DLL with UI in c#.net

i have to make an encapsulated User Login module as a dll in c# which will be used in main application. in which say an object is created as
usermodule userx=new usermodule();
// then login function will be called which will prompt a standard Ui for login and will be
// closed if login successful and will return true if login is fails after a 3 tries UI will
//automatically get closed returning false value
...
userx.Login();
......
// like wise other functions like userx.changepwd(), will prompt an UI for login and will
//return true or false accordingly
the main thing is that all function to be called from main application will not have any parameters example
Boolean Login(),etc;
also one thing that i got is confusing me
i had i conversation via mail to the programer company
and he gave me this and added more to my confusion
here it is
"
Hi,
It should have with UI. It can be application based (prefer .net 2.0 or above) or web based. Note that the system will be in .net 3.5 windows application (not likely to be WPF).
The idea for this module is to totally isolate it from other modules. Please take note on what happen when PC is disconnected or power cut off, how can the module know user is re-login and not multiple login.
"
Can you give me some hint what should i do and how i should start with my project
my problem is how dll can be used for prompting a UI.
like usermodule.login() will prompt a UI for login
uermodule.changepassword() will prompt a for chaning password
etc.

 

duplicate thread.