"that means sir when new user sine up, c# create automatically database for that particular user."
That is a spectacularly bad idea!
Creating a database or even a table for each user is a massive waste of space, memory, and time.
If you want a list of "contacts" which is specific to each user then create a table which holds cross references:
ID Name Email
1 Joe Joe@Domain.com
2 Mike Mike@OtherDomain.com
1000 Fred FredTheShred@RBS.com
ID UserID ContactID
1 1 2
2 1 1000
3 1 777
4 2 1000
5 2 666
In this example, User 1 (Joe) knows user 2 (Mike), another user with the id 777 and user 1000 (Fred).
User 2 (Mike) knows Fred, and another user with the ID 666)
To find out which contacts a user has, you select the records from the Contacts table, asking only for those records with his User ID:
SELECT ContactID FROM Contacts WHERE UserID=1