How to check list A contains any value from list B?

Question

List A:

1, 2, 3, 4

List B:

2, 5

How to check if list A contains any value from list B?

e.g. someth

Answer 1

You can check if a list is inside of another list with this

var list1 = new List<int> { 1, 2, 3, 4, 6 };
var list2 = new List<int> { 2, 3 };
bool a = list1.Any(c => list2.Contains(c));

Answer 2

For faster and short solution you can use HashSet instead of List.

a.Overlaps(b);

Overlaps documentation

This method is an O(n) instead of O(n^2) with two lists.

Answer 3

If you didn't care about performance, you could try:

a.Any(item => b.Contains(item))
// or, as in the column using a method group
a.Any(b.Contains)

But I would try this first:

a.Intersect(b).Any()

Answer 4

I've profiled Justins two solutions. a.Any(a => b.Contains(a)) is fastest.

using System;
using System.Collections.Generic;
using System.Linq;

namespace AnswersOnSO
{
    public class Class1
    {
        public static void Main(string []args)
        {
//            How to check if list A contains any value from list B?
//            e.g. something like A.contains(a=>a.id = B.id)?
            var a = new List<int> {1,2,3,4};
            var b = new List<int> {2,5};
            var times = 10000000;

            DateTime dtAny = DateTime.Now;
            for (var i = 0; i < times; i++)
            {
                var aContainsBElements = a.Any(b.Contains);
            }
            var timeAny = (DateTime.Now - dtAny).TotalSeconds;

            DateTime dtIntersect = DateTime.Now;
            for (var i = 0; i < times; i++)
            {
                var aContainsBElements = a.Intersect(b).Any();
            }
            var timeIntersect = (DateTime.Now - dtIntersect).TotalSeconds;

            // timeAny: 1.1470656 secs
            // timeIn.: 3.1431798 secs
        }
    }
}

Answer 5

You can Intersect the two lists:

if (A.Intersect(B).Any())

Answer 6

Sorry, if this is irelevant, but will return list with matches using FindAll() in case you need this:

        private bool IsContain(string cont)
    {
        List<string> ListToMatch= new List<string>() {"string1","string2"};

        if (ListToMatch.ToArray().Any(cont.Contains))
        {
            return false;
        }
        else
            return true;
    }

And usage:

List<string> ListToCheck = new List<string>() {"string1","string2","string3","string4"};
List<string> FinalList = ListToCheck.FindAll(IsContain);

The final list contains only the matched elements string1 and string2 from list to check. Can easy be switched to int List.