關于開啟子窗口的問題（代碼如下）﹐報錯

Question

寫為一個過程﹕
procedure TForm1.openchild(AForm: TForm;AFormClass: TFormClass;childcaption:string);
var
  i:integer;
begin
  for i:=0 to application.ComponentCount-1 do
  begin
    if (application.Components[i] as tform).Caption=childcaption then
    begin
      (application.Components[i] as tform).Show;
      exit;
    end
    else begin
      aform:=AFormClass.create(self);
      aform.show;
    end;  end;
end;調用﹕
rocedure TForm1.Button1Click(Sender: TObject);
begin
  openchild(form2,tform2,'form2');
end;報錯﹕  invalid class typecast問﹕如何解決

Answer 1

1 if (application.Components[i] as tform).Caption=childcaption then
改为
if （application.Components[i] is AFormClass) then
  if (application.Components[i] as tform).Caption=childcaption then
2 将
aform:=AFormClass.create(self);
aform.show;
放到循环外

Answer 2

if (Application.Components[i] is tform) then加在
if (application.Components[i] as tform).Caption=childcaption then前面有些这种代码的必要吗？

Answer 3

to: firetoucher
代碼改后如下﹕（結果出現情況﹕每單擊一次就打開一個子窗口）
procedure TForm1.Button1Click(Sender: TObject);
var
  i,j:integer;
begin
showmessage(inttostr(form1.MDIChildCount));
i:=form1.MDIChildCount;
if i>0 then
begin
  for j:=0 to i-1 do
  begin
    if form1.MDIChildren[i] is tform2 then
    begin
      form1.MDIChildren[i].Show;
      exit;
    end
    else
    begin
      form2:=tform2.Create(self);
      form2.Show;
    end;
  end;
end
else begin
  form2:=tform2.Create(self);
      form2.Show;
end;

Answer 4

又看了一遍，需要的是加一句
procedure TForm1.openchild(AForm: TForm;AFormClass: TFormClass;childcaption:string);
var
  i:integer;
begin
  for i:=0 to application.ComponentCount-1 do
  begin
if （application.Components[i] is AFormClass) then//加这一句。
    if (application.Components[i] as tform).Caption=childcaption then
    begin
      (application.Components[i] as tform).Show;
      exit;
    end
    else begin
      aform:=AFormClass.create(self);
      aform.show;
    end;  end;
end;

Answer 5

unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
    procedure openchild(AForm: TForm;AFormClass: TFormClass;childcaption:string);
  public
    { Public declarations }
  end;var
  Form1: TForm1;implementationuses Unit2;{$R *.dfm}
procedure TForm1.openchild(AForm: TForm;AFormClass: TFormClass;childcaption:string);
var
  i:integer;
begin
  for i:=0 to application.ComponentCount-1 do
  begin
    if (application.Components[i] is AFormClass) then//¼ÓÕâÒ»¾ä¡£
    if (application.Components[i] as tform).Caption=childcaption then
    begin
      (application.Components[i] as tform).Show;
      exit;
    end
    else begin
      aform:=AFormClass.create(self);
      aform.show;
    end;  end;
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
  openchild(Form2,TForm2,'Form2');
end;end.form1代码