排除Linq查询的结果,排除排除列表为空时的所有内容

Question

我有以下代码:

        public IList<Tweet> Match(IEnumerable<Tweet> tweetStream, IList<string> match, IList<string> exclude)
    {
        var tweets = from f in tweetStream
                     from m in match
                     where f.Text.ToLowerInvariant().Contains(m) 
                     select f;

        var final = from f in tweets
                    from e in exclude
                    where !f.Text.ToLowerInvariant().Contains(e.ToLowerInvariant())
                    select f;

        return final.Distinct().ToList<Tweet>();
    }

我一直在构建测试,其中没有包含final结果集并且已经愉快地匹配现在我已添加了排除如果IList<string>exclude为空则所有项目都被删除.

所以这个测试按原样通过:

        [TestMethod]
    public void Should_exclude_items_from_exclude_list()
    {
        IEnumerable<Tweet> twitterStream = new List<Tweet>
                                               {
                                                   new Tweet("I have a Mazda car"),
                                                   new Tweet("I have a ford"),
                                                   new Tweet("Mazda Rules"),
                                                   new Tweet("My Ford car is great"),
                                                   new Tweet("My renault is brill"),
                                                   new Tweet("Mazda cars are great")
                                               };
        IList<string> matches = new List<string>{"mazda","car"};
        IList<string> exclude = new List<string>{"ford"};

        Matcher target = new Matcher();
        IList<Tweet> actual = target.Match(twitterStream, matches, exclude);

        Assert.AreEqual(3, actual.Count);            
    }

但是这个测试现在失败了:

        [TestMethod]
    public void Should_match_items_either_mazda_or_car_but_no_duplicates()
    {
        IEnumerable<Tweet> twitterStream = new List<Tweet>
                                               {
                                                   new Tweet("I have a Mazda car"),
                                                   new Tweet("I have a ford"),
                                                   new Tweet("Mazda Rules"),
                                                   new Tweet("My Ford car is great"),
                                                   new Tweet("My renault is brill"),
                                                   new Tweet("Mazda cars are great")
                                               };
        IList<string> matches = new List<string>{"mazda","car"};
        IList<string> exclude = new List<string>();

        Matcher target = new Matcher();
        IList<Tweet> actual = target.Match(twitterStream, matches, exclude);

        Assert.AreEqual(4, actual.Count);
    }

我知道我错过了一些非常简单的东西,但在盯着代码一小时之后它就没有来找我.

Answer 1

好吧,我知道它为什么失败了:这是这个条款:

from e in exclude

这将是一个空集合,所以甚至没有条目甚至命中where子句.

这是另一种方法:

var final = from f in tweets
            let lower = f.Text.ToLowerInvariant()
            where !exclude.Any(e => lower.Contains(e.ToLowerInvariant())
            select f;

虽然我也考虑过msarchet的方法,但关于这个方法的好处是它最终只会评估tweetStream一次 - 所以即使从网络读取或者做了其他令人痛苦的事情,你也不必担心.在可能的情况下(和方便)我试图避免多次评估LINQ流.

当然,您可以非常轻松地完成一个查询:

var tweets = from f in tweetStream
             let lower = f.Text.ToLowerInvariant()
             where match.Any(m => lower.Contains(m.ToLowerInvariant())
             where !exclude.Any(e => lower.Contains(e.ToLowerInvariant())
             select f;

我认为即使更清洁,说实话:)