I’m going to demonstrate how to get connection string from App.config in C#.Net.

Example

App.config

<?xmlversion="1.0"encoding="utf-8" ?>
<configuration>
  <connectionStrings> 
    <addname="con1"connectionString="Server=abc\SQLEXPRESs;Database=Employee;Trusted_Connection=True"/>  
  </connectionStrings>
</configuration>

Form1.cs where we are going to use connection string of App.config

//namespace for sqlclient
using System.Data.SqlClient;
//to use connectionstring we have to use System.Configuration namespace
using System.Configuration; 
//fetching connectionstring from App.config
string strConnection= ConfigurationManager.ConnectionStrings["con1"].ConnectionString;
//creating newsqlconnection
 con= new SqlConnection(strConnection);
//opening connection
     con.Open();