problems on profile

i put the following codes in the web.config file.And i expect to collect the user information by using profile.But when i wrote codes in the .cs file,the code-intellisense did not prompt me with profilecommon. i cann't figure out why.could you kindhearted people give some clue?thank you so much!!!

<profile>
   <properties>
    <add name="f1" type="String"/>
    <add name="f2" type="String"/>
    <add name="f3" type="String"/>
    <add name="f4" type="String"/>
    <add name="f5" type="String"/>
    <add name="f6" type="String"/>
    <add name="f7" type="String"/>
   ...