GridView Paging and Sorting in ASP.Net

.aspx code:

<div>

    <asp:GridView ID="gvLocation" runat="server" AutoGenerateColumns="false"

            AllowPaging="true" AllowSorting="true" onsorting="gvLocation_Sorting"

            onpageindexchanging="gvLocation_PageIndexChanging"  >

    <Columns>

    <asp:BoundField HeaderText="Region Name" DataField="RegionName" SortExpression="RegionName" />

    <asp:BoundField HeaderText="Area Name" DataField="AreaName" SortExpression="AreaName" />

    </Columns>

    </asp:GridView>

    </div>

Here is .aspx.cs code

using System;

using System.Collections;

using System.Configuration;

using System.Data;

using System.Linq;

using System.Web;

using System.Web.Security;

using System.Web.UI;

using System.Web.UI.HtmlControls;

using System.Web.UI.WebControls;

using System.Web.UI.WebControls.WebParts;

using System.Xml.Linq;

public partial class AutoComplete : System.Web.UI.Page

{

    protected void Page_Load(object sender, EventArgs e)

    {

        if (!IsPostBack)

        {

            BindGridView();

        }

    }

    protected void BindGridView()

    {

        DataTable dtLocation = null;

        if (Session["vwLocation"] == null)

        {

            clsDAL objDal = new clsDAL();

            string query = "SELECT  RegionName, AreaName FROM TableA A INNER JOIN TableB R ON A.RegionID = R.RegionID";

            dtLocation = objDal.GetDataTable(query);

            if (dtLocation.Rows.Count > 0)

            {

                Session["vwLocation"] = dtLocation.DefaultView;

                gvLocation.DataSource = dtLocation;

                gvLocation.DataBind();

            }

        }

        else

        {

            DataView dvLocation = (DataView)Session["vwLocation"];

            gvLocation.DataSource = dvLocation;

            gvLocation.DataBind();

        }

       

    }

    protected void gvLocation_Sorting(object sender, GridViewSortEventArgs e)

    {

        string sortExpression = string.Empty;

        if (ViewState["sortExpression"] == null)

        {

            ViewState["sortExpression"] = "ASC";

            sortExpression = e.SortExpression+" ASC";

        }

        else if (ViewState["sortExpression"].Equals("ASC"))

        {

            ViewState["sortExpression"] = "DESC";

            sortExpression = e.SortExpression + " DESC";

        }

        else

        {

            ViewState["sortExpression"] = "ASC";

            sortExpression = e.SortExpression + " ASC";

        }

        DataView dvLocation = (DataView)Session["vwLocation"];

        dvLocation.Sort = sortExpression;

        BindGridView();

    }

    protected void gvLocation_PageIndexChanging(object sender, GridViewPageEventArgs e)

    {

        gvLocation.PageIndex = e.NewPageIndex;

        BindGridView();

    }