<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" BackColor="White" BorderColor="#CC9966" BorderStyle="Solid" BorderWidth="1px" CellPadding="4"  AllowSorting="True"  OnSorting="GridView1_Sorting" >
        <Columns>
            <asp:TemplateField HeaderText="Emp_ID" SortExpression="Emp_ID">
                <ItemTemplate>
                    <%#Eval("Emp_ID") %>
                </ItemTemplate>
                <ControlStyle Width="100px"></ControlStyle>
                <HeaderStyle Width="100px" />
                <ItemStyle HorizontalAlign="Center" />
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Emp_Name" SortExpression="Emp_Name">
                <ItemTemplate>
                    <%#Eval("Emp_Name") %>
                </ItemTemplate>
                <ControlStyle Width="300px"></ControlStyle>
                <HeaderStyle Width="200px" />
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Emp_Sal"  SortExpression="Emp_Sal">
                <ItemTemplate>
                    <%#Eval("Emp_Sal") %>
                </ItemTemplate>
                <ControlStyle Width="200px"></ControlStyle>
                <HeaderStyle Width="200px" />
            </asp:TemplateField>
        </Columns>
        <FooterStyle BackColor="#FFFFCC" ForeColor="#330099" />
        <HeaderStyle BackColor="#990000" Font-Bold="True" ForeColor="#FFFFCC" />
        <PagerStyle BackColor="#FFFFCC" ForeColor="#330099" HorizontalAlign="Center" />
        <RowStyle BackColor="White" ForeColor="#330099" />
        <SelectedRowStyle BackColor="#FFCC66" Font-Bold="True" ForeColor="#663399" />
        <SortedAscendingCellStyle BackColor="#FEFCEB" />
        <SortedAscendingHeaderStyle BackColor="#AF0101" />
        <SortedDescendingCellStyle BackColor="#F6F0C0" />
        <SortedDescendingHeaderStyle BackColor="#7E0000" />
    </asp:GridView>

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace Forumasp
{
    public partial class GridviewSorting : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
	{

           GridView1.DataSource = GetRecord(); //set DataSource GridView
            GridView1.DataBind(); //binding GridView
       }

        protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            GridView1.PageIndex = e.NewPageIndex;
            GridView1.DataSource = GetRecord();
                GridView1.DataBind();       
        }

        public DataTable GetRecord()
        {
            DataTable dt = new DataTable();
            string constr = ConfigurationManager.ConnectionStrings["forumConnectionString"].ToString(); // connection string
            SqlConnection con = new SqlConnection(constr);
            con.Open();
            SqlCommand com = new SqlCommand("select *from Employee", con); // table name 
            SqlDataAdapter da = new SqlDataAdapter(com);
            DataSet ds = new DataSet();
            da.Fill(ds);   
          dt=  ds.Tables[0];
          return dt;
 }

        public SortDirection direction
        {
            get
            {
                if (ViewState["directionState"] == null)
                {
                    ViewState["directionState"] = SortDirection.Ascending;
                }
                return (SortDirection)ViewState["directionState"];
            }
            set
            {
                ViewState["directionState"] = value;
            }
        }

        protected void GridView1_Sorting(object sender, GridViewSortEventArgs e)
        {
            string sortingDirection = string.Empty;
            if (direction == SortDirection.Ascending)
            {
                direction = SortDirection.Descending;
                sortingDirection = "Desc";

            }
            else
            {
                direction = SortDirection.Ascending;
                sortingDirection = "Asc";

            }
            DataView sortedView = new DataView(GetRecord());
            sortedView.Sort = e.SortExpression + " " + sortingDirection;
            Session["SortedView"] = sortedView;
            GridView1.DataSource = sortedView;
            GridView1.DataBind();  
        }  
 
    }
}