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.

Answer 1

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.