点击gridview排序的问题

gridview中有一列是文档大小的数据，点击表头的时候排序根本没有起作用，排序部分代码 public SortDirection GridViewSortDirection
        {
            get 
            {
                if (ViewState["sortDirection"] == null)
                    ViewState["sortDirection"] = SortDirection.Ascending;
                return (SortDirection)
                    ViewState["sortDirection"];
            }
            set 
            {
                ViewState["sortDirection"] = value;
            }
        }
        private void SortGridView(string sortExpression, string direction)
        {
            DataTable dt = GetGridInTable();
            DataView dv = new DataView(dt);
            dv.Sort = sortExpression + direction;
            GridView1.DataSource = dv;
            GridView1.DataBind();
        }       
        protected void GridView1_Sorting(object sender, GridViewSortEventArgs e)
        {
            string sortExpression = e.SortExpression;
            if (GridViewSortDirection == SortDirection.Ascending) //设置排序方向
            {
                GridViewSortDirection = SortDirection.Descending;
                SortGridView(sortExpression, " DESC");
            }
            else
            {
                GridViewSortDirection = SortDirection.Ascending;
                SortGridView(sortExpression, " ASC");
            }
        }
gridview中绑定数据的部分代码         string sortExpression = this.GridView1.Attributes["SortExpression"];
         string sortDirection = this.GridView1.Attributes["SortExpression"]; 
     if ((!string.IsNullOrEmpty(sortExpression)) && (!string.IsNullOrEmpty(sortDirection)))
            {
                dtTaa01.DefaultView.Sort = string.Format("{0} {1}", sortExpression, sortDirection);
            }
希望高手解惑