“List index out of bounds” on TListBox

Question

I have a TListBox on a form, and items are added with

listbox1.ItemIndex := listbox1.Items.AddObject('msg', TObject(grp));

grp is an integer. The listbox is set to lbOwnerDrawFixed.

In the onDrawItem event I get the exception EStringListError raised on the marked line:

msg := (control as Tlistbox).Items.Strings[index];           // this line works
grp := integer((control as Tlistbox).Items.Objects[index]);  // exception here

msg and grp are local string and integer variables.

Project ### raised exception class EStringListError with message 'List index out of bounds (1)'

Answer 1

You just want to store an integer, so you should change your code to

listbox1.ItemIndex := listbox1.Items.Add(IntToStr(grp));
[...]
grp := StrToInt((control as TListBox).Items[index]);

No need for storing objects here and this makes the whole thing much easier and more readable.

The exception you get now is because you can't retrieve objects using the index, but have to use the string you associated them with (the first parameter of AddObject). The correct way would be something like this:

msg := (control as Tlistbox).Items.Strings[index];
grp := integer((control as Tlistbox).Items.Objects[(control as Tlistbox).Items.IndexOf(msg)]);

Also see this tutorial about AddObject.

Answer 2

Silly mistake: I was using grp := -1 as the default group, which AddObject or Objects[index] must not like.