C# Interview Questions and Answers

A value type variable holds its data directly and is copied on assignment, while a reference type variable holds a reference to an object, so assignment copies the reference and both variables see the same object.

A string is immutable, so every change creates a new string object, while StringBuilder is a mutable buffer that modifies its contents in place, making it much faster for repeated concatenation.

A const is a compile-time constant that must be assigned where it is declared, while a readonly field can be assigned at declaration or in the constructor, so its value can be decided at runtime.

Both pass an argument by reference, but ref requires the variable to be initialised before the call, while out requires the method to assign a value before it returns.

An abstract class can hold state, constructors and partial implementation but a class can inherit only one, while an interface defines a contract that a class can implement many of.

Boxing converts a value type to object by allocating it on the heap, and unboxing converts it back; both add allocations and casts, so avoid them in hot code paths.

LINQ (Language Integrated Query) lets you query collections, databases and XML with a consistent syntax, and deferred execution means a query does not run when you define it but when you enumerate the results.

IEnumerable runs the query in memory using delegates, while IQueryable builds an expression tree that a provider such as Entity Framework translates into SQL, so filtering happens in the database.

Transient services are created every time they are requested, scoped services are created once per HTTP request, and singleton services are created once and shared for the life of the application.

A Thread is an operating-system execution unit, while a Task represents an asynchronous operation scheduled on the thread pool; await releases the current thread while the operation is pending and resumes the method when it completes.

The .NET garbage collector automatically frees managed objects that are no longer reachable, organising them into generations 0, 1 and 2 so short-lived objects are collected cheaply, and IDisposable is used to release unmanaged resources such as files and connections promptly.

Use async all the way down instead of blocking on tasks with .Result or .Wait(), and in library code use ConfigureAwait(false) so continuations do not need to return to a captured context.