Export (0) Print
Expand All

Boxing and Unboxing (C# Programming Guide)

Updated: July 2009

Boxing is the process of converting a value type to the type object or to any interface type implemented by this value type. When the CLR boxes a value type, it wraps the value inside a System.Object and stores it on the managed heap. Unboxing extracts the value type from the object. In the following example, the integer variable i is boxed and assigned to object o.

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

The object o can then be unboxed and assigned to integer variable i:

o = 123;
i = (int)o;  // unboxing

In relation to simple assignments, boxing and unboxing are computationally expensive processes. When a value type is boxed, a new object must be allocated and constructed. To a lesser degree, the cast required for unboxing is also expensive computationally. For more information, see Performance.

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

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

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
*/

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.

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 that 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 o = i;     // boxing 
int j = (int)o;  // unboxing

The following figure demonstrates the result of the previous statements.

Unboxing Conversion

UnBoxing Conversion graphic

For the unboxing of value types to succeed at run time, the item being unboxed must be a reference to an object that was previously created by boxing an instance of that value type. Attempting to unbox null causes a NullReferenceException. Attempting to unbox a reference to an incompatible value type causes an InvalidCastException.

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

  • 4.3.1 Boxing Conversions

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

  • 4.3 Boxing and Unboxing

Date

History

Reason

July 2009

Added information about exceptions to "Unboxing" section.

Customer feedback.

July 2008

Consolidated previously separate boxing and unboxing topics into this topic.

Content bug fix.

Community Additions

ADD
Show:
© 2014 Microsoft