public class MyConfigurationSection : ConfigurationSection
{
    [ConfigurationProperty("proxy", DefaultValue = "", IsRequired = false)]
    public string HttpProxy
    {
        get { return (string) this["proxy"]; }
        set { this["proxy"] = value; }
    }
}

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <configSections>
    <section name="liba"
             type="LibA.MyConfigurationSection, LibA, Version=1.0.0.0, Culture=neutral"></section>
  </configSections>
  <liba>
    <proxy>abc</proxy>
  </liba>
</configuration>

MyConfigurationSection myConfiguration = (MyConfigurationSection) ConfigurationManager.GetSection("liba");