Try the SelectedItem
property.
switch (listBox2.FindString(listBox2.SelectedItem.ToString()))//find the index of the selected string
{
case 0:
richTextBox1.Text = "0";
break;
...
}
Although, it would be more effective then to just call
//Do the same as above, then convert to string
richTextBox1.Text = listBox2.FindString(listBox2.SelectedItem.ToString()).ToString();
As this gets the selected item, finds its index, then convert the index to a string.