protected void btnConfirm_Click(object sender, EventArgs e)
        {
            Int64        readerId  = Int64.Parse(Session["ReaderId"].ToString());
            DateTime     dateAdded = DateTime.Now;
            DateTime     dateEnd   = dateAdded.AddDays(10);
            List <Int32> bookList  = new List <Int32>();

            if (Session["bookId1"] != null)
            {
                if (Session["bookId2"] != null)
                {
                    if (Session["bookId3"] != null)
                    {
                        bookList.Add(Int32.Parse(Session["bookId1"].ToString()));
                        bookList.Add(Int32.Parse(Session["bookId2"].ToString()));
                        bookList.Add(Int32.Parse(Session["bookId3"].ToString()));
                    }
                    else
                    {
                        bookList.Add(Int32.Parse(Session["bookId1"].ToString()));
                        bookList.Add(Int32.Parse(Session["bookId2"].ToString()));
                    }
                }
                else
                {
                    bookList.Add(Int32.Parse(Session["bookId1"].ToString()));
                }
            }
            else
            {
                Response.Redirect("Book.aspx");
            }
            CertificateBLL certificateBLL = new CertificateBLL(1, readerId, dateAdded, dateEnd);
            BorrowBookBLL  borrowBookBLL  = new BorrowBookBLL();

            try
            {
                borrowBookBLL.Add(certificateBLL, bookList);
                string script = "alert(\"Success!\");";
                ScriptManager.RegisterStartupScript(this, GetType(),
                                                    "ServerControlScript", script, true);
                this.btnConfirm.Visible = false;
                this.btnCancel.Visible  = false;
                Session.Abandon();
                //Response.Redirect("Reader.aspx");
            }
            catch (Exception ex)
            {
                string script = "alert(\"Error while add certificate: " + ex.Message + "\");";
                ScriptManager.RegisterStartupScript(this, GetType(),
                                                    "ServerControlScript", script, true);
            }
        }