LINQ string contain any element of a list of string (ASP net core MVC)

Question

In the Dat Access Layer of an ASP.NET Core MVC project, I need to get a list of "news" matching with any tags passed as parameter, something like:

public List<News> GetNewsByTags(List<String> tagsList)
{
    return database.News.Where(news => news.Tags.ContainsAnyElementOf(tagsList));
}

but I didn't succeed to make something like this work. For information, this is my request for only one tag:

public List<News> GetNewsByTag(String tag)
{
    return database.News.Where(news => news.Tags.Contains(tag)).ToList();
}

Is news.Tag a string or a list of strings?

Andrei
– Andrei

2018-04-11 14:53:10 +00:00
Commented Apr 11, 2018 at 14:53 — Andrei
– Andrei, Commented Apr 11, 2018 at 14:53
news.Tags is a string build like that : "tag1;tag2;tag3"

Alexis
– Alexis

2018-04-11 14:55:46 +00:00
Commented Apr 11, 2018 at 14:55 — Alexis
– Alexis, Commented Apr 11, 2018 at 14:55
Ok, that explains the error. Updated

Andrei
– Andrei

2018-04-11 14:56:30 +00:00
Commented Apr 11, 2018 at 14:56 — Andrei
– Andrei, Commented Apr 11, 2018 at 14:56

Sergey Kalinichenko · Accepted Answer · 2018-04-11 15:35:18Z

2

Since your Tags is a single string with semicolons, you should do it in two phases: first, query on the SQL side with Contains, then filter what comes back to avoid false positives:

var preliminary = database.News
    .Where(news => tagsList.Any(t => news.Tags.Contains(t)))
    .ToList();
var tagSet = new HashSet<string>(tagList);
return preliminary.Select(news => new {
    News = news
,   MatchCount = news.Tags.Split(';').Count(t => tagSet.Contains(t))
}).Where(p => p.MatchCount > 0)
.OrderByDescending(p => p.MatchCount)
.Select(p => p.News);

edited Apr 11, 2018 at 15:35

answered Apr 11, 2018 at 14:43

Sergey Kalinichenko

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Comments

Jordi Jordi · Accepted Answer · 2018-04-11 14:46:45Z

0

What about this ?

var allInCollection = news.Tags.Any(x => tagsList.Contains(x));

answered Apr 11, 2018 at 14:46

Jordi Jordi

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Comments

Andrei · Accepted Answer · 2018-04-11 15:12:00Z

0

So you need to check that any of the news tag appears in the list of tags given. That translates quite literally to LINQ:

database.News.Where(news =>
    tagsList.Any(tag => news.Tags.Contains(tag))
)

edited Apr 11, 2018 at 15:12

answered Apr 11, 2018 at 14:43

Andrei

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2 Comments

Alexis Over a year ago

I got an error on the news.Tags.Split(';') : An expression tree may not contain a call or invocation that uses optional arguments

Andrei Over a year ago

@Alexis, just figured it can be rewritten in other way, see updated version

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