Question

I think this would take O(A x B) time to execute.

(where A is the size of collectionA and B is the size of collectionB)

Am I correct?

IEnumerable<A> GetMatches(IEnumerable<A> collectionA, IEnumerable<B> collectionB)
{
    foreach (A a in collectionA)
        foreach (B b in collectionB)
            if (a.Value == b.Value)
                yield return a;
}

Is there a faster way to execute this query? (maybe using LINQ?)

Answer 1

Enumerable.Intersect will not, unfortunately, work as you’re comparing against two separate types (A and B).

This will require a bit of handling separately to get an Intersect call that will work.

You could do this in stages:

IEnumerable<A> GetMatches(IEnumerable<A> collectionA, IEnumerable<B> collectionB)
     where A : ISomeConstraintWithValueProperty
     where B : ISomeOtherConstraintWithSameValueProperty
{
    // Get distinct values in A
    var values = new HashSet<TypeOfValue>(collectionB.Select(b => b.Value));

    return collectionA.Where(a => values.Contains(a.Value));
}

Note that this will return duplicates if collectionB contains duplicates (but not collectionA), so it will have slightly different results than your looping code.

If you want unique matches (only one returned), you could change the last line to:

return collectionA.Where(a => values.Contains(a.Value)).Distinct();