Boxing in C#

Boxing is used to store value types in the garbage-collected heap. 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 type allocates an object instance on the heap and copies the value into the new object.
Consider the following declaration of a value-type variable:
The following statement implicitly applies the boxing operation on the variable i:

int i = 123;



// Boxing copies the value of i into object o. 
object o = i;

The result of this statement is creating an object reference 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. The difference between the two variables, i and o, is illustrated in the following figure.
Boxing Conversion


It also possible to perform the boxing explicitly as in the following example, but explicit boxing is never required:

int i = 123;
object o = (object)i;  // explicit boxing

Example:

class TestBoxing
{
static void Main()
{
int i = 123;

// Boxing copies the value of i into object o. 
object o = i;  

// Change the value of i.
i = 456;  

// The change in i does not effect the value stored in o.
System.Console.WriteLine("The value-type value = {0}", i);
System.Console.WriteLine("The object-type value = {0}", o);
}
}
/* Output:
The value-type value = 456
The object-type value = 123
*/

The following example demonstrates a case of invalid unboxing and the resulting InvalidCastException. Using try and catch, an error message is displayed when the error occurs.

class TestUnboxing
{
static void Main()
{
int i = 123;
object o = i;  // implicit boxing 

try
{
int j = (short)o;  // attempt to unbox

System.Console.WriteLine("Unboxing OK.");
}
catch (System.InvalidCastException e)
{
System.Console.WriteLine("{0} Error: Incorrect unboxing.", e.Message);
}
}
}

This program outputs:
Specified cast is not valid. Error: Incorrect unboxing.
If you change the statement:
int j = (short) o;
to
int j = (int) o;
the conversion will be performed, and you will get the output:
Unboxing OK.