Incorrect syntax near 'Ascending'.

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage2.master" AutoEventWireup="true" CodeFile="search.aspx.cs" Inherits="search" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    <form id="form1" runat="server">
    <asp:Label ID="Label1" runat="server"
        style="z-index: 1; left: 397px; top: 355px; position: absolute; font-family: Georgia; font-weight: 700; font-size: medium; right: 421px">Keyword:</asp:Label>
    <asp:TextBox ID="txtkey" runat="server"
        style="z-index: 1; left: 489px; top: 352px; position: absolute; width: 177px"></asp:TextBox>
    <asp:Label ID="Label2" runat="server"
        style="font-family: Georgia; font-size: medium; font-weight: 700; z-index: 1; left: 400px; top: 418px; position: absolute; right: 421px"
        Text="OrderBy:"></asp:Label>
    <asp:DropDownList ID="ddlorderby" runat="server"
        style="z-index: 1; left: 489px; top: 417px; position: absolute">
        <asp:ListItem>Name</asp:ListItem>
        <asp:ListItem>Author</asp:ListItem>
        <asp:ListItem>Category</asp:ListItem>
    </asp:DropDownList>
    <asp:DropDownList ID="ddlorder" runat="server"
        style="z-index: 1; left: 570px; top: 417px; position: absolute">
        <asp:ListItem>ascending</asp:ListItem>
        <asp:ListItem>descending</asp:ListItem>
    </asp:DropDownList>
    <asp:Button ID="btnsearch" runat="server" onclick="btnsearch_Click"
        style="z-index: 1; left: 680px; top: 411px; position: absolute; right: 183px; height: 30px"
        Text="Search" />
    <asp:GridView ID="gvbooks" runat="server" BackColor="White"
        BorderColor="#336666" BorderStyle="Double" BorderWidth="3px" CellPadding="4"
        GridLines="Horizontal"
        style="z-index: 1; left: 404px; top: 486px; position: absolute; height: 184px; width: 381px">
        <FooterStyle BackColor="White" ForeColor="#333333" />
        <HeaderStyle BackColor="#336666" Font-Bold="True" ForeColor="White" />
        <PagerStyle BackColor="#336666" ForeColor="White" HorizontalAlign="Center" />
        <RowStyle BackColor="White" ForeColor="#333333" />
        <SelectedRowStyle BackColor="#339966" Font-Bold="True" ForeColor="White" />
        <SortedAscendingCellStyle BackColor="#F7F7F7" />
        <SortedAscendingHeaderStyle BackColor="#487575" />
        <SortedDescendingCellStyle BackColor="#E5E5E5" />
        <SortedDescendingHeaderStyle BackColor="#275353" />
    </asp:GridView>
    </form>
</asp:Content>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.Configuration;
using System.Data.SqlClient;
using System.Data;

public partial class search : System.Web.UI.Page
{
    SqlConnection sqlconpubs;
    SqlDataAdapter sqldasearch;
    DataSet dsetsearch;
    DataTable dtablesearch;
    string tablename;
    string searchfield;
    protected void Page_Load(object sender, EventArgs e)
    {
        try
        {
            sqlconpubs = new SqlConnection(WebConfigurationManager.ConnectionStrings["dbcon"].ConnectionString);
            sqlconpubs.Open();
            sqldasearch = new SqlDataAdapter();
            dsetsearch = new DataSet();
            tablename = "books";
            searchfield = ddlorderby.Text;
            Additems(tablename);
            gvbooks.DataSource = null;
            gvbooks.DataBind();
        }
        catch (Exception ex)
        {
            Response.Write("Error:" + ex.Message);
        }
    }
    protected void Additems(string table)
    {
        sqldasearch = new SqlDataAdapter("select * from " + table, sqlconpubs);
        sqldasearch.Fill(dsetsearch, table);
        ddlorderby.Items.Clear();
        dtablesearch = dsetsearch.Tables[table];
        ddlorderby.DataSource = dtablesearch.Columns;
        ddlorderby.DataBind();
    }
    protected void btnsearch_Click(object sender, EventArgs e)
    {
        try
        {
            string selectcommand;
            if (txtkey.Text == "")
                selectcommand = "select * from " + tablename + " order by " + searchfield + " " + ddlorder.SelectedItem.Value;
            else
                selectcommand = "select * from " + tablename + " where " + searchfield + " like '" + txtkey.Text + "%' order by " + searchfield + " " + ddlorder.SelectedItem.Value;
            sqldasearch = new SqlDataAdapter(selectcommand, sqlconpubs);
            dsetsearch.Clear();
            sqldasearch.Fill(dsetsearch, tablename);
            dtablesearch = dsetsearch.Tables[tablename];
            gvbooks.DataSource = dtablesearch;
            gvbooks.DataBind();
            ddlorderby.Text = searchfield;
        }
        catch (Exception ex)
        {
            Response.Write("error:" + ex.Message);
        }


    }
}