is 与 as 运算符举例
unit Unit1;

interface

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

type
TForm1 = class(TForm)
Button1: TButton;
Edit1: TEdit;
procedure FormCreate(Sender: TObject);
private
procedure MyPro(Sender: TObject);
{ Private declarations }
public
{ Public declarations }
end;

var
Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
Button1.OnClick := MyPro;
Edit1.OnClick := MyPro;
end;

procedure TForm1.MyPro(Sender: TObject);
begin
if Sender is TButton then
ShowMessage((Sender as TButton).Caption);

if Sender.InheritsFrom(TEdit) then
ShowMessage((Sender as TEdit).Text);
end;

end.