asp.net - On Click Twice the Button Shows "ConnectionString property has not been initialized" -

i have following code:

using system; using system.collections.generic; using system.data.sqlclient; using system.data; using system.web; using system.web.ui; using system.web.ui.webcontrols;  public partial class supervissions_pages_jwelorder : system.web.ui.page {     #region variable     classdata clsdt = new classdata();     sqldataadapter da;     dataset ds;     #endregion     #region events     protected void page_load(object sender, eventargs e)     {         if (!ispostback)         {             bindgrid();          }         session["mbrid"] = "lym47336163";     }      protected void grdview_rowcancelingedit(object sender, gridviewcancelediteventargs e)     {         grdview.editindex = -1;         bindgrid();     }     protected void grdview_rowcommand(object sender, gridviewcommandeventargs e)     {         if (e.commandname.equals("add"))         {             dropdownlist item = (dropdownlist)grdview.footerrow.findcontrol("ddlparticular_f");             textbox quantity = (textbox)grdview.footerrow.findcontrol("txtquantity_f");             textbox rate = (textbox)grdview.footerrow.findcontrol("txtrate_f");             textbox serialcode = (textbox)grdview.footerrow.findcontrol("txtserial_f");              addjwelorder(txtmbrid.text, txtdate.text, item.text, quantity.text, rate.text, serialcode.text);                 grdview.editindex = -1;             bindgrid();         }     }     protected void grdview_rowdeleting(object sender, gridviewdeleteeventargs e)     {      }     protected void grdview_rowediting(object sender, gridviewediteventargs e)     {         grdview.editindex = e.neweditindex;         bindgrid();         dropdownlist item = (dropdownlist)grdview.footerrow.findcontrol("ddlparticular_f");         ddparticular(item);     }     protected void grdview_rowupdating(object sender, gridviewupdateeventargs e)     {      }     protected void txtmbrid_textchanged(object sender, eventargs e)     {         using (clsdt.sqlcnn)         {             clsdt.sqlcnn.open();             sqlcommand cmd = new sqlcommand("select mbr_nm mbr_mst mbr_id = @name", clsdt.sqlcnn);             cmd.parameters.add("@name", txtmbrid.text);             datatable dtt = new datatable();             string strname = (string)cmd.executescalar();             if (!string.isnullorempty(strname))             {                 txtmbrnm.text = strname;             }         }     }     protected void txtdate_textchanged(object sender, eventargs e)     {         bindgrid();     }      protected void grdview_rowcreated(object sender, gridviewroweventargs e)     {         if (e.row.rowtype == datacontrolrowtype.footer)         {             var particular = e.row.findcontrol("ddlparticular_f") dropdownlist;             if (null != particular)             {                 ddparticular(particular);             }         }     }     #endregion     #region functions     protected void bindgrid()     {         using (clsdt.sqlcnn)         {             clsdt.sqlcnn.open();             sqlcommand cmd = new sqlcommand("usp_crud_jwelorders", clsdt.sqlcnn);             cmd.parameters.add(new sqlparameter("@operation", sqldbtype.varchar, 20));             cmd.parameters["@operation"].value = "display";             cmd.commandtype = commandtype.storedprocedure;             //grdview.datasource = cmd.executereader();             //grdview.databind();              datatable dt = new datatable();             dt.load(cmd.executereader());             grdview.datasource = dt;             grdview.databind();         }     }     protected void addjwelorder(string strmemberid, string strdate, string strparticular, string strquantity, string strrate, string serialcode)     {         using (clsdt.sqlcnn)         {             clsdt.sqlcnn.open();             sqlcommand cmd = new sqlcommand("usp_crud_jwelorders", clsdt.sqlcnn);             cmd.parameters.add(new sqlparameter("@operation", sqldbtype.varchar, 10));             cmd.parameters.add(new sqlparameter("@memberid", sqldbtype.varchar, 20));             cmd.parameters.add(new sqlparameter("@orderdt", sqldbtype.datetime));             cmd.parameters.add(new sqlparameter("@item", sqldbtype.varchar, 3));             cmd.parameters.add(new sqlparameter("@orderqty", sqldbtype.varchar,20));             cmd.parameters.add(new sqlparameter("@rte", sqldbtype.varchar, 8));             cmd.parameters.add(new sqlparameter("@serealcode", sqldbtype.varchar, 20));              cmd.parameters.add(new sqlparameter("@franchise_code", sqldbtype.varchar, 20));             cmd.parameters.add(new sqlparameter("@orderid", sqldbtype.varchar, 20));              cmd.parameters["@operation"].value = "add";             cmd.parameters["@memberid"].value = strmemberid;             cmd.parameters["@orderdt"].value = strdate;             cmd.parameters["@item"].value = strparticular;             cmd.parameters["@orderqty"].value = strquantity;             cmd.parameters["@rte"].value = strrate;             cmd.parameters["@serealcode"].value = serialcode;             cmd.parameters["@franchise_code"].value = session["mbrid"].tostring();             cmd.parameters["@orderid"].value = clsdt.getdatatable("select right(1000000+(max(jwl_orid) + 1),6) orderid jwl_ord_mst").rows[0][0].tostring();             cmd.commandtype = commandtype.storedprocedure;              da = new sqldataadapter(cmd);             ds = new dataset();             da.fill(ds);         }     }     protected void ddparticular(dropdownlist ddlparticular)     {         datatable dt = new datatable();         ddlparticular.datasource = clsdt.getdatatable("select com_cmcd,com_cmnm com_mst com_cmcd = (select com_cmcd com_typ com_ctnm = 'jewellery')");         ddlparticular.datatextfield = "com_cmnm";         ddlparticular.datavaluefield = "com_cmcd";         ddlparticular.databind();     }      protected void delete(string id)     {         using (clsdt.sqlcnn)         {             clsdt.sqlcnn.open();             sqlcommand cmd = new sqlcommand("usp_crud_jwelorders", clsdt.sqlcnn);             cmd.parameters.add(new sqlparameter("@operation", sqldbtype.varchar, 10));             cmd.parameters.add(new sqlparameter("@si", sqldbtype.int));              cmd.parameters["@operation"].value = "delete";             cmd.parameters["@si"].value = id;             cmd.commandtype = commandtype.storedprocedure;             da = new sqldataadapter(cmd);             ds = new dataset();             da.fill(ds);         }     }     protected void updatemail(string strsi, string strparticular, string strquantity, string strrate, string serialcode)     {         using (clsdt.sqlcnn)         {             clsdt.sqlcnn.open();             sqlcommand cmd = new sqlcommand("usp_crud_jwelorders", clsdt.sqlcnn);             cmd.parameters.add(new sqlparameter("@operation", sqldbtype.varchar, 10));             cmd.parameters.add(new sqlparameter("@item", sqldbtype.varchar, 3));             cmd.parameters.add(new sqlparameter("@orderqty", sqldbtype.int));             cmd.parameters.add(new sqlparameter("@rte", sqldbtype.varchar, 8));             cmd.parameters.add(new sqlparameter("@serealcode", sqldbtype.varchar, 20));             cmd.parameters.add(new sqlparameter("@si", sqldbtype.int));             cmd.parameters["@operation"].value = "update";             cmd.parameters["@item"].value = strparticular;             cmd.parameters["@orderqty"].value = strquantity;             cmd.parameters["@rte"].value = strrate;             cmd.parameters["@serealcode"].value = serialcode;             cmd.parameters["@si"].value = strsi;             cmd.commandtype = commandtype.storedprocedure;             da = new sqldataadapter(cmd);             ds = new dataset();             da.fill(ds);         }     }      #endregion  } 

