Today I learned how to implement a custom Enumerable.DistinctBy extension method that returns distinct elements from a sequence according to a specified key selector function.

.NET 6 and its successors have the method built in within LINQ, but I needed it in a .NET Standard 2.0 class library, so I was out of luck. My implementation is simple, not different from others I found online, and should also work fine with old .NET releases. Here it is:

    public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        var keys = new HashSet<TKey>();
        foreach (var element in source)
            if (keys.Contains(keySelector(element))) continue;
            yield return element;

In the following usage example, I will get back all unique objects from the original sequence, distinct by their Name property:

    var uniques = mySequenceOfObjects.DistinctBy(x => x.Name);

I later went to check the official .NET 6+ implementation. They support an optional equality comparer , which I don’t need, but their base implementation is similar to mine (they use deferred execution as well).

By the way, years after its open-sourcing, I still get thrills when I realize I can always look at, let alone contribute to, the .NET source code.