if not Assigned(Modeless) then Assigned()什么意思！
assigned 是用来判断某一指针(pointer)或过程引用是否为nil(空)，如果为空则返回假（false）。
用法示例(防止窗体被实例化多次)：
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;
var
Form1: TForm1;
implementation
uses Unit2;
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
begin
if (Not assigned(form2)) then
begin
form2:=Tform2.Create(Self);
end;
form2.show;
end;
end.
－－－－－－－－－－－－－－－－－－－－－－－
unit Unit2;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs;
type
TForm2 = class(TForm)
procedure FormClose(Sender: TObject; var Action: TCloseAction);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form2: TForm2;
implementation
{$R *.dfm}
procedure TForm2.FormClose(Sender: TObject; var Action: TCloseAction);
begin
Action:=caFree;
form2:=nil; //这句比较重要，窗体被释放时，窗体变量并不会自动变空
end;
end.
很详细呢，不过不是我的原创，我转的，希望对你有用啦。。。。。
另一篇网络文章
关于Delphi中的Assigned
2008-04-05 10:53
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关于Delphi中的Assigned
function Assigned(var P): Boolean;
Description
Use Assigned to determine whether the pointer or procedure referenced by P is nil. P must be a variable reference of a pointer or procedural type. Assigned(P) corresponds to the test P<> nil for a pointer variable, and @P <> nil for a procedural variable.
Assigned returns False if P is nil, True otherwise.
检查指针指向的参考变量或过程是否为nil
每次我通常的处理方法都是：
if assigned(frm) then frm.close; 但是当下次调用时就会出错。为什么呢，直到咋天我才知道原因
frm.close;frm.free; 只是指定这块内存可以重写，并未释放为NIL 因此当下次调用时即使frm.free已经
执行过assigned(frm)仍为TRUE；
正确的处理方法：
if assigned(frm) then
begin
frm.close;
frm:=nil;
end;
或:
if assigned(frm) then
begin
frm.close;
freeandnil(frm);
end;
freeandnil的说明：
procedure FreeAndNil(var Obj);
Description
Use FreeAndNil to ensure that a variable is nil after you free the object it references. Pass any variable that represents an object as the Obj parameter.
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好的代码像粥一样，都是用时间熬出来的
来源：https://www.cnblogs.com/jijm123/p/10818999.html |
发表于 2019-5-6 13:11:00