I am looking for file based storage solutions that I can use with a .Net project. THey need to have a sql-like interface for storing and retrieving data. They need to have
The only other option I have any experience with is Microsoft SQL Server Compact if you don't want to install anything on the user side. SQL Server Express works as well if you did want to install something. It really all has to do with what features you want to have in your embedded database.
The one advantage of SQL Compact is that when its on a local drive i.e. not on a network drive it natively supports multiple connections. If you want it to write to a database on a network drive though you'll have problems with it. From your brief description it sounds like SQL Compact is what you want. If you have more specific requirements the answer might be different.