overview

Boxing & Unboxing are performed by the CLR because it uses the Unified Type System in which a value of any type is treated an object.

boxing

• Converting a value type (struct, enum, numeric types, floats, bool, char) to object (or an interface type implemented by this value).
• The CLR “boxes” the value inside an object and stores it on the heap.
• A new object is allocated, constructed, and stored. This is expensive.
• The operation is implicit.

unboxing

• Extracting the value type from the object.
• The operation is explicit.

example

int i = 123;
object o = i; // i is now boxed in o.

Note that o is not a reference to i:

i = 456; // This does not change o.

Unboxing

o = 123;
i = (int)o;

performance

Boxing stores the value type on the garbage-collected heap. Because a new object must be allocated and constructed, this is computationally expensive. The cast required to unbox is also expensive, but less so.

Any time a type can store both value and reference types (like ArrayList), it will box/unbox the value types into objects.