Gridview sort

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!this.IsPostBack)
        {
           GridView1.DataSource= getDate();
           GridView1.DataBind();
        }
    }

    public DataTable getDate()
    {
        DataTable dt;
         string conString="server=server01;database=test;uid=sa;pwd=sql";
         using (SqlConnection con = new SqlConnection(conString))
         { 
             con.Open();
             using(SqlCommand cmd = new SqlCommand("select * from tab",con))
             {
                using(SqlDataReader reader=cmd.ExecuteReader())
                {
                    dt = new DataTable();
                    //填充数据
                    dt.Load(reader, LoadOption.OverwriteChanges);
                }
             }
         }
         return dt;
         
    }
    protected void GridView1_Sorting(object sender, GridViewSortEventArgs e)
    {
        if ((string)this.ViewState["Sort"]!= e.SortExpression)
        {
            this.ViewState["Sort"] = e.SortExpression;
        }
        else
        {
            this.ViewState["Sort"] = e.SortExpression + " " + "DESC";
        }

        DataTable dt = new DataTable();
        dt = getDate();
        dt.DefaultView.Sort = ViewState["Sort"].ToString();
        GridView1.DataSource = dt.DefaultView;
        GridView1.DataBind();
         
    }
}