c# - Input string was not in correct format? -

when run c# code, error: input string not in correct format. , highlights code:

       thetwavalue=convert.toint32(thetwalabel.text); 

why happening?

additional info:

the entire c# function:

    <script  runat="server">                 protected void yourlistview_load(object sender, eventargs e) {     label thetwalabel;  int thetwavalue; foreach (listviewitem item in yourlistview.items) {         thetwalabel = (label)item.findcontrol("twalabel");     thetwavalue = convert.toint32(thetwalabel.text);     if (thetwavalue >= 0)     {         if (thetwavalue < 90)             thetwalabel.forecolor = system.drawing.color.yellow;         else             thetwalabel.forecolor = system.drawing.color.red;     }  }     }                  </script> 

here rest of code:

<asp:sqldatasource id="sqldatasource1" runat="server" connectionstring="<%$ connectionstrings:2007 soundassist ver 1.0.5  05-12-2011 ( 2013-06-24)connectionstring %>" providername="<%$ connectionstrings:2007  soundassist ver 1.0.5  05-12-2011 ( 2013-06-24)connectionstring.providername %>" selectcommand="select [plant] [plant]">   </asp:sqldatasource>   <asp:dropdownlist id="dropdownlist1" runat="server" autopostback="true" datasourceid="sqldatasource1" datatextfield="plant" datavaluefield="plant" height="85px" width="393px">   </asp:dropdownlist>   <asp:sqldatasource id="sqldatasource2" runat="server" connectionstring="<%$ connectionstrings:2007  soundassist ver 1.0.5  05-12-2011 ( 2013-06-24)connectionstring %>" providername="<%$ connectionstrings:2007  soundassist ver 1.0.5  05-12-2011 ( 2013-06-24)connectionstring.providername %>" selectcommand="select [plant], [group no#] column1, [group] [temp table contains twa values] ([plant] = ?)">       <selectparameters>           <asp:controlparameter controlid="dropdownlist1" name="plant" propertyname="selectedvalue" type="string" />       </selectparameters>   </asp:sqldatasource>   <asp:dropdownlist id="dropdownlist2" runat="server" autopostback="true" datasourceid="sqldatasource2" datatextfield="group" datavaluefield="column1" height="30px" width="394px">   </asp:dropdownlist>   <asp:sqldatasource id="sqldatasource3" runat="server" connectionstring="<%$ connectionstrings:2007  soundassist ver 1.0.5  05-12-2011 ( 2013-06-24)connectionstring %>" providername="<%$ connectionstrings:2007  soundassist ver 1.0.5  05-12-2011 (2013-06-24)connectionstring.providername %>" selectcommand="select [plant], [group no#] column1, [group], [job code] job_code, [job function] job_function, [job classification] job_classification [temp table contains twa values] (([plant] = ?) , ([group no#] = ?))">       <selectparameters>           <asp:controlparameter controlid="dropdownlist1" name="plant" propertyname="selectedvalue" type="string" />           <asp:controlparameter controlid="dropdownlist2" name="column1" propertyname="selectedvalue" type="string" />       </selectparameters>   </asp:sqldatasource>   <asp:dropdownlist id="dropdownlist3" runat="server" autopostback="true" datasourceid="sqldatasource3" datatextfield="job_classification" datavaluefield="job_classification" height="17px" width="384px">   </asp:dropdownlist>   <asp:listview id="yourlistview" onload="yourlistview_load" runat="server"  datasourceid="sqldatasource4">       <itemtemplate>           <span style="">plant:           <asp:label id="plantlabel" runat="server" text='<%# eval("plant") %>' />           <br />           column1:           <asp:label id="column1label" runat="server" text='<%# eval("column1") %>' />           <br />           group:           <asp:label id="grouplabel" runat="server" text='<%# eval("group") %>' />           <br />           job_code:           <asp:label id="job_codelabel" runat="server" text='<%# eval("job_code") %>' />           <br />           job_classification:           <asp:label id="job_classificationlabel" runat="server" text='<%# eval("job_classification") %>' />           <br />           job_function:           <asp:label id="job_functionlabel" runat="server" text='<%# eval("job_function") %>' />           <br />           job_description:           <asp:label id="job_descriptionlabel" runat="server" text='<%# eval("job_description") %>' />           <br />           twa:           <asp:label id="twalabel" runat="server" text='<%# eval("twa") %>' />           <br />           <br />           </span>       </itemtemplate>        </asp:listview>   <asp:sqldatasource id="sqldatasource4" runat="server" connectionstring="<%$ connectionstrings:2007  soundassist ver 1.0.5  05-12-2011 ( 2013-06-24)connectionstring %>" providername="<%$ connectionstrings:2007  soundassist ver 1.0.5  05-12-2011 ( 2013-06-24)connectionstring.providername %>" selectcommand="select [plant], [group no#] column1, [group], [job code] job_code, [job classification] job_classification, [job function] job_function, [job description] job_description, [twa] [temp table contains twa values] (([plant] = ?) , ([group no#] = ?) , ([job classification] = ?))">       <selectparameters>           <asp:controlparameter controlid="dropdownlist1" name="plant" propertyname="selectedvalue" type="string" />           <asp:controlparameter controlid="dropdownlist2" name="column1" propertyname="selectedvalue" type="string" />           <asp:controlparameter controlid="dropdownlist3" name="job_classification" propertyname="selectedvalue" type="string" />       </selectparameters>   </asp:sqldatasource> 

you have check input valid -- use tryparse instead of convert function:

if (int.tryparse(thetwalabel.text, out thetwavalue)) { } 

update post-edit

i wrong, don't think values of label controls posted server -- explain why they're showing empty. in case, setting colors "loaded" event seems unnecessarily complicated. why not create helper method , bind directly?

protected color getcolorforlabel(string text) {     int thetwavalue;     if (text != null && int.tryparse(text, out thetwavalue) && thetwavalue >= 0)     {         return (thetwavalue < 90) ? system.drawing.color.yellow : system.drawing.color.red;     }     return system.drawing.color.green; } 

then use bind foreground property:

<asp:label id="twalabel" runat="server"      text = '<%# eval("twa") %>'      foreground = '<%# getcolorforlabel( eval("twa") string ) %>' />