I have used VistaDB for many in-memory database apps. It would be nice to work with it using Entity Developer. (This is for Entity Framework)
http://www.gibraltarsoftware.com/VistaDB
Feature Request: support for VistaDB
Re: Feature Request: support for VistaDB
Thank you for your feedback. We will consider adding this feature in Entity Developer, but there is no timeframe at the moment.