using System.IO;

using System;

public class Program

{

    public static void Main()

    {

        string fileName = @"C:\mydir.old\myfile.ext|||sheet1";

        string extension;

        char[] illigal;

​

        extension = Path.GetExtension(fileName);

        Console.WriteLine("GetExtension('{0}') returns '{1}'",

            fileName, extension);

​

    }

}