试试带参数的 Exit

unit Unit1;

interface

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

type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
end;

var
Form1: TForm1;

implementation

{$R *.dfm}

function fun(str: string): string;
begin
if str = '' then Exit('空'); {Delphi 2009 支持}
Result := str + str;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
s: string;
begin
s := fun('ABC');
ShowMessage(s); {ABCABC}

s := fun('');
ShowMessage(s); {空}
end;

end.