This post is an aide-memoire as I learn more about boxing and unboxing in C# and is based upon this part of the C# docs.

Boxing

Is the process of converting a value type (such as int or bool) to the type Object or to any interface type implemented by this value type. When the CLR boxes a value type, it wraps the value inside a System.Object and stores it in the managed heap.

Boxing is implicit.

int i = 10; 
// this line boxes i
object o = i;

Although it is possible to perform the boxing explicitly it is not required.

int i = 10;
// explicit boxing
object o = (object)i;

Unboxing

Extracts the value type from the object.

Unboxing is explicit.

int i = 10; 
// boxes i
object o = i;
// unboxes the object to the int value type named j
int j = (int)o;

Performance

Both boxing and Unboxing are computationally expensive operations.