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From: Arian_Intellect
Message 2 in Discussion
Read this
Boxing And Unboxing C#
Boxing and unboxing is a essential concept in C#�s type system. With Boxing
and unboxing one can link between value-types and reference-types by allowing
any value of a value-type to be converted to and from type object. Boxing and
unboxing enables a unified view of the type system wherein a value of any type
can ultimately be treated as an object.
Converting a value type to reference type is called Boxing.Unboxing is an
explicit operation.
C# provides a �unified type system�. All types�including value types�derive
from the type object. It is possible to call object methods on any value, even
values of �primitive� types such as int.
The example using System;
class Test
{
static void Main() {
Console.WriteLine(3.ToString());
}
}
calls the object-defined ToString method on an integer literal.
The example class Test
{
static void Main() {
int i = 1;
object o = i; // boxing
int j = (int) o; // unboxing
}
}
An int value can be converted to object and back again to int.
This example shows both boxing and unboxing. When a variable of a value type
needs to be converted to a reference type, an object box is allocated to hold
the value, and the value is copied into the box.
Unboxing is just the opposite. When an object box is cast back to its
original value type, the value is copied out of the box and into the appropriate
storage location.
Boxing conversions
A boxing conversion permits any value-type to be implicitly converted to the
type object or to any interface-type implemented by the value-type. Boxing a
value of a value-type consists of allocating an object instance and copying the
value-type value into that instance.
For example any value-type G, the boxing class would be declared as follows: class vBox
{
G value;
G_Box(G g) {
value = g;
}
}
Boxing of a value v of type G now consists of executing the expression new
G_Box(v), and returning the resulting instance as a value of type object.
Thus, the statements
int i = 12;
object box = i;
conceptually correspond to
int i = 12;
object box = new int_Box(i);
Boxing classes like G_Box and int_Box above don�t actually exist and the
dynamic type of a boxed value isn�t actually a class type. Instead, a boxed
value of type G has the dynamic type G, and a dynamic type check using the is
operator can simply reference type G. For example,
int i = 12;
object box
= i;
if (box is int) {
Console.Write("Box contains an int");
}
will output the string �Box contains an int� on the console.
A boxing conversion implies making a copy of the value being boxed. This is
different from a conversion of a reference-type to type object, in which the
value continues to reference the same instance and simply is regarded as the
less derived type object.
For example, given the declaration struct Point
{
public int x, y;
public Point(int x, int y) {
this.x = x;
this.y = y;
}
}
the following statements
Point p = new Point(10, 10);
object box =
p;
p.x = 20;
Console.Write(((Point)box).x);
will output the value 10 on
the console because the implicit boxing operation that occurs in the assignment
of p to box causes the value of p to be copied. Had Point instead been declared
a class, the value 20 would be output because p and box would reference the same
instance.
Unboxing conversions
An unboxing conversion permits an explicit conversion from type object to any
value-type or from any interface-type to any value-type that implements the
interface-type. An unboxing operation consists of first checking that the object
instance is a boxed value of the given value-type, and then copying the value
out of the instance. unboxing conversion of an object box to a value-type G
consists of executing the expression ((G_Box)box).value.
Thus, the statements object box = 12;
int i = (int)box;
conceptually correspond to
object box = new int_Box(12);
int i =
((int_Box)box).value;
For an unboxing conversion to a given value-type to
succeed at run-time, the value of the source argument must be a reference to an
object that was previously created by boxing a value of that value-type. If the
source argument is null or a reference to an incompatible object, an
InvalidCastException is thrown.
CONCLUSION :
This type system unification provides value types with the benefits of
object-ness without introducing unnecessary overhead.
For programs that don�t need int values to act like objects, int values are
simply 32-bit values. For programs that need int values to behave like objects,
this capability is available on demand. This ability to treat value types as
objects bridges the gap between value types and reference types that exists in
most languages.
Shaju Thomas
e-Cosmos Technologies Ltd.,
Mail: [EMAIL PROTECTED]
Phone(Off)
: 51217038/39 Extn: 132
Mobile: +91 98455 21794
----- Original Message -----
From:
BalajiKrishnan
To: BDOTNET
Sent: Thursday, October 16, 2003 3:31
PM
Subject: .NET Concepts........
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From:
BalajiKrishnan
Message 1 in Discussion
Dear All, Can any
one explain me what is a.Monitor Class b.Boxing and
Unboxing c.Threading. and also tell me where it can be
used...... Thanks Balaji.
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