.NET Full Stack Last-Minute Revision

30 quick revision notes for .NET Full Stack

1. Modern .NET is cross-platform and open source; .NET 10 is the current LTS release, while .NET Framework 4.8.1 is Windows-only legacy.
2. Value types store data directly and are copied on assignment; reference types store a reference to a heap object.
3. Boxing wraps a value type in an object on the heap; generic collections avoid boxing and extra allocations.
4. Strings are immutable, so use StringBuilder when appending many times inside a loop.
5. const is compile-time and copied into callers; readonly is set at runtime in the declaration or constructor.
6. Use throw, not throw ex, to rethrow an exception without losing the original stack trace.
7. Records give value-based equality, readable ToString and with-expressions, ideal for DTOs and immutable data.
8. An abstract class can hold state and constructors; a class can implement many interfaces but inherit only one class.
9. SOLID means single responsibility, open/closed, Liskov substitution, interface segregation and dependency inversion.
10. LINQ uses deferred execution, so a query runs only when enumerated with foreach, ToList, Count or First.
11. IQueryable builds an expression tree translated to SQL; IEnumerable runs filters in application memory.
12. Dictionary and HashSet give O(1) average lookups; List.Contains is O(n) and slows down large comparisons.
13. async and await free the thread during I/O, which improves scalability, not the speed of a single request.
14. Blocking with .Result or .Wait() can cause deadlocks and thread pool starvation; stay async all the way.
15. Avoid async void except for event handlers, because callers cannot await it or catch its exceptions.
16. Use Task.WhenAll to run independent I/O calls concurrently and SemaphoreSlim to limit concurrency in async code.
17. Middleware order matters: exception handler, HTTPS, routing, CORS, authentication, authorization, then endpoints.
18. Transient creates a new instance each time, scoped once per request, singleton once per application.
19. Never inject a scoped service such as DbContext into a singleton; use IServiceScopeFactory or IDbContextFactory.
20. Use IHttpClientFactory or typed clients instead of creating HttpClient per request to avoid socket exhaustion.
21. 401 means not authenticated and 403 means authenticated but not allowed to access the resource.
22. DbContext is lightweight and not thread-safe; register it as scoped and run one operation at a time.
23. Fix the N+1 problem with Include or, better, a Select projection that fetches only needed columns.
24. Use AsNoTracking for read-only queries and ExecuteUpdate or ExecuteDelete for set-based bulk changes.
25. A rowversion concurrency token makes EF Core throw DbUpdateConcurrencyException when another user changed the row.
26. A table has one clustered index that orders the data; non-clustered indexes are separate structures with pointers.
27. ROW_NUMBER numbers every row, RANK leaves gaps after ties, and DENSE_RANK does not leave gaps.
28. WHERE filters rows before grouping; HAVING filters groups after GROUP BY and can use aggregate functions.
29. Always pass user input as parameters to prevent SQL injection, whether using ADO.NET, EF Core or Dapper.
30. Angular signals and OnPush reduce unnecessary change detection; React hooks manage state and side effects in components.

Night-before checklist

• Revise your saved questions on this page once, aloud.
• Read the quick notes and mark any you cannot explain in one line.
• Rehearse a two-minute self-introduction and two project stories.
• Keep CV, ID, laptop charger and interview link ready; sleep on time.