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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 Types (C# Reference) 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
BoxingConversion graphic

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

This example converts an integer variable i to an object o by means of 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

C# Language Specification

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

  • 4.3.1 Boxing Conversions

See Also

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