My client has a running web application which has been online for more than a year. Recently the client has introduced smart cards for his employees. Because of the difficulty in integrating smart card with its api on a web interface(i will post another detailed question on this later) we are planning to have desktop interface for this. There are 10-20 terminals which will use the desktop interface. 3 approaches for doing this that I have considered :

1. Direct connection and operations on DB-Not using this because of data integrity and consistency issues.
2. Build web service end points and use it from desktop interface
3. Build a dll with common functions and use from both web and desktop

Questions: 1. What are your opinions based on 2 and 3 approach? 2. Any other approach that I should consider?

Note: I am using .Net framework, web application in asp.net