How to use Linq to check if a list of strings contains any string in a list

EDIT: This answer assumed that tags was a collection of strings...

It sounds like you might want:

var list = new List<string> { ... };
var query = query.Where(x => x.tags.Any(tag => list.Contains(tag));

Or:

var list = new List<string> { ... };
var query = query.Where(x => x.tags.Intersect(list).Any());

(If this is using LINQ to SQL or EF, you may find one works but the other doesn't. In just LINQ to Objects, both should work.)

To get the count, you'd need something like:

var result = query.Select(x => new { x, count = x.tags.Count(tag => list.Contains(tag)) })
                  .Where(pair => pair.count != 0);

Then each element of result is a pair of x (the item) and count (the number of matching tags).

I've done something like this before:

var myList = new List<string>();
myList.Add("One");
myList.Add("Two");

var matches = query.Where(x => myList.Any(y => x.tags.Contains(y)));

I am not quite sure from your question if x.tags is a string or list, if it is a list Jon Skeet's answer is correct. If I understand you correctly though x.tags is a string of strings. If so then the solution is:

list.Any(x => x.tags.IndexOf(x) > -1)

to count them do

list.Count(x => x.tags.IndexOf(x) > -1)

like this:

List<string> list = new List<string>();
list.Add("One");
list.Add("Two");

 var result = query.Where(x => list.Contains(x.tags));

  var t = new List<string> { "a", "b", "c" };

var y = "a b d";

var res = y.Count(x => t.Contains(x.ToString()));

I faced a similar problem recently and here's how I managed to work it out:

var list = [list of strings];
if (list != null && list.Any())
{
    queryable = queryable.Where(x => x.tags != null);
    var tagQueries = new List<IQueryable<WhateverTheDbModelIs>>();
    foreach (var element in list)
    {
        tagQueries.Add(queryable.Where(x => x.tags.Contains(element)));
    }
    IQueryable<WhateverTheDbModelIs> query = tagQueries.FirstOrDefault();
    foreach (var tagQuery in tagQueries)
    {
        query = query.Union(tagQuery);
    }
    queryable = queryable.Intersect(query);
}

probably not the best option but something a less experienced developer can understand and use