GET random Unique OTP(One Time Password) in C#.NET

Use this Code for random Unique OTP(One Time Password) generation:

private string generatePassword()
    {
        int lenthofpass = 6;
        string allowedChars = "";
        allowedChars = "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,";
        allowedChars += "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,";
        allowedChars += "1,2,3,4,5,6,7,8,9,0,!,@,#,$,%,&,?";
        char[] sep = { ',' };
        string[] arr = allowedChars.Split(sep);
        string passwordString = "";
        string temp = "";
        Random rand = new Random();
        for (int i = 0; i < lenthofpass; i++)
        {
            temp = arr[rand.Next(0, arr.Length)];
            passwordString += temp;
        }
        return passwordString;
    }

Send Mail with Attachment in asp.net

 MailAddress SendFrom = new MailAddress("brijesh.singh879@gmail.com", "Brijesh");
        MailAddress SendTo = new MailAddress("brijesh.singh879@gmail.com");
        System.Net.Mail.MailMessage MyMessage = new System.Net.Mail.MailMessage(SendFrom, SendTo);
        MyMessage.IsBodyHtml = true;
        MyMessage.Subject = "Send Mail with attachment ";
        MyMessage.Body = "Body";
        if (fupResume.HasFile)
        {
            string FileName = Path.GetFileName(fupResume.PostedFile.FileName);
            MyMessage.Attachments.Add(new Attachment(fupResume.PostedFile.InputStream, FileName));
        }
        System.Net.NetworkCredential authentication = new System.Net.NetworkCredential("brijesh.singh879@gmail.com", "password");
        SmtpClient client = new SmtpClient("smtp.gmail.com", 25);//587
        client.EnableSsl = true;
        client.UseDefaultCredentials = true;
        client.Credentials = authentication;
        client.Send(MyMessage);