public override object Execute(Expression expression)
        {
            String             url     = GetQueryText(expression);
            IEnumerable <Post> results = PostHelper.PerformWebQuery(url);

            return(results);
        }
        public override String GetQueryText(Expression expression)
        {
            SearchCriteria criteria;

            // 翻译查询条件
            criteria = new PostExpressionVisitor().ProcessExpression(expression);

            // 生成URL
            String url = PostHelper.BuildUrl(criteria);

            return(url);
        }