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));

You can do it, but in the current form, you'd want to use the Where extension method.

var results = original.Where(x => yourEnumerable.Contains(x.ID));

Intersect on the other hand will find elements that are in both IEnumerable's. If you are looking for just a list of ID's, you can do the following which takes advantage of Intersect

var ids = original.Select(x => x.ID).Intersect(yourEnumerable);

A simple way would be:

IEnumerable<ClassA> result = original.Where(a => idsToFind.contains(a.ID));

Use the Where method to filter the results:

var result = original.Where(o => idsToFind.Contains(o.ID));

Naming things is important. Here is an extension method base on the Join operator:

private static IEnumerable<TSource> IntersectBy<TSource, TKey>(
    this IEnumerable<TSource> source,
    IEnumerable<TKey> keys,
    Func<TSource, TKey> keySelector)
        => source.Join(keys, keySelector, id => id, (o, id) => o);

You can use it like this var result = items.IntersectBy(ids, item => item.id).

I've been tripping up all morning on Intersect, and how it doesn't work anymore in core 3, due to it being client side not server side.

From a list of items pulled from a database, the user can then choose to display them in a way that requires children to attached to that original list to get more information.

What use to work was:

itemList = _context.Item
        .Intersect(itemList)
        .Include(i => i.Notes)
        .ToList();

What seems to now work is:

itemList = _context.Item
        .Where(item => itemList.Contains(item))
        .Include(i => i.Notes)
        .ToList();

This seems to be working as expected, without any significant performance difference, and is really no more complicated than the first.