The SqlConnection object is an object that derive of DbConnection. It opens the connection between the application and the database. It’s also inherit of IDisposable because of DbConnection.

var connection = new SqlConnection();
connection.ConnectionString = @"Data Source=PATRICK-PC\SQLEXPRESS;Initial Catalog=Northwind;Integrated Security=True";
connection.Open();
connection.Close();

or

using (var connection = new SqlConnection())
{
	connection.ConnectionString = @"Data Source=PATRICK-PC\SQLEXPRESS;Initial Catalog=Northwind;Integrated Security=True";
	connection.Open();
	connection.Close();
}

These two snippets of code illustrate the creation of a connection. Both of them were using connection string from a string but the value can be directly loaded from the app.config or web.config.

ConnectionStringSettings connectionStringSettings = ConfigurationManager.ConnectionStrings["ApplicationServices"];
using (var connection = new SqlConnection(connectionStringSettings.ConnectionString))
{
    connection.Open();
    connection.Close();
}

<configuration>
  <connectionStrings>
    <add name="ApplicationServices"
         connectionString="Data Source=PATRICK-PC\SQLEXPRESS;Initial Catalog=Northwind;Integrated Security=True"
         providerName="System.Data.SqlClient" />
  </connectionStrings>
  ...
  ...