What are the main differences between .NET Framework and .NET (formerly .NET Core)?

Start by describing platform scope and cross-platform support, then mention performance and deployment models. Explain that .NET Framework is Windows-only and mature, while modern .NET is cross-platform, open source, and has unified runtime versions across workloads. Give a practical example: if you need to run on Linux containers or use newer C# language features, choose modern .NET; if you maintain a legacy Windows-only app with heavy reliance on older libraries, you may stay on .NET Framework. Discuss migration considerations like third-party library compatibility and API changes when planning an upgrade. Tip: avoid saying one is universally better, and focus on constraints like OS, library needs, and team experience. When asked in an interview, clarify the project context before recommending migration or platform choice.

How does dependency injection work in ASP.NET Core and why should you use it?

Explain the concept: dependency injection means you provide a class its dependencies rather than creating them inside the class, which improves testability and separation of concerns. In ASP.NET Core you register services in Startup or Program using AddTransient, AddScoped, and AddSingleton, and the framework injects them into constructors. Give a concrete example: register an IEmailService with AddScoped and accept IEmailService in your controller constructor, then mock IEmailService in unit tests to isolate controller logic. Mention lifetime choices and how a scoped service is created per HTTP request while singleton lives for the app lifetime. Avoid overusing singletons for stateful services that hold request data, since that can cause concurrency issues. When answering, emphasize testability and maintainability as primary benefits rather than buzzwords.

Explain async and await in C#, and when to use them.

Describe async and await as language keywords that let you run asynchronous work without blocking a thread, returning Task or Task from methods. Explain that async methods compose asynchronous operations and keep the main thread responsive, which is important for I/O bound work like database calls, HTTP requests, and file access. Provide an example: make database queries using EF Core methods that return Task, await them in a service method, and return the result to a controller without blocking. Note common pitfalls such as using async void outside of event handlers, which makes error handling and awaiting completion difficult. Tip: prefer async for I/O-bound operations, but avoid async for CPU-bound work unless you offload to Task.Run with care. In interviews, show you understand deadlock scenarios by explaining ConfigureAwait(false) when appropriate in libraries.

What is garbage collection in .NET and how does it affect application performance?

Explain that the .NET runtime garbage collector automatically reclaims memory for objects no longer in use, organizing objects into generations to optimize collection. Discuss how GC pauses can affect latency, with short collections for young objects and longer full collections for older generations. Give a tangible example: creating many short-lived objects in a tight loop increases Gen0 collections and may be fine, while keeping large objects alive can promote them to Gen2 and cause more expensive collections. Mention tools such as dotnet-counters, PerfView, or Visual Studio diagnostics to measure allocations and identify hotspots. Avoid manual memory management workarounds unless necessary, and do not suggest forcing full GCs in production. When answering, emphasize measurement and targeted fixes like reducing allocations or reusing buffers to improve performance.

Can you explain SOLID principles with examples in C#?

Briefly define each principle: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion, and relate them to maintainable object-oriented design. Focus on small concrete examples that show how each principle prevents common design problems such as rigid classes or tightly coupled code. For example, apply Single Responsibility by splitting a UserService that both validates input and sends emails into separate Validator and EmailSender classes. Use Dependency Inversion by depending on IEmailSender in your service instead of a concrete EmailSender to make testing easier and swap implementations later. When discussing SOLID in interviews, tie principles to real tradeoffs, since over-applying abstractions can add complexity. Use concise code snippets if asked, and explain how applying one principle influenced your design decisions in a past project.

What is Entity Framework and when should you use EF Core versus Dapper or raw ADO.NET?

Explain that Entity Framework is an object-relational mapper that maps database tables to C# classes and handles SQL generation, change tracking, and migrations. State that EF Core works well when you want productivity, maintainability, and a domain model, while micro-ORMs like Dapper are better for raw SQL performance and simpler mapping needs. Give a scenario: use EF Core for a business app with complex relationships and frequent schema evolution, and use Dapper in performance-critical reporting queries where you need fine-grained SQL control. Mention that raw ADO.NET may be suitable when you need the lowest level of control or minimal dependencies. Tip: discuss how you measured performance or maintained code when you chose one approach over another. In interviews, be ready to show a migration path or optimization steps you took when EF queries were slow.

