<!-- Aqui é o webconfig com a sua connectionstring -->
<?xml version="1.0"?>
.....
<configuration>
<appSettings/>
<connectionStrings>
<add name="clientesConnectionString" connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=C:\_aspn\artigos\App_Data\clientes.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True" providerName="System.Data.SqlClient"/>
</connectionStrings>
<system.web>
....
<authentication mode="Windows"/>
</system.web>
</configuration>

----------------------------------

//Aqui obtenho a connectionstring do webconfig

using System.Configuration;

SqlConnection sqlconn = new SqlConnection(ConfigurationManager.ConnectionStrings("clientesConnectionString").ConnectionString);