public static IEnumerable<TSource> DistinctBy<TSource, TKey>

    (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)

{

    HashSet<TKey> seenKeys = new HashSet<TKey>();

    foreach (TSource element in source)

    {

        if (seenKeys.Add(keySelector(element)))

        {

            yield return element;

        }

    }

}