c# - Using Binding converter to display and update a property in silverlight -

i using converter class display property called noofweeks based on normal working hours(40), example if value stored in database 40 displaying 1 in datagrid column, if 80 displaying 2, here xaml , converter code

<sdk:datagridtemplatecolumn width="*"  canuserreorder="false" headerstyle="{staticresource datagridbaseheaderstyle}" header="fte">                 <sdk:datagridtemplatecolumn.celltemplate>                     <datatemplate>                         <grid>                             <textblock  text="{binding hours, converter={staticresource fteconverter}}"  verticalalignment="stretch" margin="4,2" horizontalalignment="stretch"></textblock>                         </grid>                     </datatemplate>                 </sdk:datagridtemplatecolumn.celltemplate>                 <sdk:datagridtemplatecolumn.celleditingtemplate>                     <datatemplate>                         <textbox text="{binding hours,mode=twoway, converter={staticresource fteconverter}}"  verticalalignment="top" margin="4,2"                                           maxlength="50" horizontalalignment="stretch" >                         </textbox>                     </datatemplate>                 </sdk:datagridtemplatecolumn.celleditingtemplate>             </sdk:datagridtemplatecolumn>   public object convert(object value, type targettype, object parameter, system.globalization.cultureinfo culture)       {           if (value == null)               return false;           else           {               // assumes rowtype has been passed bound value               double hours= (double)value;                        return math.round(hours/40,0);               }           } 

so data grid userid 1 , 2 no of hours in database 80 , 120 respectively looks like

 userid  noofweeks        standardhours(not visible user)       1        2                  80    2        3                  120 

this working fine expected, want update hours in database based on noofweeks modifed user.

for example in above table if user userid updates noofweeks 4 want save hours 160 (4 *40).

userid noofweeks standardhours(not visible user) 1 4 160 2 3 120 getting bit confused here how update property?

for new mvvm , converters, did use convertback method convert user input hours

 public object convertback(object value, type targettype, object parameter, system.globalization.cultureinfo culture)       {            if (value == null)               return false;           else           {               // assumes rowtype has been passed bound value               string val = (string)value;               double doublevalue = double.tryparse(val, out doublevalue) ? doublevalue : 0;               return doublevalue * 40;               }           }       } 

one thing note if user enters text or besides numbers converting value 0. might not suit requirements , can use idataerrorinfo in property setters stop , notify users entering values not allowed.