What is boxing and unboxing?

Boxing is an implicit conversion of a value type to the type object or to any interface type implemented by this value type. Boxing a value of a value allocates an object instance and copies the value into the new object. Consider the following declaration of a value-type variable:
int i = 123;
The following statement implicitly applies the boxing operation on the variable i:
object o = i;
The result of this statement is creating an object o, on the stack, that references a value of the type int, on the heap. This value is a copy of the value-type value assigned to the variable i.
Unboxing is an explicit conversion from the type object to a value type or from an interface type to a value type that implements the interface. An unboxing operation consists of:
• Checking the object instance to make sure it is a boxed value of the given value type.
• Copying the value from the instance into the value-type variable.
The following statements demonstrate both boxing and unboxing operations:
int i = 123; // A value type
object box = i; // Boxing
int j = (int)box; // Unboxing