I accually wrote my own SMS library in C#, back in 2005. My aim was to create a commercial library out of it, but it just never happened; besides this is probably my 1000 th project that has never been released... Anyway I decided to change that - because of this question - and will now start to release much of my superfine code under MIT license (free, free, free).
https://github.com/johanssonrobotics/FJR.SmsSolution (click the link, then the "Zip" button on GitHub to download)
Anyway, as I said, I wrote most of the code in 2005, but did some heavy refactoring to it about a year ago, so it should be solid. Anyway, feel free to test it. Comes with a demo Windows-forms application.
For instance, this could be used to connect a phone via COM-cable and send/receive SMS-messages. Easily.
Have fun & hope this helps!