Pointer to Control - delphi

Question

I would like to create array of all listboxes and access them. I tried to do it using pointers, however my program crashes during runtime with error access violation at address...
type ControlsCount = 4;
type PLB = ^TListBox;
var listBoxes: array of PLB;
procedure TExport.FormCreate(Sender: TObject);
var i: word; n: integer;
begin
with FormExport do
begin
ListRowHeight := List_sex.height;
List_sex.items.add('---');
List_sex.items.add('Man');
List_sex.items.add('Woman');
List_sex.onmousemove:=ListMouseMove;
setLength(listBoxes, ControlsCount);
n := -1;
for i := 0 to ControlsCount - 1 do
if Components[i] is TWinControl then
if TWinControl(Components[i]).CanFocus then
begin
inc(n);
// mistake here: should be listBoxes[n] not listBoxes[i]
listBoxes[i] := PLB(Components[i]);
end;
realControlsCount := n;
end;
end;
procedure TExport.resetListBoxes;
var i: word;
begin
for i := 0 to realControlsCount-1 do
begin
TListBox(listBoxes[i]^).height := ListRowHeight;
end;
end;
So here I try to set the pointer of the control to listBoxes[i].
listBoxes[i] := PLB(Components[i]);
and here I try to access it:
TListBox(listBoxes[i]^).height := ListRowHeight;
this is the line where it generates error.
What am I doing wrong?

Answer 1

Just remove all pointer stuff and check whether control is really TListBox. Also you misused ControlsCount while accessed another list Components[i]
var listBoxes: array of TListBox;
...
for i := 0 to ControlsCount - 1 do
if Controls[i] is TListBox then //note strict constraint
listBoxes[n] := Controls[i] as TListBox;
...
listBoxes[i].height := ListRowHeight;
Aslso consider using TList<TListBox> instead of array

Answer 2

Regarding the answere MBo gave this is how I would use it with a TList
unit Unit1;
interface
uses
System.SysUtils, System.Types, System.UITypes, System.Classes, System.Variants,
FMX.Types, FMX.Controls, FMX.Forms, FMX.Graphics, FMX.Dialogs, Generics.Collections,
FMX.Layouts, FMX.ListBox, FMX.Controls.Presentation, FMX.StdCtrls;
type
TForm1 = class(TForm)
lst1: TListBox;
lst2: TListBox;
lst3: TListBox;
btn1: TButton;
pnl1: TPanel;
procedure FormCreate(Sender: TObject);
procedure FormDestroy(Sender: TObject);
procedure btn1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
listboxes: TList<TListBox>; // Define list that will contain listboxes
implementation
{$R *.fmx}
procedure TForm1.btn1Click(Sender: TObject);
var
lstbx: TListBox;
begin
for lstbx in listboxes do
ShowMessage(lstbx.Height.ToString); Loop through all listboxes and show their height
end;
procedure TForm1.FormCreate(Sender: TObject);
var
control: TControl;
begin
listboxes := TList<TListBox>.Create; // Create the TList
for control in pnl1.Controls do
begin
if control is TListBox then
listboxes.Add(control as TListBox); // Loop through all listboxes on a panel and add then to the list if they are a listbox
end;
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
listboxes.Free; // Free the list
end;
end.