<?xml version="1.0" encoding="utf-8" ?>
    <configuration>
        <connectionStrings>
            <add name="CONSTRING" connectionString="Data Source=SQL-PC;Initial Catalog=DATABASE;Integrated Security=True"/>
        </connectionStrings>
    </configuration>

using System.Windows.Forms;
using System.Data.SqlClient;
using System.Configuration;

namespace Sample {
    public partial class Sample: Form
    {
        public string conn = ConfigurationManager.ConnectionStrings["CONSTRING"].ConnectionString;
    }

var conString =System.Configuration.ConfigurationManager.ConnectionStrings["connectionStringName"];
string strConnString = conString.ConnectionString;
SqlConnection con = new SqlConnection(strConnString);
SqlCommand cmd = new SqlCommand(String.Format("SELECT * FROM Table;"), con);
con.Open();
SqlDataReader reader = cmd.ExecuteReader();

<connectionStrings>
<add name="DefaultConnection" connectionString="Data Source=(localdb)v11.0;Initial Catalog=database;Integrated Security=True; MultipleActiveResultSets=True"
     providerName="System.Data.SqlClient" />
</connectionStrings>

static string ConStr = "Data Source=DataSourceName;Initial Catalog=DatabaseName;Integrated Security=True";
SqlConnection con = new SqlConnection(ConStr);

using System.Configuration;

var conString = System.Configuration.ConfigurationManager.ConnectionStrings["CONSTRING"];
string strConnString = conString.ConnectionString;
SqlConnection con = new SqlConnection(strConnString);

<connectionStrings>
<add name="CONSTRING" connectionString="Data Source=MONAIT-PC;Initial Catalog=CMS;Integrated Security=True" providerName="System.Data.SqlClient"/>

</connectionStrings>

***{Sql Server Authentication}***

***{Windows Server Authentication}***