strings have a handy property called Length. if the ones digit is at the end of the string, just call a substring for the last character:
string s = "12341"
string ones = s.Substring(s.Length-1)
(don't need the second argument of substring because we are already at the end of the string)
Your other option, of course, would be to parse it to an integer, then perform % 10, then convert the result back to a string.