These are based on what’s commonly asked at product companies, fintech, SaaS, and cloud-heavy teams.

1. C# / .NET Core

Q1. Difference between Task and ValueTask?

Answer:
Task is a reference type allocated on the heap. ValueTask is a struct designed to reduce allocations when an operation often completes synchronously.

Use ValueTask only when:

  • The method is hot-path

  • It frequently completes synchronously

  • You control both producer and consumer

Downsides:

  • More complex usage

  • Can only be awaited once unless converted to Task

  • Misuse can hurt performance

Rule: Default to Task. Use ValueTask only after profiling.

Q2. Explain how Garbage Collection works in .NET

Answer:
.NET uses a generational GC:

  • Gen 0: Short-lived objects

  • Gen 1: Survivors from Gen 0

  • Gen 2: Long-lived objects

Large objects (>85KB) go to the LOH and are expensive to collect.

Server GC is optimised for throughput (used in ASP.NET), while Workstation GC is optimised for responsiveness.

Key optimisations:

  • Minimise allocations

  • Reuse objects

  • Avoid LOH allocations

Q3. What happens when you use async / await?

Answer:
The compiler transforms the method into a state machine.

  • Execution runs synchronously until the first await

  • The thread is returned to the pool

  • Continuation runs when the awaited task completes

No new thread is created.

Common mistake:
Blocking with .Result or .Wait() → deadlocks (especially in ASP.NET).

2. ASP.NET Core

Q4. Explain the ASP.NET Core request pipeline

Answer:
Requests pass through a chain of middleware.
Each middleware can:

  • Handle the request

  • Modify it

  • Short-circuit the pipeline

Order matters:

  • Exception handling → routing → auth → endpoints

Middleware is best for cross-cutting concerns; filters are better for MVC-specific logic.

Q5. Middleware vs Filters

Answer:

Middleware Filters
Global MVC-only
Executes for all requests Only controllers/actions
Runs before routing Runs after routing
Good for logging/auth Good for validation

Q6. Explain DI lifetimes

Answer:

  • Singleton: One instance per app

  • Scoped: One per request

  • Transient: New every resolution

Common pitfall:
Injecting Scoped into Singleton → memory leaks / runtime errors.

3. Databases & EF Core

Q7. EF Core tracking vs no-tracking

Answer:

  • Tracking: Change detection, higher memory usage

  • No-tracking: Faster reads, no change tracking

Use no-tracking for:

  • Read-only queries

  • APIs

  • Reporting

context.Users.AsNoTracking().ToList();

Q8. How do you improve EF Core performance?

Answer:

  • Use AsNoTracking

  • Avoid lazy loading

  • Use projection (Select)

  • Add proper indexes

  • Use compiled queries

  • Use Dapper for complex queries

4. Concurrency & Threading

Q9. How do you avoid deadlocks?

Answer:

  • Use async all the way

  • Avoid .Wait() / .Result

  • Avoid blocking locks with async code

  • Use ConfigureAwait(false) in libraries

Q10. When to use lock vs SemaphoreSlim?

Answer:

  • lock: Sync, in-memory, fast

  • SemaphoreSlim: Async-friendly, throttling

Use SemaphoreSlim for async APIs or rate limiting.

5. Architecture

Q11. Explain Clean Architecture

Answer:
Core rules:

  • Business logic is independent of frameworks

  • Dependencies point inward

  • Infrastructure is replaceable

Layers:

  • Domain

  • Application

  • Infrastructure

  • Presentation

Benefits:

  • Testability

  • Maintainability

  • Technology independence

Q12. Monolith vs Microservices?

Answer:
Monolith:

  • Simpler

  • Easier deployment

  • Faster development early

Microservices:

  • Independent scaling

  • Fault isolation

  • Higher complexity

Rule: Start monolith, extract services when scaling requires it.

6. Performance & Resilience

Q13. How do you handle retries and failures?

Answer:
Use Polly:

  • Retry with backoff

  • Circuit breaker

  • Timeout

Never retry:

  • Non-idempotent operations

  • Authentication failures

Q14. What is backpressure?

Answer:
Backpressure prevents overload by slowing producers when consumers are overwhelmed.

Examples:

  • Bounded channels

  • Rate limiting

  • Queue depth monitoring

7. Security

Q15. How does JWT authentication work?

Answer:

  • Client authenticates

  • Server issues JWT

  • Client sends token in headers

  • Server validates signature and claims

JWT is stateless but:

  • Needs token expiration

  • Requires refresh strategy

  • Must be sent over HTTPS

8. Testing

Q16. Difference between unit and integration tests?

Answer:

  • Unit: Fast, isolated, mocks dependencies

  • Integration: Real dependencies, slower, higher confidence

A senior balances both.

9. Behavioural / Seniority

Q17. How do you handle production incidents?

Answer:

  • Stabilise first

  • Rollback or mitigate

  • Communicate clearly

  • Root cause analysis

  • Prevent recurrence

No blame, focus on learning.

Q18. How do you mentor juniors?

Answer:

  • Code reviews with explanation

  • Pair programming

  • Encourage ownership

  • Teach fundamentals, not shortcuts