sorting gridview column when column header clicks

<asp:boundfield datafield="ProductCode" headertext="Product Code" xmlns:asp="#unknown">SortExpression="ProductCode" /></asp:boundfield>

//sorting Populate and load GridView on Page_Load Event
       private DataTable BindGridView()
       {

           DataTable dtGrid = new DataTable();
           string strSelect = "YOUR SELECT QUERY";
           OleDbCommand cmd = new OleDbCommand(strSelect, con);
           OleDbDataAdapter dAdapter = new OleDbDataAdapter(cmd);
           dAdapter.Fill(dtGrid);
           return dtGrid;

       }

       //Create Public Property of SortDirection type and store direction in ViewState.
       public SortDirection dir
       {

           get
           {
               if (ViewState["dirState"] == null)
               {

                   ViewState["dirState"] = SortDirection.Ascending;
               }

               return (SortDirection)ViewState["dirState"];

           }
           set
           {
               ViewState["dirState"] = value;
           }

       }
       //Check Gridview's current direction from ViewState and set new sort direction in Sorting Event.

       protected void gvDetails_Sorting(object sender, GridViewSortEventArgs e)
       {

           string sortingDirection = string.Empty;
           if (dir == SortDirection.Ascending)
           {
               dir = SortDirection.Descending;
               sortingDirection = "Desc";
           }

           else
           {
               dir = SortDirection.Ascending;
               sortingDirection = "Asc";
           }
           DataView sortedView = new DataView(BindGridView());
           sortedView.Sort = e.SortExpression + " " + sortingDirection;
           Productlist.DataSource = sortedView;
           Productlist.DataBind();
       }