Boxing
and Unboxing (C# Programming Guide)
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. Boxing is implicit;
unboxing is explicit. The concept of boxing and unboxing underlies the C#
unified view of the type system in which a value of any type can be treated as
an object.
In the following
example, the integer variable i is boxed and assigned
to object o.
C#
int i = 123;
// The following line boxes i.
object o = i;
The object o can then be unboxed
and assigned to integer variable i:
C#
o = 123;
i = (int)o; // unboxing
The following examples
illustrate how boxing is used in C#.
C#
// String.Concat example.
// String.Concat has many versions. Rest the mouse pointer on
// Concat in the following statement to verify that the
version
// that is used here takes three object arguments. Both 42
and
// true must be boxed.
Console.WriteLine(String.Concat("Answer",
42, true));
// List example.
// Create a list of objects to hold a heterogeneous collection
// of elements.
List<object> mixedList = new List<object>();
// Add a string element to the list.
mixedList.Add("First
Group:");
// Add some integers to the list.
for (int j = 1; j < 5; j++)
{
// Rest the mouse pointer over j to verify that you are
adding
// an int to a list of objects. Each element j is boxed when
// you add j to mixedList.
mixedList.Add(j);
}
// Add another string and more integers.
mixedList.Add("Second
Group:");
for (int j = 5; j < 10; j++)
{
mixedList.Add(j);
}
// Display the elements in the list. Declare the loop variable by
// using var, so that the compiler assigns its type.
foreach (var item in mixedList)
{
// Rest the mouse pointer over item to verify that the
elements
// of mixedList are objects.
Console.WriteLine(item);
}
// The following loop sums the squares of the first group of
boxed
// integers in mixedList. The list elements are objects, and
cannot
// be multiplied or added to the sum until they are unboxed.
The
// unboxing must be done explicitly.
var sum = 0;
for (var j = 1; j < 5; j++)
{
// The following statement causes a compiler error: Operator
// '*' cannot be applied to operands of type 'object' and
// 'object'.
//sum += mixedList[j] * mixedList[j]);
// After the list elements are unboxed, the computation does
// not cause a compiler error.
sum += (int)mixedList[j] * (int)mixedList[j];
}
// The sum displayed is 30, the sum of 1 + 4 + 9 + 16.
Console.WriteLine("Sum:
" + sum);
// Output:
// Answer42True
// First Group:
// 1
// 2
// 3
// 4
// Second Group:
// 5
// 6
// 7
// 8
// 9
// Sum: 30
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:
C#
int i = 123;
The following
statement implicitly applies the boxing operation on the variable i:
C#
// 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 is also possible to
perform the boxing explicitly as in the following example, but explicit boxing
is never required:
C#
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.
C#
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.
C#
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:
C#
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
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