How to add dll in web application with executing code from it

Revision 2
I have written a class library file and use the dll of this file in web application

i write some code in this dll like generating connection string as well as appSettings in web.config file

but problem is that.... i want to generate this connection string and appsettings data while adding this dll in web application instead of running the application

so plz help me...

some frnz are not understand my question
let me clear..

when i add dll in my application it added successfully
no problem about it.
when i run my application then connection string generate in web.config

bt what happen with this...
all sessions are cleared

hence i want to generate this connection string at the time of adding this dll in application

so is there any event handler or loader is available which will help to get me result