protected void gridView_Sorting(object sender, GridViewSortEventArgs e)

{

    DataTable dataTable = grid.DataSource as DataTable;

    if (dataTable != null)

    {

        DataView dataView = new DataView(dataTable);

        dataView.Sort = GetSortExpression(e.SortExpression);

        grid.DataSource = dataView;

        grid.DataBind();

    }

}

private string GetSortExpression(string sortExpression) 
{

   if (sortExpression == GridViewSortExpression && GridViewSortDirection == "ASC")

   {

       GridViewSortDirection = "DESC";

   }

   else 

   {

       GridViewSortDirection = "ASC";

   }

   GridViewSortExpression = sortExpression;

   return sortExpression + " " + GridViewSortDirection;