GridView手动双向排序并加上排序图标

前台代码并没有什么就是一个GridView控件
主要是后台代码：

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;
using System.Data.SqlClient;

public partial class Sort : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            GridView1.DataSource = this.GetDataset();
            GridView1.DataKeyNames = new String[] { "id" };
            GridView1.DataBind();
            ViewState["OrderDire"] = " Desc";
        }
    }
    private string gvSortExpression
    {
        get
        {
            if (ViewState["SortOrder"] != null && ViewState["SortOrder"].ToString() != string.Empty)
            {
                return ViewState["SortOrder"].ToString();
            }
            else
            {
                return "";
            }
        }
        set
        {
            ViewState["SortOrder"] = value;
        }
    }
    private DataSet  GetDataset()
    {
        SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["Personal"].ConnectionString);
        conn.Open();
        SqlDataAdapter adp = new SqlDataAdapter("select * from information", conn);
        DataSet dataset = new DataSet();
        adp.Fill(dataset, "information");
        conn.Close();
        return dataset;
    }
    protected void GridView1_Sorting(object sender, GridViewSortEventArgs e)
    {
        gvSortExpression = e.SortExpression;
            if (ViewState["OrderDire"].ToString() == " Desc")
            {
                ViewState["OrderDire"] = " ASC";
            }
            else
            {
                ViewState["OrderDire"] = " Desc";
            }  
        DataView dataview = this.GetDataset().Tables[0].DefaultView;
        string sort = gvSortExpression + " " + ViewState["OrderDire"].ToString();
        dataview.Sort = sort;
        GridView1.DataSource = dataview;
        GridView1.DataKeyNames = new String[] { "id" };
        GridView1.DataBind();
    }
    protected void ShowSortedIcon(GridView gvData, GridViewRow gvRow)
    {
            for (int index = 0; index <= gvData.Columns.Count - 1; index++)
            {
                if ((gvData.Columns[index].SortExpression == gvSortExpression) && (gvData.Columns[index].SortExpression !=""))
                {
                    Image img = new Image();
                    if ((string)ViewState["OrderDire"] == " ASC")
                    {
                        img.ImageUrl = "~/Images/SortUp.gif";
                        img.ToolTip = "递增排列";
                    }
                    else
                    {
                        img.ImageUrl = "~/Images/SortDown.gif";
                        img.ToolTip = "递减排列";
                    }
                    gvRow.Cells[index].Controls.Add(img);
                }
            }
    }
    protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.Header)
        {
            ShowSortedIcon(GridView1, e.Row);
        }
    }
}