BaseDir in app.config file, like WCF baseadress

Question

Hi would like to use an attribute within another attribute, like WCF baseadress. My app.config file is like below. I've added a new section named BaseDir and then I would like to use it in an other section.

 <section name="BaseDir" type="System.Configuration.DictionarySectionHandler"/>
 <BaseDir>
    <add key="PreprocessorBase" value="C:\Users\MyUser\Desktop\FormProcessing\Preprocessor\"/>
</BaseDir>

And then I try to use it like below.

  <Folders>
    <add key="FtpInput" value="${BaseDir.PreprocessorBase}Input\FtpInput"/>
    <add key="FtpBackup" value="${BaseDir.PreprocessorBase}Backup\FtpInput"/>
</Folders>

Thanks.