How would you secure an ASP.NET Core Web API?

Outline a layered approach: authenticate users, authorize actions, validate input, and protect data in transit and at rest. Mention common mechanisms such as JWT bearer tokens for stateless APIs, OAuth2 for delegated access, HTTPS enforcement, and role or policy-based authorization for fine-grained control. Give an example setup: configure JWT authentication in Program, add Authorization policies for admin routes, validate model state to avoid injection attacks, and store secrets in a secure vault rather than appsettings. Also use rate limiting, logging, and input sanitization for additional defense in depth. Avoid suggesting homegrown crypto or token schemes; prefer well-tested libraries and standards. When answering, reference concrete middleware and configuration steps you have implemented in past projects to show practical experience.

What are delegates and events in C# and how do you use them?

Define delegates as type-safe function pointers and events as a pattern that uses delegates to notify subscribers when something happens. Explain that you declare a delegate type or use built-in Action and Func types, then expose an event to let other classes subscribe with handler methods. Provide an example: declare public event EventHandler OrderPlaced; then raise the event in your order processing code after validating and saving the order, checking for null before invoking. Mention common pitfalls such as invoking events on a background thread without proper synchronization and leaking subscriptions if you do not unsubscribe when appropriate. Tip: prefer EventHandler for custom event data, and consider using weak references or explicit unsubscription in long-lived publishers to prevent memory leaks. In interviews, give a short code pattern to demonstrate correct event raising and exception handling in subscribers.

What is middleware in ASP.NET Core and how do you write custom middleware?

Explain that middleware are components in the request pipeline that can inspect, modify, or short-circuit HTTP requests and responses. Describe that middleware are added in order using app.UseX methods and each middleware calls the next delegate to continue the pipeline. Give a basic example: create a class with an InvokeAsync(HttpContext context, RequestDelegate next) method to log request timing, call await next(context), and then log response time. Remember to register the middleware in Program with app.UseMiddleware () and consider exception handling and ordering when adding middleware. Avoid putting heavy logic or blocking calls in middleware, since they run on the request path and affect latency. When explaining middleware in an interview, mention how ordering impacts behavior and give a specific ordering rationale from your experience.

How do you approach exception handling and logging in a .NET application?

Describe a strategy that centralizes exception handling with middleware for web apps, catches and logs unexpected errors, and preserves meaningful error information for diagnostics. Recommend structured logging libraries like Serilog or Microsoft.Extensions.Logging and show how to add context such as correlation IDs and user identifiers to logs. Offer an example: implement a global error handling middleware that catches exceptions, logs error details including stack traces and request context, and returns a sanitized error response to the client. Combine that with scoped logging in services and custom exception types for domain errors to separate expected from unexpected failures. Avoid returning raw exception details to clients or swallowing errors silently in catch blocks. In interviews, emphasize how you used logs and metrics to find the root cause of real incidents and the steps you took to improve observability.

Tell me about a time you faced a tight deadline and how you handled it.

Situation: We had a critical bug discovered two days before a major release that could block customer onboarding. Task: As the senior developer on the team, you needed to triage, fix, and get the change reviewed and deployed quickly while minimizing risk to other features. Action: You organized a focused triage session, reproduced the bug, created a minimal failing test, and split the work into a small fix plus quick integration tests; you also paired with another engineer for the review to speed up validation. Result: The patch was merged and deployed within 24 hours, automated tests caught a related regression, and the release proceeded on schedule with no customer impact.

Describe a situation where you disagreed with a technical decision and how you resolved it.

Situation: On a project, the team planned to adopt a third-party library that you believed added unnecessary complexity and licensing risk. Task: You needed to present an alternative that addressed the same problem while convincing the team and stakeholders to reconsider. Action: You prototyped a lightweight in-house implementation, documented tradeoffs, and prepared a short comparison showing performance, maintenance, and license implications; you then presented both options in a technical review meeting. Result: The team agreed to the lighter approach for the initial release and adopted the third-party library later only when a clear need and budget for support existed.

