C and C# file encryption method and particular solution

C and C# file encryption method and particular solution

I have small system on Windows 7 OS which consists of two application (C
and C# applications) and a database which is a text or binary file in
fact.
C application only read from database, C# application could read and
write. I would like to defend this database file from outward impacts.
I tend to use Public-key cryptography method, RSA for example. Could you
offer a better one method or a better particular solution. I suggest I
need some open source, commercial-friendly licensed libraries (like
OpenSSL) for C and C# languages. However my own implementation could be a
good choice too.
UPD: I could change both applications code and I want to encrypt the
entire file.