After that code I clear the dropdown
Session["StyleNo"] = ddlStyleNo.SelectedValue.ToString().Trim();
Session["DefectNo"] = ddlDefectType.SelectedValue.ToString().Trim();
ScriptManager.RegisterStartupScript(this, typeof(string), "OPEN_WINDOW", "var Mleft = (screen.width/2)-(760/2);var Mtop = (screen.height/2)-(700/2);window.open( 'Reports/Smt_StyleWiseReportWindow.aspx?', null, 'height=700,width=760,status=yes,toolbar=no,scrollbars=yes,menubar=no,location=no,top=\'+Mtop+\', left=\'+Mleft+\'' );", true);
ddlStyleNo.Items.Clear();
ddlDefectType.Items.Clear();