Value vs Reference, Boxing, and Identity

Two families, two behaviors

A value type (struct, enum, numeric types, bool, char) carries its data inline. Assignment copies the whole value. A reference type (class, record class, interface, delegate, array, string) stores a reference; the object lives on the heap, and assignment copies only the reference — two variables then alias one object.

struct PointS { public int X; }
class  PointC { public int X; }

var s1 = new PointS { X = 1 };
var s2 = s1; s2.X = 9;              // s1.X is still 1 — copy

var c1 = new PointC { X = 1 };
var c2 = c1; c2.X = 9;              // c1.X is now 9 — alias

Neither is "better". Use structs for small, immutable, leaf values (coordinates, money amounts, identifiers); classes for entities with identity and lifetime. record struct gives you value semantics with generated equality; record (class) gives reference semantics with value-based equality — handy but distinct things.

Boxing: the invisible heap allocation

Boxing wraps a value type in a heap object so it can travel where a reference is expected. It happens silently in three common places:

object o = 42;                       // explicit box
IComparable<int> c = 7;              // interface box
Console.WriteLine(string.Join(" ", 1, 2, 3));   // params object[]: each int boxes

Unboxing is the reverse cast — it must name the EXACT type: unboxing a boxed int as double throws InvalidCastException, even though int → double converts implicitly in normal code.

Boxing costs allocation and hurts in hot loops. Console.WriteLine(42) does NOT box (it has an int overload) — but string.Format("{0}", x) with an object[] path does. Equality gets subtle too: boxed values compare by value with Equals, but == on object compares references:

object a = 1000, b = 1000;
a.Equals(b)      // true  — boxed int compares by value
a == b           // false — different boxes
(object)a == (object)1000   // false again

Check your understanding

• DateTime is a struct: after d2 = d1; d2 = d2.AddDays(1), what is d1? (Unchanged — AddDays returns a new value.)
• Why does ArrayList (pre-generics) feel slow for ints? (Every Add boxes.)
• What does (short)(object)intVar throw? (InvalidCastException — exact-type rule.)