procedure TForm1.btn1Click(Sender: TObject);
begin
{$Define ss}    (*  用于定义一个条件符号，一旦定义，条件符号就为真 *)
{$IFDEF ss}     (* 对条件符号进行判断，为真则编译源文件 *)
  ShowMessage('ss Has defined!!');
  Text := 'ss Has defined!!';
  Form1.Color := clLime;
{$ELSE}          (* 与$IFDEF配合使用，如果$IFDEF条件为假，则只对源文件$ELSE后一小部分进行编译 *)
  ShowMessage('ss Has not defined!!');
  Text := 'ss Has NOT defined!!'
  Form1.Color := clRed;
{$ENDIF}         (* 结束一个以$IF开始的条件段 *)


{$UNDEF ss}      (* 撤消以前的条件符号定义 *)

{$IFDEF ss}   (*  因为已经撤销ss的定义 *)
  ShowMessage('ss Has defined!!');
  Text := 'ss Has defined!!';
  Form1.Color := clLime;
{$ELSE}
  ShowMessage('ss Has not defined!!');  (* 因此，将执行以下语句 *)
  Text := 'ss Has NOT defined!!';
  Form1.Color := clRed;
{$ENDIF}


end;