I want to use any free SMS service to send the SMS (free) from my application. I am developing the application in C# + asp.net. How can I do the code for that.
It is very easy. You have to download AT commands (contain message & other command of mobile) of your mobile. Use the predefined format to send message at modem port where your mobile is connected (may be wired or through Bluetooth). Before sending a messagemake a connection to a port.
It depends where you are based. SMS is likely to be a country specific thing, and thus an telco provided thing, and ultimately they're not likely to be free as the telecos make tonnes of cash from SMS. Even if you find a free service (there seems to be a lot in Western Europe) you contend with the service provider not providing an API or if you resort to screen scraping it will likely be against the sites' usage policies.
Take a look at
Free Software/Tools and Libraries for Sending and Receiving SMS Messages with a Computer / PC
If you manage to find a SMS service which allows you to send that many SMSs via an API then the provider should also have somewhere this API documented. If there is just a Webinterface then you have to simulate the HTTP communication (most probably by filling out form-fields) with your application. You have to find out what form-fields and login-data the SMS service expects and build your HTTP request accordingly.
Providing code for that is not possible since every SMS provider has its own interface and the communication for that has to be aligned for that.
Most mobile/cell providers provide a free email to SMS gateway so if you know the phone number's service provider and you're only catering to a select market (e.g. UK only) then that may be an option.
See SMS from web application for more details or try https://stackoverflow.com/questions/tagged/c%23%20sms