Why is there no Linq method to return distinct values by a predicate?

It’s annoying, certainly. It’s also part of my “MoreLINQ” project which I must pay some attention to at some point 🙂 There are plenty of other operations which make sense when acting on a projection, but returning the original – MaxBy and MinBy spring to mind.

As you say, it’s easy to write – although I prefer the name “DistinctBy” to match OrderBy etc. Here’s my implementation if you’re interested:

    public static IEnumerable<TSource> DistinctBy<TSource, TKey>
        (this IEnumerable<TSource> source,
         Func<TSource, TKey> keySelector)
    {
        return source.DistinctBy(keySelector,
                                 EqualityComparer<TKey>.Default);
    }

    public static IEnumerable<TSource> DistinctBy<TSource, TKey>
        (this IEnumerable<TSource> source,
         Func<TSource, TKey> keySelector,
         IEqualityComparer<TKey> comparer)
    {
        if (source == null)
        {
            throw new ArgumentNullException("source");
        }
        if (keySelector == null)
        {
            throw new ArgumentNullException("keySelector");
        }
        if (comparer == null)
        {
            throw new ArgumentNullException("comparer");
        }
        return DistinctByImpl(source, keySelector, comparer);
    }

    private static IEnumerable<TSource> DistinctByImpl<TSource, TKey>
        (IEnumerable<TSource> source,
         Func<TSource, TKey> keySelector,
         IEqualityComparer<TKey> comparer)
    {
        HashSet<TKey> knownKeys = new HashSet<TKey>(comparer);
        foreach (TSource element in source)
        {
            if (knownKeys.Add(keySelector(element)))
            {
                yield return element;
            }
        }
    }