C#

<br />
program Project;<br />
<br />
{$APPTYPE CONSOLE}<br />
<br />
uses<br />
  SysUtils, Classes;<br />
  <br />
type<br />
  // TMyBook is like an basic adapter. It's decendants provide the application<br />
  // with various functions and variables.<br />
  TMyBook = class<br />
  private<br />
    FName: string;<br />
  public<br />
    constructor Create(const AName: string); virtual;<br />
    property Name: string read FName;<br />
  end;<br />
<br />
  // TMyBookClass - a metaclass of TMyBook type ??? <br />
  TMyBookClass = class of TMyBook;<br />
  // A decendant of TMyBook, will override functions from TMyBook.<br />
  TMyNewBook = class(TMyBook);<br />
<br />
  // Basicly contains information to create, access and<br />
  // manage the TMyBook instance.<br />
  TBookInfo = record<br />
    Name: String;<br />
    BookInstance: TMyBook;<br />
    BookClass: TMyBookClass;<br />
  end;<br />
<br />
var<br />
 // The currently active TMyBook is accessed throught this.<br />
 ActiveBook: TMyBook;<br />
 // An array where all the TBookInfos are stored. <br />
 Books: array of TBookInfo;<br />
<br />
constructor TMyBook.Create(const AName: string);<br />
begin<br />
  FName := AName;<br />
end;<br />
<br />
// Finds a TMyBook by name if there is one present.<br />
function FindBook(const AName: string): Integer;<br />
begin<br />
  for Result := 0 to Length(Books) - 1 do<br />
    if CompareText(Books[Result].Name, AName) = 0 then Exit;<br />
  Result := -1;<br />
end;<br />
<br />
// Fills the TBookInfo with information about the TMyBook.<br />
// It also determines the class type witch is going to be used.<br />
procedure RegisterBookInfo(const AName: string; ABookClass: TMyBookClass);<br />
var<br />
  Index: Integer;<br />
begin<br />
  Index := Length(Books);<br />
  SetLength(Books, Index + 1);<br />
  with Books[Index] do<br />
  begin<br />
    Name := AName;<br />
    BookClass := ABookClass;<br />
    BookInstance := nil;<br />
  end;<br />
end;<br />
<br />
// Returns the instance of a book by name. The hard part here<br />
// is that the instance is created of BookClass type.<br />
function GetBook(const AName: string): TMyBook;<br />
var<br />
  Index: Integer;<br />
begin<br />
  Index := FindBook(AName);<br />
  with Books[Index] do<br />
  begin<br />
    Assert(BookInstance = nil);<br />
    BookInstance := BookClass.Create(Name);<br />
    Result := BookInstance;<br />
  end;<br />
end;<br />
<br />
// Sets up a book instance and assigns it to ActiveBook.<br />
procedure SetActiveBook(const ABookName: string);<br />
begin<br />
  // ReleaseActiveBook;<br />
  ActiveBook := GetBook(ABookName);<br />
end;<br />
<br />
begin<br />
  // First I register the info about the book, usually at program start.<br />
  RegisterBookInfo('New Book', TMyNewBook);              <br />
  // And if I have a need, I activate it. I might as well use any other book<br />
  // and switch between them at runtime with this clause.<br />
  SetActiveBook('New Book');<br />
end.<br />