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

Question

I want to get the distinct values in a list, but not by the standard equality comparison.

What I want to do is something like this:

return myList.Distinct( (x, y) => x.Url == y.Url ); 

I can't, there's no extension method in Linq that will do this - just one that takes an IEqualityComparer.

I can hack around it with this:

return myList.GroupBy( x => x.Url ).Select( g => g.First() ); 

But that seems messy. It also doesn't quite do the same thing - I can only use it here because I have a single key.

I could also add my own:

public static IEnumerable<T> Distinct<T>(      this IEnumerable<T> input, Func<T,T,bool> compare ) {     //write my own here } 

But that does seem rather like writing something that should be there in the first place.

Anyone know why this method isn't there?

Am I missing something?

Answer 1

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;             }         }     } 

Answer 2

But that seems messy.

It's not messy, it's correct.

• If you want Distinct Programmers by FirstName and there are four Amy's, which one do you want?
• If you Group programmers By FirstName and take the First one, then it is clear what you want to do in the case of four Amy's.

I can only use it here because I have a single key.

You can do a multiple key "distinct" with the same pattern:

return myList   .GroupBy( x => new { x.Url, x.Age } )   .Select( g => g.First() );