A multicolumn listbox is probabbly not what you need but it is possible to use.
You probably need a 2 d string array infact.
To use a listbox and acces the content of the cells instead of the selected elements you need the property node "item names".
The names are the content of the string in the listbox.
Further you don't need a type conversion here but a double for loop to get all the strings.
If you need just one selectde value the other option is to index the item names with the indexes from the listbox
You have a single selection listbox so only the selected cell will be available or no value when the index =-1.
For some reason, I did not investigate, you only can select items in the left row, I leave that to you.
greetings from the Netherlands