C# Interview Questions

1. What is C# and how does it relate to the .NET runtime and CLR?

2. What is the difference between a value type and a reference type in C#?

3. What is the difference between a class and a struct in C#?

4. What are the boxing and unboxing operations in C# and why do they matter?

5. What is the difference between managed and unmanaged code in C#?

6. How does garbage collection work in C# and what are its generations?

7. What is the IDisposable interface and the using statement in C#?

8. What is the difference between const, readonly, and static in C#?

9. What are properties in C# and how do they differ from fields?

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

11. What are delegates in C# and how do they enable callbacks?

12. What is the difference between an event and a delegate in C#?

13. What are lambda expressions and how do they relate to delegates in C#?

14. What is LINQ in C# and how does deferred execution work?

15. What is the difference between IEnumerable and IQueryable in C#?

16. What is async/await in C# and how does the Task-based model work?

17. What is the difference between Task and Thread in C#?

18. What are generics in C# and what problems do they solve?

19. What is the difference between ref, out, and in parameters in C#?

20. What are extension methods in C# and how do they work?

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

22. What are nullable reference types in C# and why were they introduced?

23. What is the difference between String and StringBuilder in C#?

24. What are records in C# and how do they differ from classes?

25. What is pattern matching in C# and how has it evolved?

26. What is the difference between throw and throw ex in C# exception handling?

27. What are indexers in C# and when should you use them?

28. What is the difference between covariance and contravariance in C# generics?

29. What is dependency injection in C# and how does the built-in container work?

30. What is the difference between first-chance and second-chance exceptions in C#?

31. Why does mixing blocking calls with async/await deadlock, and what does ConfigureAwait(false) do?

32. When should you return ValueTask instead of Task, and what are the rules for consuming one?

33. What are Span<T> and Memory<T> in C#, and when do you use each?

34. What is IAsyncDisposable and when must you use await using instead of using?

35. What is the Large Object Heap, and how do server GC and workstation GC change your application's behaviour?

36. What is a captive dependency in .NET dependency injection, and how do you detect and prevent it?

37. What are expression trees in C#, and how do LINQ providers use them?

38. How do you implement cooperative cancellation with CancellationToken, and what are the common mistakes?

39. When does using a struct hurt performance, and what do readonly struct, ref struct and in parameters fix?

40. What is thread pool starvation in .NET, and how do you diagnose and fix it?

41. What are C# source generators, and when should they replace runtime reflection?

42. Why can string comparison give different results on different machines, and how do you choose the right StringComparison?

43. Why does creating a new HttpClient per request cause socket exhaustion, and what does IHttpClientFactory fix?

44. How do exception filters (catch ... when) differ from catch-and-rethrow, and why does that matter in production?

45. When do you actually need a finalizer in C#, and why is SafeHandle preferred?

46. How do System.Threading.Channels support producer/consumer pipelines, and when would you choose them over BlockingCollection?

47. What problem does IAsyncEnumerable<T> solve, and how do you cancel and consume one correctly?

48. How does type initialisation work in C#, and what are the TypeInitializationException and deadlock traps?

49. How can a .NET application leak memory despite garbage collection, and how do you find the cause?

50. How do you handle polymorphism and custom conversion in System.Text.Json, and what breaks when migrating from Newtonsoft.Json?

51. When do you use lock, SemaphoreSlim or Interlocked, and why can you not await inside a lock?

52. What breaks when you publish a .NET application trimmed or with Native AOT, and how do you make a library compatible?

53. What does the C# compiler actually generate for an async method, and why does that matter for performance?

54. Why is async void dangerous, and what happens to an exception in a task nobody awaits?

55. If you await Task.WhenAll and three of the tasks fail, what exception do you see and how do you get the rest?

56. What is multiple enumeration of an IEnumerable, and how do you find and fix it in production code?

57. Explain the full dispose pattern — why is there a protected virtual Dispose(bool disposing)?

58. Nullable reference types are enabled, so why is a null still reaching production?

59. Two records with identical list contents compare as unequal. Why, and what else about record equality catches people out?

60. When would you use stackalloc versus ArrayPool for a temporary buffer, and what are the risks?

61. You mutate a struct stored in a List and the change disappears. What is going on?