Q: A junior developer writes a loop that adds 100,000 int values to a non-generic System.Collections.ArrayList. A senior colleague says this will be much slower than using List<int. Why?

Correct answer: ArrayList stores object references, so each int is boxed on insertion. ArrayList is untyped and stores everything as object. Adding an int boxes it (heap allocation + copy) on every Add, and reading it back unboxes it. List<int is generic and stores the ints inline in a backing int[] array — no boxing at all. For 100,000 elements this difference in allocations and GC pressure is significant.

Question 1

In C, which of these is a value type by default?

Accepted Answer

Correct answer: struct Point { }. struct declares a value type in C — instances are stored inline (on the stack, or inline inside whatever contains them) and are copied by value on assignment. class, string, and object are all reference types: variables of those types hold a reference to data on the heap.

Question 2

What does this print?

Accepted Answer

Correct answer: 1. Point is a struct (value type), so var p2 = p1; copies the entire value into p2 — p2 is an independent copy, not a reference to p1. Mutating p2.X has no effect on p1, so p1.X is still 1.

Question 3

What does this print?

Accepted Answer

Correct answer: 99. Box is a class (reference type). var b2 = b1; copies only the reference, so b1 and b2 both point to the same object on the heap. Changing b2.Value changes the same underlying object that b1 refers to, so b1.Value is also 99.

Question 4

What does the line object boxed = number; do?

Accepted Answer

Correct answer: It allocates a new object on the heap and copies the value of number into it (boxing). Assigning a value type to an object (or any interface it implements) triggers boxing: the CLR allocates a new object on the managed heap and copies the value type's data into it. The original number variable and the boxed copy are then independent — this is why boxing has a real allocation and copy cost.

Question 5

What is unboxing in C?

Accepted Answer

Correct answer: Extracting the value type back out of a boxed object (e.g. casting object back to int). Unboxing is the reverse of boxing: taking a reference stored as object (which holds a boxed value type) and casting it back to the original value type, e.g. int i = (int)boxedObj;. This copies the value out of the heap-allocated box into a value-type variable; an invalid cast throws InvalidCastException.

Question 6

A junior developer writes a loop that adds 100,000 int values to a non-generic System.Collections.ArrayList. A senior colleague says this will be much slower than using List<int. Why?

Accepted Answer

Correct answer: ArrayList stores object references, so each int is boxed on insertion. ArrayList is untyped and stores everything as object. Adding an int boxes it (heap allocation + copy) on every Add, and reading it back unboxes it. List<int is generic and stores the ints inline in a backing int[] array — no boxing at all. For 100,000 elements this difference in allocations and GC pressure is significant.

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