Question

Having the following:

public List<int> List1 { get; set; }
...
var x = GiveMeObject(); // x.List2 --> each element on list2 has an Id (int).
...
bool containsAtLeastOne = ???

What is the easiest/fastest/shortest way (in linq) to verify if at least 1 element of list1 is in the list2 ?

Thanks

Answer 1

alternative: Intersect

bool containsAtLeastOne = List1.Intersect(x.List2.Select(e => e.Id)).Any()

If your collections are getting large, you should use Intersect instead of Contains, since Intersect is at least as fast as Contains. Depending on your collecions, Contains can get slow quickly.

If your collections are quite small (< 1000 elements), this difference would probably not matter.

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If you don’t mind a non-LINQ way and some more lines of code, you could use

var tmp = new HashSet<int>(x.List2.Select(e => e.ID));
tmp.IntersectWith(list1);
bool containsAtLeastOne = tmp.Any();

which will probably be faster than the LINQ approach.