What is this namespace ITypeOfObjectsBoundToListBox ? Couldn't find it

Question

I took the answer to my problem from here: What i want to do is to set different Tooltip text for each item in a listbox.

ITypeOfObjectsBoundToListBox

This is the method from the other answer in the link i'm trying to use:

private ITypeOfObjectsBoundToListBox DetermineHoveredItem()
        {
            Point screenPosition = ListBox.MousePosition;
            Point listBoxClientAreaPosition = listBox1.PointToClient(screenPosition);

            int hoveredIndex = listBox1.IndexFromPoint(listBoxClientAreaPosition);
            if (hoveredIndex != -1)
                return listBox1.Items[hoveredIndex] as ITypeOfObjectsBoundToListBox;
            else
                return null;
        }

But ITypeOfObjectsBoundToListBox is not exist.

Error: Error 2 The type or namespace name 'ITypeOfObjectsBoundToListBox' could not be found (are you missing a using directive or an assembly reference?)