如何通过窗体的句柄来访问窗体中的控件，如果是EDIT则返回EDIT的TEXT

Question

procedure TForm1.Button1Click(Sender: TObject);
var
 h,h1:thandle;
 a:string;
 i:integer;
begin
h1:=findwindow(nil,'form1')    ;
h:=findWindowEx(h1,0,'Tedit',nil);
SendMessage(h, WM_GETTEXT,sizeof(a),LongInt(PChar(a))) ;
showessage(a);
end;
以上程序为什么a的值是空？

Answer 1

procedure TForm1.SpeedButton1Click(Sender: TObject);
var
 h,h1:thandle;
 a:array[0..255] of char;
 i:integer;
begin
h1:=findwindow(nil,'form1')    ;
h:=findWindowEx(h1,0,'TEdit','Edit1');
SendMessage(h, WM_GETTEXT,256, Integer(@a[0])) ;
showmessage(a);
end;

Answer 2

h:=findWindowEx(h1,0,'TEdit','Edit1');
中应该把'Edit1'换为nil