Hi
Maybe you did not using System.Configuration namespace :
<configuration>
<connectionstrings>
<add name="DBConnectionString">
connectionString="Data Source=Manoj;Initial Catalog=Test;Persist Security Info=True;User ID=sa;Password=admin123"
providerName="System.Data.SqlClient" />
</add></connectionstrings>
</configuration>
using System.Configuration;
string nwConn = System.Configuration.ConfigurationManager.ConnectionStrings[1].ToString();
However you have many different solutions for reading connection string from app.config for example follow of these links:
Connection string App.Config
Connection Strings and Configuration Files
connection string and app config
Best Regards.