How to ensure that the «if» statement gets parsed _before_ a given resource/class?

It is a an answer using virtual resources, that lavaman suggested.

I don't like the fact, that it forces me to put code with "VirtualBox" to the class, that I think should be logically independent to it. But I think it can be worked-around using delegated classes (like userdocs::virtualbox, defined completely elsewhere).

class userdocs ( $sourcetype) {
  case $sourcetype {
    'cryptpartition' : {
      @file { "${userhome}/.VirtualBox": ensure=>link, ...
      }
      ...
    }
    'partition' : {
      @file { "${userhome}/.VirtualBox": ensure=>link, ...
      }
      ...
    }
    'none' : {
      @file { "${userhome}/.VirtualBox": ensure=>directory, ...
      }
      ...
    }
  }
  ...
}

and

class vbox {
  realize File["${userhome}/.VirtualBox"]
  ...

}