help.. error on connection string

Question

hi ...

i have a database with sql server 2012 authontication called "box" 

i build an aplplication in c#.. what i want to do is :

attach this database file  into client matchine to run an application without setup sql server.

1- from mycomputer --> manage--> i stoped the sql server serviice  

2- i copy database files (box.mdf and box_log.ldf) inside application  directory.

3- build connection string to connect with database as: 

(

this work correctly: conn.ConnectionString = "Data Source=BASIM-PC\\SQLSERVER2012;Initial Catalog=box;Persist Security Info=True;User ID=basimbox;Password=f1977";

but this error:conn.ConnectionString = @"Data Source=.\sqlexpress;Initial Catalog=box;Persist Security Info=True;User ID=basimbox;Password=f1977";

) 

 plz my friend help me!!!