2 <clause number="11.3.1" title="Boxing conversions">
3 <paragraph>A boxing conversion permits any <non_terminal where="11.1">value-type</non_terminal> to be implicitly converted to the type object or to any <non_terminal where="11.2">interface-type</non_terminal> implemented by the <non_terminal where="11.1">value-type</non_terminal>. Boxing a value of a <non_terminal where="11.1">value-type</non_terminal> consists of allocating an object instance and copying the <non_terminal where="11.1">value-type</non_terminal> value into that instance. </paragraph>
4 <paragraph>The actual process of boxing a value of a <non_terminal where="11.1">value-type</non_terminal> is best explained by imagining the existence of a boxing class for that type. <example>[Example: For any <non_terminal where="11.1">value-type</non_terminal> T, the boxing class behaves as if it were declared as follows: <code_example><![CDATA[
12 ]]></code_example></example></paragraph>
14 <example>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 <code_example><![CDATA[
17 ]]></code_example>conceptually correspond to <code_example><![CDATA[
19 object box = new int_Box(i);
20 ]]></code_example>end example]</example>
22 <paragraph>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. 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>[Example: For example, <code_example><![CDATA[
26 Console.Write("Box contains an int");
28 ]]></code_example>will output the string "Box contains an int" on the console. end example]</example> </paragraph>
29 <paragraph>A boxing conversion implies making a copy of the value being boxed. This is different from a conversion of a <non_terminal where="11.2">reference-type</non_terminal> to type object, in which the value continues to reference the same instance and simply is regarded as the less derived type object. <example>[Example: For example, given the declaration <code_example><![CDATA[
33 public Point(int x, int y) {
38 ]]></code_example>the following statements <code_example><![CDATA[
39 Point p = new Point(10, 10);
42 Console.Write(((Point)box).x);
43 ]]></code_example>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]</example> </paragraph>