ConfigSettings class for accessing the config file ... a nice clean way. It provides a Require()

config-settings.cs

    public class ConfigSettings : System.Dynamic.DynamicObject
    {
        private static readonly System.Lazy<ConfigSettings> _settings = new Lazy<ConfigSettings>(() => new ConfigSettings());

        private ConfigSettings()
        { }

        public static dynamic Instance { get { return _settings.Value; } }

        public override bool TryGetMember(System.Dynamic.GetMemberBinder binder, out object result)
        {
            result = ConfigurationManager.AppSettings[binder.Name];
            return true;
        }

        public static string Require(string key)
        {
            if (ConfigurationManager.AppSettings[key] == null)
                throw new ApplicationException(string.Format("Config File Key: {0} not found", key));

            return ConfigurationManager.AppSettings[key];
        }

        public static string Optional(string key, string defaultValue)
        {
            return ConfigurationManager.AppSettings[key] ?? defaultValue;
        }
    }