Give an example of a time you improved application performance.

Situation: A reporting endpoint was taking too long under load and blocking user workflows during peak times. Task: Your goal was to reduce response time and keep data accurate without a full rewrite of the reporting service. Action: You profiled the endpoint, identified a hot SQL query and excessive object allocations, added targeted indexes, refactored the query to fetch only needed columns, and introduced streaming results to avoid large in-memory collections. Result: Response time dropped from 6 seconds to 800 milliseconds under load, CPU usage decreased, and user complaints dropped significantly after deployment.

.net developer Interview Questions: Complete Guide

Expect a mix of coding tests, system design, and behavioral questions when preparing for .net developer interview questions. Interviews often include live coding or take-home assignments, architecture discussions, and questions about your past projects, so prepare examples and practice explaining tradeoffs clearly.

Common Interview Questions

Behavioral Questions (STAR Method)

STAR Method: Structure your answers using Situation, Task, Action, and Result to tell compelling stories about your experience.

Questions to Ask the Interviewer

Show your interest by asking thoughtful questions

•What does success look like in this role after six months, and what metrics would you use to measure it?
•Can you describe the team structure and how this role collaborates with product and QA?
•What are the biggest technical challenges the team is facing right now, and what work is planned to address them?
•How do you handle code reviews and deployment processes, and what tools do you use for CI/CD?
•What opportunities are there for learning and ownership, such as owning a service or leading a migration?

Interview Preparation Tips

Practice explaining your design decisions clearly, including tradeoffs and alternatives, using a recent project as your example.

Do a timed live coding exercise and explain your thought process aloud; interviewers want to see your approach to problem solving.

Prepare 2-3 concrete stories using the STAR format about difficult bugs, deadlines, and teamwork to answer behavioral prompts.

Read the job description and align your examples to the tech stack and responsibilities they list, showing direct relevance to the role.

Overview

This guide helps you prepare for . NET developer interviews at junior, mid, and senior levels.

It focuses on practical topics interviewers ask about most often: C# language features, . NET runtime behavior, ASP.

NET Core web apps, Entity Framework, performance tuning, and system design. Expect three common interview stages: a screening call (15–30 minutes), a technical round with live coding or take-home tests (45–120 minutes), and a senior-level design/leadership interview (60–90 minutes).

Interviewers typically evaluate three skill areas: coding fluency (algorithms, data structures, clean C#), platform knowledge (memory management, async model, DI), and application-level concerns (security, database design, API contracts). For example, a mid-level role might include 2–4 coding problems on LeetCode-style arrays/strings and one task to optimize an API endpoint’s throughput by 20–50%.

Real-world prep should include: 1) solving 30 focused algorithm problems (easy/medium split about 40/60), 2) building or reviewing 2–3 small projects (API + EF Core + unit tests), and 3) studying 5 architecture patterns (e. g.

, CQRS, repository, adapter).

Use targeted practice: simulate a 90-minute live coding round, then review code for readability and edge cases. Track progress with simple metrics: time-to-solve, number of test cases covered, and percentage improvement in runtime complexity.

Actionable takeaway: schedule 6–8 weeks of preparation, focus on 30 algorithm problems, and complete at least two end-to-end . NET projects to show in interviews.

Subtopics to Master (with Examples)

Organize study into focused subtopics and concrete tasks. Below are high-value areas with sample questions and task-based practice.

•C# Language & Core APIs
•Key items: value vs reference types, pattern matching, records, nullable reference types.
•Sample question: "How does 'ref struct' differ from a normal struct– Show a short example and explain stack safety.
•Practice: implement 5 small utilities using Span<T> and generics.

•.NET Runtime & Memory
•Key items: garbage collection generations, finalizers vs IDisposable, memory leaks.
•Sample question: "How would you diagnose a 30% memory increase in production– Describe using dotMemory and memory dumps.

