I know that there a plenty of questions here on how to disable the single-click behaviour of the Pantheon file manager. But, in fact, I really like it, and I'm rather confused by the fact, that this behaviour is implemented inconsistently in Freya: Particularly, we still have to use a double click to choose a file in a dialog. This is highly inconvenient. Can the single-click behaviour be enabled for the file chooser dialogs too?
The Pantheon file-chooser still uses the native Gtk Filechooser widget and this seems to hard code the double click behaviour. So it is not possible to change this at present by changing a setting.
It would be possible however to amend the coding for Pantheon filechooser so that it sets the behaviour to match that setting for Files.
A bug report will need to be raised concerning this at https://bugs.launchpad.net/pantheon-files