c# - Dynamic Controls in PlaceHolder were lost after an action is performed -

i have placeholder in webform master page code

<asp:dropdownlist id="noofsubjectlist" runat="server"           autopostback="true" onselectedindexchanged="noofsubject_index_changed">                     <asp:listitem>1</asp:listitem>                     <asp:listitem>2</asp:listitem>                     <asp:listitem>3</asp:listitem>                     <asp:listitem>4</asp:listitem>                     <asp:listitem>5</asp:listitem>                     <asp:listitem>6</asp:listitem>                     <asp:listitem>7</asp:listitem>                     <asp:listitem>8</asp:listitem>                     <asp:listitem>9</asp:listitem>                     <asp:listitem>10</asp:listitem>                     <asp:listitem>11</asp:listitem>                     <asp:listitem>12</asp:listitem> </asp:dropdownlist> <asp:placeholder id="placeholder" runat="server" />     

based on number(n) selected dropdownlist (1), dynamically creating 2 sets of dropdownlist (2,3) selected n numbers. in first dropdownlist loads data database. code shown below,

protected void noofsubject_index_changed(object sender, eventargs e)     {       int value = convert.toint32(noofsubjectlist.selecteditem.text.tostring());       dataset ds1 = new dataset();       mysql.data.mysqlclient.mysqlconnection mysqlconnection = new            mysql.data.mysqlclient.mysqlconnection           ("database=school;server=localhost;uid=root;pwd=;");       mysql.data.mysqlclient.mysqlcommand mysqlcommand = new            mysql.data.mysqlclient.mysqlcommand           ("select dept tabledept status='active'", mysqlconnection);       mysql.data.mysqlclient.mysqldataadapter mysqladaptor = new             mysql.data.mysqlclient.mysqldataadapter(mysqlcommand);       mysqladaptor.fill(ds1);       (int = 0; <= value - 1; i++)       {          dropdownlist _sublist = new dropdownlist();          _sublist.id = "sublist" + (i + 1);          _sublist.width= 75;          dropdownlist _deptlist = new dropdownlist();          _deptlist.id = "deptlist" + (i + 1);                        _deptlist.width = 75;          _deptlist.selectedindexchanged += deptlist_index_changed;          literal _spacer = new literal();          _spacer.text = "<br />";          literal _spacerlbl = new literal();          _spacerlbl.text = "&nbsp;&nbsp;";          label _lbl = new label();          _lbl.id = "lbl_subject" + i;          _lbl.text = "subject" + (i+1);          label _lbl1 = new label();          _lbl1.id = "lbl_dept" + i;          _lbl1.text = "department";          _deptlist.datasource = ds1;          _deptlist.datatextfield = ds1.tables[0].columns["dept"].columnname;          _deptlist.datavaluefield = ds1.tables[0].columns["dept"].columnname;          _deptlist.databind();          _deptlist.autopostback = true;          placeholder.controls.add(_lbl1);          placeholder.controls.add(_spacerlbl);          placeholder.controls.add(_deptlist);          placeholder.controls.add(_spacerlbl);          placeholder.controls.add(_lbl);          placeholder.controls.add(_sublist);          placeholder.controls.add(_spacer);       }   } protected void deptlist_index_changed(object sender, eventargs e)   {      \\based on selection in dropdownlist2, list loaded           \\database dropdownlist3         }     

the above event triggered based on selection of number n dropdownlist (1). if select item dropdownlist (2) data generated database , added dropdownlist (3) . problem when try select item dropdownlist (2), dynamically created controls lost. how overcome issue?

i googled , found out missing called postback in code. not able find out relevant resource learn

i got link shows easy example of creating controls after postback.

creating dynamic textbox controls using c#