產生64位隨机無重復碼

public static string GetKeyX(string source)
        {
            StringBuilder retstr=new StringBuilder("");
            Random rand;
            char dd;
            int j;
            int len=source.Length;
            for(int i=0;i<len;i++)
            {
                rand=new Random((int)((i+1)*DateTime.Now.Ticks)%Int32.MaxValue);
                j=rand.Next(retstr.Length+1)+1;
                dd=source[i];
                if(retstr.Length<=0) 
                    retstr.Append(dd);
                else if(j>retstr.Length)
                    retstr.Append(dd);
                else
                    retstr.Insert(j-1,dd);
                
            }
            return retstr.ToString();
        }
        public static string GetKeyX()
        {
            return GetKeyX("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ+=");
        }