gistfile1.cs

    // your code, shortened
    public class FilterFactory
    {
        public Expression<Func<Message, bool>> Filter(FilterGroupParameter filterGroupParameter)
        {
            var factory = FiltererFactoryFactory.Get(filterGroupParameter.Key);
            return factory.Get(filterGroupParameter);
        }
    }

    // my code, and you'll going to hate me for it..

    interface IFiltererFactory
    {
        Expression<Func<Message, bool>> Get(FilterGroupParameter param);
    }

    class FiltererFactoryFactory
    {
        public static IFiltererFactory Get(FilterParameterKeysEnum bla)
        {
            switch (bla)
            {
                case FilterParameterKeysEnum.Content:
                    return new ContentFilterFactory();
                case FilterParameterKeysEnum.FeedId:
                    return new FeedIdFilterFactory();
            }

            throw new NotImplementedException();
        }
    }

    class ContentFilterFactory : IFiltererFactory
    {
        public Expression<Func<Message, bool>> Get(FilterGroupParameter param)
        {
            var contentFilter = Filterer.Get(param.Key, message => message.Body, null);
            return contentFilter.FilterString(param.Value);
        }
    }

    class FeedIdFilterFactory : IFiltererFactory
    {
        public Expression<Func<Message, bool>> Get(FilterGroupParameter param)
        {

            var feedidFilter = Filterer.Get(param.Operator, null, message => message.TeamFeedId);
            return feedidFilter.FilterInt(int.Parse(param.Value));
        }
    }


    // some temp fakes to make code work, partially..
    public struct FilterGroupParameter
    {
        public FilterParameterKeysEnum Key;
        public FilterParameterOperatorEnum Operator;
        public string Value;
    }

    public enum FilterParameterKeysEnum
    {
        Content,
        FeedId
    }