how can i solved . error on connection string in c# : 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!!!