Simple .DistinctBy Extension

LINQ’s Distinct extension has largely been a disappointment to me. Sure, it’s nice when I’m working with a collection of integers, but more often than not, I’m working with a collection of objects and don’t have an IEqualityComparer<TSource> available to me. I know I could just create one, but I just want to use a lambda like just about everything else I do with LINQ!

To the internet!, right? I learned I could use the following trick to accomplish what I want:

collection
  .GroupBy(x => x.key)
  .Select(x => x.First());

Works like a charm, but I got tired of dot-GroupBy-dot-Select-ing and adding a comment about what I was doing for future maintainers, and I think it’s a lot better to just chuck it into an extension method.

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(
    this IEnumerable<TSource> source, 
    Func<TSource, TKey> keySelector
{
    return
        source
            ?.GroupBy(keySelector)
            .Select(grp => grp.First());
}

Ahh, nice! Alternatively, could score this functionality by adding MoreLINQ to your project. On a neat side-note, you can also cherry-pick which MoreLINQ functionality you want by installing individual packages.

Author: Adam Prescott

I'm enthusiastic and passionate about creating intuitive, great-looking software. I strive to find the simplest solutions to complex problems, and I embrace agile principles and test-driven development. View all posts by Adam Prescott