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September 06, 2008

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What is boxing in .net?

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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 H, the boxing class would be declared as follows:class vBox {   H value;   H_Box(H h)   {    value = h;   } }Boxing of a value v of type H now consists of executing the expression new H_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 H_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 declarationstruct 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.

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