program ClassTest;
type
  TBase = class(TObject)
  private
    FCount: Integer;
  public
    constructor Create;
    destructor Destroy; override;
    procedure SetCount(V: Integer);
    function GetCount: Integer;
    property Count: Integer read GetCount write SetCount default 0;
  end;
  THelper = class helper for TBase
    function Doubled: Integer;
  end;
constructor TBase.Create;
begin
  FCount := 0;
end;
destructor TBase.Destroy;
begin
  inherited;
end;
procedure TBase.SetCount(V: Integer);
begin
  FCount := V;
end;
function TBase.GetCount: Integer;
begin
  GetCount := FCount;
end;
function THelper.Doubled: Integer;
begin
  Doubled := Count * 2;
end;
var B: TBase;
begin
  B := TBase.Create;
  B.Count := 5;
end.