Code:

unit untMain;

interface

uses

Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,

StdCtrls;

type

TForm1 = class(TForm)

   Button1: TButton;

   procedure Button1Click(Sender: TObject);

private

   { Private declarations }

public

   { Public declarations }

end;

var

Form1: TForm1;

implementation

function calc_double ( r: real ): real; stdcall; external 'ExDouble.dll';

{$R *.DFM}

procedure TForm1.Button1Click(Sender: TObject);

begin

    // в окошке сообщения будет цифра 21

    showMessage ( floatToStr ( calc_double ( 10.5 ) ) );

end;

end.