using static System.Console;

​

public class Program {

    public static void Main() {

        WriteLine(FormataRG("12345678x"));

        WriteLine(FormataRG("123456789"));

    }

    public static string FormataRG(string texto) => texto.Substring(0, 2) + "." + texto.Substring(2, 3) + "." + texto.Substring(5, 3) + "-" + texto.Substring(8, 1).ToUpper();

}

​

//https://pt.stackoverflow.com/q/55017/101