Boxing Conversion (C# Programming Guide)

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:

int i = 123;

The following statement implicitly applies the boxing operation on the variable i:

object o = i;  // implicit boxing

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

This example converts an integer variable i to an object o by using boxing. Then, the value stored in the variable i is changed from 123 to 456. The example shows that the original value type and the boxed object use separate memory locations, and therefore can store different values.

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

        i = 456;  // change the contents of i

        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
*/

For more information, see the following sections in the C# Language Specification:

• 4.3.1 Boxing Conversions

C# Programming Guide

Boxing and Unboxing (C# Programming Guide)

Unboxing Conversion (C# Programming Guide)