Is there a difference between sql server connection string and the express vesion?!
By default (though I don't recommend it1), Sql Express is installed as a named instance. That means you must connect like Server=servername.com\SQLEXPRESS
as opposed to just Server=servername.com
.
As always, connectionstrings.com has the goodies.
1 I don't recommend named instances because they run on a different port, which makes firewalling a PITA. Also, it makes little sense to me to run more than 1 MSSQL Server on a computer when you could just host multiple databases instead - so I'm not really sure what the point is.