Get objects which are in both lists based on specific comparator [duplicate]

Question

I have two List objects, listA and listB, which contain a collection of User objects.

Each User object has a property ID.

I want to get a list of users which are present in both listA and listB based on their ID property.

Here is my code so far:

listA.Where(a => listB.Any(b => b.ID == a.ID));

Is there a better way to do this? It feels like it could be inefficient, especially if listB is large.

The User object does not implement IEquatable.

Answer 1

If User implements IEquatable<User>, assumption is that two User are the same if ID are the same, then you can use LINQ Intersect. For example:

listA.Intersect(listB);

If User does not implement IEquatable<User> you can invoke Intersect with IEqualityComparer<User>. For Example:

listA.Intersect(listB, new UserEqualityComparer());

where

UserEqualityComparer : IEquatable<User> {...}