sda.SelectCommand.Connection = con 提示 未将对象引用设置到对象的实例,该如何处理

            SqlConnection con = Ares.WebSite.App_Code.DB.CreateConn();            con.Open();            SqlDataAdapter sda = new SqlDataAdapter();            sda.SelectCommand.Connection = con;            sda.SelectCommand.CommandText = "select * from M_SalesTab";            DataSet ds = new DataSet();            sda.Fill(ds);            Response.Write(ds.Tables[0].Rows[0][1].ToString());