Connect to SQL Server 2012 Database with C# (Visual Studio 2012)

In your connection string replace server=localhost with "server = Paul-PC\\SQLEXPRESS;"

I tested all the answers here, but for me, none worked. So I studied a bit the problem, and finally I found the connection string needed. To get this string, you do:
1. in you project name:
a. right click the project name,
b. click Add,
c. select SQL Server Database (obviously you can rename it as you wish).
Now the new desired database will be added to your project.
2. The database is visible in the Server Explorer window.
3. Left click the database name in the Server Explorer window; now check the Solution Explorer window, and you will find the "Connection String", along side with Provider, State, Type, Version.
4. Copy the connection string provided, and put it in the Page_Load method:

string source = @"Data Source=(LocalDB)\v11.0;AttachDbFilename=c:\x\x\documents\visual studio 2013\Projects\WebApplication3\WebApplication3\App_Data\Product.mdf;Integrated Security=True";SqlConnection conn = new SqlConnection(source);conn.Open();//your code here;conn.Close();

I renamed my database as Product. Also, in the "AttachDbFilename", you must replace "c:\x\x\documents\" with your path to the phisical address of the .mdf file.

It worked for me, but I must mention this method works for VS2012 and VS2013. Don't know about other versions.

Try:

SqlConnection myConnection = new SqlConnection("Database=testDB;Server=Paul-PC\\SQLEXPRESS;Integrated Security=True;connect timeout = 30");