using System;
using System.Collections.Generic;
using System.Text;

namespace ring
{
    class Program
    {
        private static char[] constant =  {
        '0','1','2','3','4','5','6','7','8','9',
        'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z',
        'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'
        };
        public static string GetRandomStringByLength(int Length)
        {
            StringBuilder newRandom = new StringBuilder();
            Random rd = new Random();
            for (int i = 0; i < Length; i++)
            {
                newRandom.Append(constant[rd.Next(62)]);
            }
            return newRandom.ToString();
        }
        static void Main(string[] args)
        {
            //Get 10 random caracters string
            Console.WriteLine(GetRandomStringByLength(10));
            Console.ReadLine();
        }
    }
}
http://www.cnblogs.com/Ring1981/archive/2006/11/30/577400.html