From Xojo Documentation
The number of the selected row.
If no item is selected, SelectedRowIndex returns ListBox.NoSelection. If the ListBox is in multiple-row selection mode, then the number of the lowest selected row is returned. For example, if rows 1, 3, and 4 are selected, SelectedRowIndex returns 1.
You can put the ListBox into multiple-row selection mode by changing the RowSelectionType property.
The following code in the Change event handler displays the text in the first column of the selected row:
The following code displays a message if no row is selected: