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判断一个对象是否存在(赋值)的三种办法如下:
if obj <> nil then ...
if Boolean(obj) then ...
if Assigned(obj) then ...

通过下面的测试, 结论是 obj <> nil 最慢; Boolean(obj) 与 Assigned(btn) 相当!

测试图:



测试源码:
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}

var
  btn: TButton;
  n: Cardinal;

procedure TForm1.Button1Click(Sender: TObject);
var
  i: Integer;
begin
  n := GetTickCount;
  for i := 0 to 100000000 do if btn <> nil then ;
  n := GetTickCount - n;
  Text := IntToStr(n);

  n := GetTickCount;
  for i := 0 to 100000000 do if Boolean(btn) then ;
  n := GetTickCount - n;
  Text := Text + ' - ' + IntToStr(n);

  n := GetTickCount;
  for i := 0 to 100000000 do if Assigned(btn) then ;
  n := GetTickCount - n;
  Text := Text + ' - ' + IntToStr(n);
end;

end.

posted on 2008-03-12 09:03  万一  阅读(...)  评论(...编辑  收藏