Client's app connecting to host?

Hello, I’m a PHP dev, but playing with C# (Visual Studio 2010) right now, and I have a question about connecting to any hosted database.

Let’s say I own a site called and I want to develop an app that will allow users to check in-box without a browser.

Can application connecting from client’s OS to host database be totally secure? What about sniffers, hexeditors and cracking?

What do you think? Any ideas (excepting db user privileges)?

That does not give them access to the database, it allows them to interact with the database through a set of rules defined in the API.
Meaning they cannot request anything they like (as they could using your proposed method).

Well, but API is going to have a login form and all passwords and logins are stored in the database…

You should never allow users to connect to your database!
What you should do instead is create an API.