Boxing and unboxing in C#

Boxing and unboxing is an essential concept in C#. With Boxing and unboxing one can link between value-types and reference-types by allowing any value of a value-type to be converted to and from type object. Boxing and unboxing enables a unified view wherein a value of any type can ultimately be treated as an object. Converting a value type to reference type is called Boxing. Unboxing is an explicit operation.

C# provides a unified type system. All types including value types derive from the type object. It is possible to call object methods on any value, even values of primitive types such as int.

Example:

class Test
{
static void Main() {
Console.WriteLine(3.ToString());
}
}

class Test
{
static void Main() {
int i = 1;
object o = i; // boxing
int j = (int) o; // unboxing
}
}

The above example shows both boxing and unboxing. When a variable of a value type needs to be converted to a reference type, an object box is allocated to hold the value, and the value is copied into the box. A boxing conversion permits any value-type to be implicitly converted to the type object or to any interface-type implemented by the value-type. Boxing a value of a value-type consists of allocating an object instance and copying the value-type value into that instance.

Unboxing is just the opposite. When an object box is cast back to its original value type, the value is copied out of the box and into the appropriate storage location.