Assuming that the .dll is going to be distributed to your users somehow, you can call is using the following:
However, if you are not planning to distribute the .dll to your users, then you must consider the following: 1) If the .dll is part of a standard system install, you can just make use of it, assuming that it is present. It may not be a bad idea to test the validity of your object before trying to call its methods, just in case. 2) If the .dll is custom, you must make it available to your users in some form. If you don't distribute it, you will have to put it in server-side code or use it client-side via RDS or a similar technology that doesn't require your users to have a local copy.