and design is:

the problem when click add button first time data saved sucessfully... second time when click add button shows me error below.

classdata code is:

imports microsoft.visualbasic imports system.data.sqlclient imports system.data public class classdata     private sqlstring string     private com sqlcommand     private dmlstring string     private sqldr sqldatareader     public sqlcnn new sqlconnection(configurationmanager.connectionstrings("constring").tostring())       public function getrandomval() string         dim str string         dim rnd new random         str = rnd.next(111111, 999999)         return str     end function      public function getdatatable(byval qry string) datatable          if sqlcnn.state <> connectionstate.open sqlcnn.open()         dim sqlcmd new sqlcommand(qry, sqlcnn)          dim dt new datatable         ' dim sqldt new sqldataadapter(qry, sqlcnn)         ' com.connection = sqlcnn         sqlcmd.commandtimeout = 150000          sqldr = sqlcmd.executereader()         ' sqldt.fill(dt)         dt.load(sqldr)         sqldr.close()         if sqlcnn.state = connectionstate.open sqlcnn.close()         return dt     end function      public function executequery(byval strqry string) integer          if sqlcnn.state <> connectionstate.open sqlcnn.open()         dim sqlcmd new sqlcommand(strqry, sqlcnn)         sqlcmd.commandtimeout = 1500         ' com.connection = sqlcnn         'com.commandtimeout = 15000         'sqldr = sqlcmd.executereader()         dim int = sqlcmd.executenonquery()         if sqlcnn.state = connectionstate.open sqlcnn.close()         return int     end function     'public function executequery(byval strqry string) datatable     '    dim dt new datatable     '    if sqlcnn.state <> connectionstate.open sqlcnn.open()     '    dim sqlcmd new sqlcommand(strqry, sqlcnn)     '    dt = sqlcmd.executereder()     '    if sqlcnn.state = connectionstate.open sqlcnn.close()     '    return     'end function end class