Layer
• persistence ignorance principle
• DDD trilemma (purity / encapsulation / performance)
• aggregates creation (constructor / static method / factory)
• DDD Layers
  - Application layer

Microservices

junior

• 🤷‍♂️

middle

• what are microservices? what is the use case for using them?
• microservice vs monolith
• can one microservice access another microservice’s database?

senior

• correlation id
• strong consistency vs eventual consistency
• synchronous communication (HTTP vs GraphQL vs gRPC)
• asynchronous communication (AMQP)
• what is a proxy server? API gateway? BFF? load balancers? Ocelot
• load balancing strategies (sticky session)
• message brokers. pull vs push models. publish/subscriber
• message queues. dead-letter queues
• transactions management
  - transactional Outbox
  - state machine, SAGA (orchestration vs choreography)
  - compensation events
  - two-phase commit
• vertical vs horizontal scaling
  - which to choose?
  - what are the limits?
• distributed locks
• distributed cache
• idempotency in microservices
• service aggregator pattern
• queue-based load leveling
• inbox pattern
• outbox pattern
• messaging guarantees
  - at most once (uniqueness)
  - at least once (delivery)
  - exactly once (delivery, uniqueness)
• backplane pattern
• circuit-breaker pattern
• retry HTTP calls
  - which HTTP status codes should be retried and which shouldn’t?
  - how do you avoid a self-inflicted DDoS attack?
  - why is exponential backoff preferable over contact timeout?
  - what is jitter and why do we need it?
  - can you retry every operation? what is idempotency?
• ETL
• tracing requests in a distributed system
• contract testing. PactNet

Cloud computing

junior

• 🤷‍♂️

middle

• on-premise vs cloud deployment
• benefits and drawbacks of going cloud
• cloud models (PaaS vs IaaS vs SaaS)

senior

• serverless concept
• vertical/horizontal scaling
• cold start problem in lambdas
• blob storage
  types of resources (an account, a container, a blob)
  access tiers (hot, cool, cold, archive)
• vendor lock-in

DevOps

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junior

• 🤷‍♂️

middle

• CI Definition and Pipeline
• distinguish CI/CD steps

senior

• shell scripts, .ps1 scripts
• CD strategies
  - Recreate
  - Blue/Green
  - A/B
  - Canary
  - Feature Flag
• can configure CI/CD pipeline
• can provision resources with Infrastructure as Code
• understand database migrations based on CD strategy

Containers

junior

• 🤷‍♂️

middle

• VM vs container
• docker
• dockerfile
• image
• container
• docker CLI
  - image pull and push
  - run an image in the container
  - execute a command in the container
• how to access a container from your PC? (expose port)

senior

• docker compose
• Docker Hub
• multi-stage build
• volumes
• how to access an application on your PC from the container? (network_mode: host)

Software development methodologies

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junior

• basics of work organization using ticket management software (i.e., Jira or Trello)

middle

• SCRUM basics: roles, meetings, processes, artifacts
• Kanban basics: Kanban board, tickets processing
• pair programming
• task hierarchy
  - epic
  - user story
  - task

senior

• set-up Scrum or Kanban on the project
• work breakdown structure (WBS)

Estimation

junior

• knows fibonacci numbers
• knows T-shirt sizes

middle

• pocker estimation. What is the etalon story (a reference user story or a baseline story)?

senior

• critical path method
• three-point estimation
• knows when an investigation is required
• include in the estimate
  - task estimate
  - risk estimate
  - tests
  - investigation/documentation

Project documentation

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junior

• functional specification
• can gather requirements from stakeholders

middle

• understand technical specifications (how to setup/configure project, development process, branch names)
• understand functional specification (sequence diagrams, use-case diagrams, etc)
• release-notes
• when task considered to be completed? acceptance criteria
• UML. different types of diagrams
• Entity–Relationship model

senior

• non-functional specification
• deployment specification
• can write project documentation
• C4 model. Difference between Container and Components
• Architecture Decision Record
• GDPR compliance

For the End

I hope I didn’t upset anyone because the list does not match their expectation 😅. It’s simply my personal perspective, and it’s perfectly fine if you have a different view.

Do not worry if you don’t know something, and do not get arrogant if you master a section before somebody else. Remember that everybody learns stuff at a different pace. The labels of junior, mid-level, and senior do not mean anything. As practice shows, what a junior developer has not learned yet, a senior developer has already forgotten 😁.

💬 Let me know in the comment sections what other topics deserve to be on the list

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