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4.3.1 Boxing conversions

Visual Studio .NET 2003

A boxing conversion permits a value-type to be implicitly converted to a reference-type. The following boxing conversions exist:

  • From any value-type (including any enum-type) to the type object.
  • From any value-type (including any enum-type) to the type System.ValueType.
  • From any value-type to any interface-type implemented by the value-type.
  • From any enum-type to the type System.Enum.

Boxing a value of a value-type consists of allocating an object instance and copying the value-type value into that instance.

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. For any value-type T, the boxing class behaves as if it were declared as follows:

sealed class T_Box: System.ValueType
{
   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);

Boxing classes like T_Box and int_Box above do not actually exist and the dynamic type of a boxed value is not actually a class type. 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. 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.

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 been declared a class instead, the value 20 would be output because p and box would reference the same instance.

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