TStrings vs TStringList in TCheckListBox

Question

In Delphi 7, I'm using a TCheckListBox. I want it to use a TStringList rather than a TStrings, so I can set Duplicates to dupIgnore, and Sorted to TRUE.

Can I just do this:

Form1 = class(TObject
    CheckListBox1: TCheckListBox;  // created by the IDE
end;

procedure TForm1.FormCreate
begin
   CheckListBox1.Items.Free;
   CheckListBox1.Items := TStringList.Create;
   CheckListBox1.Items.Sorted := TRUE;
   CheckListBox1.Items.Duplicates := dupIgnore;
end;

Is this safe? Any caveats or suggestions?

EDIT: Removed declaration for MyStringList and added .Items to the last two assignment lines.

EDIT 2: Trying to compile the above, it looks like I'd have to cast the two final lines like this:

        TStringList(CheckListBox1.Items).Sorted := TRUE;
        TStringList(CheckListBox1.Items).Duplicates := dupIgnore;

Although I might be able to get this to run, I'm asking the question because just getting it to run doesn't mean it will always run or is safe.

I dont think it's a good idea. The `TStrings` created by `TCheckListBox` are actually of type `"TListBoxStrings"` which overrides Put, Get etc methods. the `Sorted` property works fine without the use of TStringList. — kobik, Dec 11 '11 at 17:14
@kobik, what is `TListBoxStrings` ? As I know, [`TCheckListBox.Items`](http://docwiki.embarcadero.com/VCL/en/CheckLst.TCheckListBox.Items) is declared as [`TStrings`](http://docwiki.embarcadero.com/VCL/en/StdCtrls.TCustomListBox.Items) — TLama, Dec 11 '11 at 17:17
yes, but they are created as `FItems := TListBoxStrings.Create;` in `constructor TCustomListBox.Create(AOwner: TComponent);` — kobik, Dec 11 '11 at 17:18

Ondrej Kelle · Accepted Answer · 2011-12-11T17:34:01.117

9

You don't control what class TCheckListBox uses to store its items. Assigning the Items property a value only assigns its items to the internal storage.

Also, you shouldn't call Items.Free;. TCheckListBox depends on its internal instance of TListBoxStrings.

To answer your edits in your question: Don't hard-cast the Items property to TStringList, either. The typecast is wrong (the instance exposed by Items is not a TStringList) and will only cause problems.

Edit, to suggest a workaround for what you seem to try to achieve: To keep the checklistbox sorted, you can set its Sorted property to True. To avoid duplicates, you can check the list before adding an item in code.

edited Dec 11 '11 at 17:34

answered Dec 11 '11 at 17:22

Ondrej Kelle

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1

You can use an external `TStringList` for storage. In this case, you'll have to create it as virtual, and retrieve the data on need. Note that there is an issue with huge number of items - see http://stackoverflow.com/questions/7144966/how-to-get-rid-of-tlistbox-vertical-scroll-limit – Arnaud Bouchez Dec 11 '11 at 18:35
I've taken your advice and pre-sorted my data in a TStringList before adding to the TCheckListbox. But, could you clarify why my approach didn't work. I thought that the whole point of OO was that, here: TStringList was a descendant of TStrings, so I'd be able to substitute a TStringList – RobertFrank Dec 11 '11 at 22:59
1

If B inherits from A then B is A but A may not be B. (E.g. a dog is an animal but an animal may not be a dog.) In your case, `Items` is declared as `TStrings` so you can safely assume that it's actually a `TStrings` descendant, not more. Assuming `TStringList` is assuming too much, and in this case it's wrong (`Items` does not actually point to a `TStringList` instance). – Ondrej Kelle Dec 11 '11 at 23:07
@Robert - From an OO point of view you can substitute any `TStrings` descendant for `Items`. From a practical point of view you can't, because `FItems` is private to the class (unless you resort to horrible hacks). In any case, `TCheckListBox` depends on the functionality provided by its items class: `TListBoxStrings` (another `TStrings` descendant). For instance, TListBoxStrings sends a `LB_ADDSTRING` to the underlying api control after adding an item. TStringList does not do that, it has no notion of an owning ListBox. For this reason a StringList won't serve to your purpose. – Sertac Akyuz Dec 12 '11 at 00:10

TStrings vs TStringList in TCheckListBox

1 Answers1