sigo con app.config

hola!

Quiero leer la clave *servidor* y también *ejemplo*. para ello uso este
código que he visto:


using System.Configuration;


cadena = ConfigurationManager.ConnectionStrings["Servidor"].ToString();
cadena = ConfigurationManager.AppSettings["Ejemplo"].ToString();


Siempre me da error de *referencia a objeto no establecida*.


mi fichero es el siguiente:

<?xml version="1.0" encoding="utf-8" ?>

<configuration>

<configSections>

<sectionGroup name="userSettings"
type="System.Configuration.UserSettingsGroup, System, Version=2.0.0.0,
Culture=neutral, PublicKeyToken·7a5c561934e089" >

<section name="Factura.Properties.Settings"
type="System.Configuration.ClientSettingsSection, System, Version=2.0.0.0,
Culture=neutral, PublicKeyToken·7a5c561934e089"
allowExeDefinition="MachineToLocalUser" requirePermission="false" />

</sectionGroup>

<sectionGroup name="applicationSettings"
type="System.Configuration.ApplicationSettingsGroup, System,
Version=2.0.0.0, Culture=neutral, PublicKeyToken·7a5c561934e089" >

<section name="Factura.Properties.Settings"
type="System.Configuration.ClientSettingsSection, System, Version=2.0.0.0,
Culture=neutral, PublicKeyToken·7a5c561934e089"
requirePermission="false" />

</sectionGroup>

</configSections>

<userSettings>

<Factura.Properties.Settings>

<setting name="Ejemplo" serializeAs="String">

<value>algo de ejemplo</value>

</setting>

</Factura.Properties.Settings>

</userSettings>

<applicationSettings>

<Factura.Properties.Settings>

<setting name="Servidor" serializeAs="String">

<value>(GRANDE)\SqlExpress</value>

</setting>

</Factura.Properties.Settings>

</applicationSettings>

</configuration>