Boxing, unboxing, and hidden allocations

Boxing, unboxing, and hidden allocations

Boxing wraps a value type in a heap object so it can travel as object, through an interface, or into a non-generic collection. Unboxing copies it back out. The box is a real allocation — gen0 pressure, not theoretical cost:

int x = 42;
object o = x;          // BOX: 24-byte heap object (header + value)
int y = (int)o;        // UNBOX: copy back out

The silent ones are what bite in production:

IComparable c = 5;          // box at the interface
string.Join(",", list);     // boxes each struct element
Func<int> f = s.GetHashCode; // struct receiver copied/captured
list.Add(structPoint);      // ArrayList: box per element

Where boxing is EXPECTED in this course: object boundaries you design deliberately (serialization, homogeneous storage), and once per logical operation — not per element in a hot loop. ToString() on a struct boxes if not overridden. Watch for enum boxed into Enum.HasFlag patterns on hot paths.

Grade your own code the way the runtime does: GC.GetAllocatedBytesForCurrentThread() before and after the loop. Bytes that grow per iteration are the truth; your intention is not.