Intersect LINQ query

If I have an IEnumerable where ClassA exposes an ID property of type long. Is it possible to use a Linq query to get all instances of ClassA with ID belonging to a second IEnumerable?

In other words, can this be done?

IEnumerable<ClassA> = original.Intersect(idsToFind....)?

where original is an IEnumerable<ClassA> and idsToFind is IEnumerable<long>.

Yes.

As other people have answered, you can use Where, but it will be extremely inefficient for large sets.

If performance is a concern, you can call Join:

var results = original.Join(idsToFind, o => o.Id, id => id, (o, id) => o);

If idsToFind can contain duplicates, you'll need to either call Distinct() on the IDs or on the results or replace Join with GroupJoin (The parameters to GroupJoin would be the same).

I will post an answer using Intersect.

This is useful if you want to intersect 2 IEnumerables of the same type.

First we will need an EqualityComparer:

    public class KeyEqualityComparer<T> : IEqualityComparer<T>
    {
        private readonly Func<T, object> keyExtractor;

        public KeyEqualityComparer(Func<T, object> keyExtractor)
        {
            this.keyExtractor = keyExtractor;
        }

        public bool Equals(T x, T y)
        {
            return this.keyExtractor(x).Equals(this.keyExtractor(y));
        }

        public int GetHashCode(T obj)
        {
            return this.keyExtractor(obj).GetHashCode();
        }
    }

Secondly we apply the KeyEqualityComparer to the Intersect function:

var list3= list1.Intersect(list2, new KeyEqualityComparer<ClassToCompare>(s => s.Id));