Unit1.pas

unit Unit1;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls;

type

  { TForm1 }

  TForm1 = class(TForm)
    Button1: TButton;
    Edit1: TEdit;
    procedure Button1Click(Sender: TObject);
  private

  public

  end;



var
  Form1: TForm1;

implementation

uses
  Unit2;

{$R *.lfm}

{ TForm1 }

procedure TForm1.Button1Click(Sender: TObject);
var
  aManager: TManagerClass;
begin
  aManager:=TManagerClass.Create;
  Edit1.Text:=aManager.Item.GetAlias;
  aManager.Free;
end;

end.

Unit2.pas

unit Unit2;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils;

type

  { TBaseClass }

  TBaseClass = class
  public
    class function GetAlias: String;
  end;

  { TDescendantOfBaseClass }

  TDescendantOfBaseClass = class(TBaseClass)
  public
    class function GetAlias: String;
  end;

  { TManagerClass }

  TManagerClass = class
  private
    FItem: TBaseClass;
    procedure SetItem(AValue: TBaseClass);
  public
    constructor Create;
    destructor Destroy; override;
    property Item: TBaseClass read FItem write SetItem;
  end;

implementation

{ TManagerClass }

procedure TManagerClass.SetItem(AValue: TBaseClass);
begin
  if FItem=AValue then Exit;
  FItem:=AValue;
end;

constructor TManagerClass.Create;
begin
  FItem:=TDescendantOfBaseClass.Create;
end;

destructor TManagerClass.Destroy;
begin
  FItem.Free;
  inherited Destroy;
end;

{ TDescendantOfBaseClass }

class function TDescendantOfBaseClass.GetAlias: String;
begin
  Result:='Descendant';
end;

{ TBaseClass }

class function TBaseClass.GetAlias: String;
begin
  Result:=EmptyStr;
end;

end.