
11 Types
11.3 Boxing and unboxing
11.3.1 Boxing conversions
Paragraph 11 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. 2 Boxing a value of a value-type consists of allocating an object instance and copying the value-type value into that instance.
Paragraph 21 The actual process of boxing a value of a value-type is best explained by imagining the existence of a boxing class for that type. [Example: For any value-type T, the boxing class behaves as if it were declared as follows:
sealed class T_Box
{
T value;
public T_Box(T t) {
value = t;
}
}
Boxing of a value v of type T now consists of executing the expression new T_Box(v), and returning the resulting instance as a value of type object. Thus, the statements int i = 123; object box = i;conceptually correspond to
int i = 123; object box = new int_Box(i);end example] Paragraph 31 Boxing classes like T_Box and int_Box above don't actually exist and the dynamic type of a boxed value isn't actually a class type. 2 Instead, a boxed value of type T has the dynamic type T, and a dynamic type check using the is operator can simply reference type T. [Example: For example,
int i = 123;
object box = i;
if (box is int) {
Console.Write("Box contains an int");
}
will output the string "Box contains an int" on the console. end example]
Paragraph 41 A boxing conversion implies making a copy of the value being boxed. 2 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. [Example: 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 been declared a class instead, the value 20 would be output because p and box would reference the same instance. end example]
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