structures can be passed by reference.

they can be returned as pointers.

MDI (Multiple Document Interface) is a Microsoft Windows programming interface for creating an application that enables users to work with multiple documents at the same time. Each document is in a separate space with its own controls for scrolling. The user can see and work with different documents such as a spreadsheet, a text document, or a drawing space by simply moving the cursor from one space to another.

An MDI application is something like the Windows desktop interface since both include multiple viewing spaces. However, the MDI viewing spaces are confined to the application's window or client area . Within the client area, each document is displayed within a separate child window . MDI applications can be used for a variety of purposes - for example, working on one document while referring to another document, viewing different presentations of the same information, viewing multiple Web sites at the same time, and any task that requires multiple reference points and work areas at the same time

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 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.