•Asynchronous Programming
•Key items: async/await, Task vs ValueTask, deadlocks in synchronization contexts.
•Example task: convert a blocking I/O call to async and measure latency reduction.

•Web & Data Access (ASP.NET Core + EF Core)
•Key items: middleware, model binding, connection pooling, EF Core change tracking.
•Sample question: "Explain optimistic concurrency and implement it in EF Core."

•Design Patterns & Architecture
•Key items: Dependency Injection, Repository, CQRS, microservices boundaries.
•Practice: design a payment API with idempotency and list 3 failure modes.

•Testing, CI/CD, DevOps
•Key items: unit/integration tests, pipelines (GitHub Actions/Azure DevOps), Docker.
•Task: add a GitHub Actions pipeline that runs tests and builds a Docker image.

Actionable takeaway: pick 8 subtopics, practice 3 hands-on tasks per topic, and log performance metrics (time, test coverage, latency improvements).

Resources and Study Plan

Use a mix of documentation, hands-on platforms, books, and sample repos. Below is a concise list with specific uses and a short 8-week plan.

•Official Docs
•Microsoft Docs (.NET, ASP.NET Core, EF Core): authoritative API details and migration guides.

•Books (targeted reads)
•"C# in Depth" by Jon Skeet — focus on language nuances and real examples.
•"Pro ASP.NET Core" (latest edition) — for middleware, routing, and hosting.

•Online Courses & Tutorials
•Pluralsight or Udemy for structured tracks: prioritize courses with coding labs.
•Microsoft Learn: free modules on Azure, authentication, and containerization.

•Practice Platforms
•LeetCode: solve 30 priority problems (arrays, strings, trees). Track time per problem.
•Exercism (C#) and HackerRank: focus on language-specific idioms.

•Sample Projects & Repos
•eShopOnWeb (Microsoft) or similar sample apps to study architecture, DI, and EF Core patterns.

•Tools
•Visual Studio/JetBrains Rider, dotnet CLI, dotTrace/dotMemory, Postman, Docker, Azure CLI.

8-week Plan (practical)

•Weeks 1–2: C# fundamentals + 10 small kata problems.
•Weeks 3–4: ASP.NET Core + EF Core project (API + tests).
•Weeks 5–6: Algorithms/practice on LeetCode (20 problems) and profiling exercises.
•Weeks 7–8: System design prep, mock interviews, and polishing GitHub portfolio.

Actionable takeaway: follow the 8-week plan, use Microsoft Docs for clarifications, and maintain a practice log with metrics (time, passed tests) to measure readiness.

.net developer Interview Questions: Complete Guide

Michael Rodriguez

Common Interview Questions

Behavioral Questions (STAR Method)

Questions to Ask the Interviewer

Interview Preparation Tips

Overview

Subtopics to Master (with Examples)

Resources and Study Plan

Interview Prep Checklist

Build your job search toolkit

.net developer Interview Questions: Complete Guide

Michael Rodriguez

Common Interview Questions

Q1What are the main differences between .NET Framework and .NET (formerly .NET Core)?

Q2How does dependency injection work in ASP.NET Core and why should you use it?

Q3Explain async and await in C#, and when to use them.

Q4What is garbage collection in .NET and how does it affect application performance?

Q5Can you explain SOLID principles with examples in C#?

Q6What is Entity Framework and when should you use EF Core versus Dapper or raw ADO.NET?

Q7How would you secure an ASP.NET Core Web API?

Q8What are delegates and events in C# and how do you use them?

Q9What is middleware in ASP.NET Core and how do you write custom middleware?

Q10How do you approach exception handling and logging in a .NET application?

Behavioral Questions (STAR Method)

B1Tell me about a time you faced a tight deadline and how you handled it.

B2Describe a situation where you disagreed with a technical decision and how you resolved it.

B3Give an example of a time you improved application performance.

Questions to Ask the Interviewer

Interview Preparation Tips

Overview

Subtopics to Master (with Examples)

Resources and Study Plan

Interview Prep Checklist

Build your job search toolkit