SharePoint GridView 排序 表头有图标

代码如下：

  #region GridView1 Setting
        this.GridView1.PageIndexChanging += new GridViewPageEventHandler(GridView1_PageIndexChanging);
        this.GridView1.Sorting += new GridViewSortEventHandler(GridView1_Sorting);
        this.GridView1.RowCreated += new GridViewRowEventHandler(GridView1_RowCreated);

        this.GridView1.PagerTemplate = null;
        this.GridView1.AutoGenerateColumns = false;
        this.GridView1.EnableViewState = false;
        //排序
        this.GridView1.AllowSorting = true;
        //分页
        this.GridView1.AllowPaging = true;
        this.GridView1.PageSize = PageSize;

        #endregion

 #region GridView1 Method
    void GridView1_RowCreated(object sender, GridViewRowEventArgs e)
    {
        GridView senderGridView = (GridView)sender;

        if (e.Row != null && e.Row.RowType == DataControlRowType.Header)
        {
            foreach (TableCell cell in e.Row.Cells)
            {
                if (cell.HasControls())
                {
                    LinkButton button = cell.Controls[0] as LinkButton;
                    if (button != null)
                    {
                        Image image = new Image();
                        image.ImageUrl = "/_layouts/images/blank.gif";
                        if (ConvertNULL(ViewState["SortExpression11"]) == button.CommandArgument)
                        {
                            if (ConvertNULL(ViewState["SortDirection11"]) == SortDirection.Ascending.ToString())
                                image.ImageUrl = "/_layouts/images/sortup.gif";
                            else
                                image.ImageUrl = "/_layouts/images/sortdown.gif";
                        }
                        cell.Controls.Add(image);
                    }
                }
            }
        }
    }

    void GridView1_Sorting(object sender, GridViewSortEventArgs e)
    {
        ViewState["SortExpression11"] = e.SortExpression;
        switch (ConvertNULL(ViewState["SortDirection11"]))
        {
            case "Ascending":
                ViewState["SortDirection11"] = SortDirection.Descending.ToString();
                break;
            case "Descending":
                ViewState["SortDirection11"] = SortDirection.Ascending.ToString();
                break;
            default:
                ViewState["SortDirection11"] = e.SortDirection;
                break;
        }

        dt11 = ViewState["DT11"] as DataTable;
        if (dt11 != null)
        {
            if (ConvertNULL(ViewState["SortDirection11"]) == SortDirection.Ascending.ToString())
            {
                dt11.DefaultView.Sort = e.SortExpression + " asc";
            }
            else
            {
                dt11.DefaultView.Sort = e.SortExpression + " desc";
            }
            ViewState["DT11"] = dt11.DefaultView.ToTable();

            this.GridView1.DataSource = dt11.DefaultView;
            this.GridView1.DataBind();
        }
    }


    void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        this.GridView1.PageIndex = e.NewPageIndex;
        this.GridView1.DataSource = ViewState["DT11"] as DataTable;
        this.GridView1.DataBind();
    }

    #endregion