interface's demo

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

  ITest = interface 
    procedure Say;
  end;

  TSay = class(TInterfacedObject, ITest)
  private
    procedure Say;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

{ TSay }

procedure TSay.Say;
begin
  ShowMessage('hello');
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  a: ITest;
begin
  a := TSay.Create;
  a.Say;
  a := nil;
end;

end.