UnionBy Linq Implementation

Question

I needed an implementation on Union that compares a property of an object, rather the objects themselves. I came up with the following:

public static IEnumerable<TSource> UnionBy<TSource, TKey>(
    this IEnumerable<TSource> first,
    IEnumerable<TSource> second,
    Func<TSource, TKey> keySelector,
    IEqualityComparer<TKey> keyComparer = null)
{
    HashSet<TKey> keys = new HashSet<TKey>(keyComparer);
    foreach (TSource element in first)
    {
        if (keys.Add(keySelector(element)))
        {
            yield return element;
        }
    }
    foreach (TSource element in second)
    {
        if (!keys.Add(keySelector(element)))
        {
            continue;
        }
        yield return element;
    }
}

which I can use by saying something along the lines of:

result = first.UnionBy(second, x => x.Property1);

Which works for me, but I was wondering if there wasn't something already implemented in Linq that I was missing (other than implementing my own EqualityComparer which to me seems less intuitive).

Since I won't be using the same properties every time I want this union I would either have to make multiple EqualityComparer's for each situation which doesn't seem correct to me, or make some generic EqualityComparer that would take in a property selector Func. It seemed less intuitive to me than just providing a generic Linq extension that accepted the property selector itself.

Answer 1

Yes you can write it as follows instead:

var q = first.Concat(second).GroupBy(x => x.Property1).Select(x => x.First());