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API proposal: Comparison by key #33877
Description
Before
You have to write out the selector twice:
names.Sort((first, second) => string.Compare(
first.Identifier.GetIdentifierText(),
second.Identifier.GetIdentifierText(),
StringComparer.OrdinalIgnoreCase));(Comparer.Sequence is #33873)
statements.OrderBy(GetNameSegments, Comparer.Sequence(
Comparer.Create((first, second) => string.Compare(
first.Identifier.GetIdentifierText(),
second.Identifier.GetIdentifierText(),
StringComparer.OrdinalIgnoreCase)))After
// Proposal 1
names.Sort(Comparer.By(
(SimpleNameSyntax name) => name.Identifier.GetIdentifierText(),
StringComparer.OrdinalIgnoreCase));
// Proposal 2
names.Sort(Comparer<SimpleNameSyntax>.By(
name => name.Identifier.GetIdentifierText(),
StringComparer.OrdinalIgnoreCase))// Proposal 1
statements.OrderBy(GetNameSegments, Comparer.Sequence(Comparer.By(
(SimpleNameSyntax name) => name.Identifier.GetIdentifierText(),
StringComparer.OrdinalIgnoreCase));
// Proposal 2
statements.OrderBy(GetNameSegments, Comparer.Sequence(Comparer<SimpleNameSyntax>.By(
name => name.Identifier.GetIdentifierText(),
StringComparer.OrdinalIgnoreCase))Proposal 1
namespace System.Collections
{
public sealed class Comparer : IComparer, ISerializable
{
+ public static IComparer<T> By<T, TKey>(
+ Func<T, TKey> keySelector,
+ IComparer<TKey>? keyComparer = null);
// No other static members are declared by Comparer
}
}Proposal 2
namespace System.Collections.Generic
{
public abstract partial class Comparer<T> : IComparer, IComparer<T>
{
+ public static IComparer<T> By<TKey>(
+ Func<T, TKey> keySelector,
+ IComparer<TKey>? keyComparer = null);
// Other static members declared by Comparer:
public static Comparer<T> Create(Comparison<T> comparison);
}
}Draft implementation
Click to expand
private sealed class KeyComparer<T, TKey> : IComparer<T>
{
private readonly Func<T, TKey> keySelector;
private readonly IComparer<TKey> keyComparer;
public KeyComparer(Func<T, TKey> keySelector, IComparer<TKey> keyComparer)
{
this.keySelector = keySelector;
this.keyComparer = keyComparer;
}
public int Compare([AllowNull] T x, [AllowNull] T y)
{
return keyComparer.Compare(
keySelector.Invoke(x!),
keySelector.Invoke(y!));
}
}Reacted by Petr Onderka, GSPP, lbmaian, ProphetLamb, Joe Amenta, Johan Larsson, Eirik Tsarpalis, Mr. TA, NAGAYAMA Toshiaki and Tau Gärtli
Activity
svick commentedon Apr 11, 2020
I definitely like this idea, though I think it becomes much more valuable if you add the ability to compare multiple keys. That would mean instead of:
you could write something like:
A NuGet package that implements this already exists, though, to my dismay, it's not very popular.
jnm2 commentedon Apr 11, 2020
@svick One thing I've been favoring even with LINQ instead of ThenBy is to select a tuple:
The only thing this doesn't let you do is specify a comparer for each tuple element, but that's a proposal I have on my list to file separately from this because I want it without selectors too.
eiriktsarpalis commentedon Nov 23, 2020
I would probably prefer proposal 2, since the untyped Comparer type lives in the
System.Collectionsnamespace.lbmaian commentedon Dec 1, 2020
A similar API for
EqualityComparerwould also be a nice counterpart to this proposal. Just replaceIComparerwithIEqualityComparerand have an internalKeyEqualityComparerclass implement the latter.eiriktsarpalis commentedon Apr 19, 2021
FWIW this feels like it could be an overload for Comparer.Create, e.g.
eiriktsarpalis commentedon Oct 29, 2021
I think we might need to submit an API proposal that merges this with #33873 so that we can land on a holistic design.
jnm2 commentedon Oct 29, 2021
@eiriktsarpalis I also want to propose a value tuple comparer so that it's possible to e.g. specify OrdinalIgnoreCase for
Dictionary<(string A, int B), Foo>. Should that be part of the holistic design? I can write it up if so.ValueTupleComparer.Create(StringComparer.OrdinalIgnoreCase, EqualityComparer<int>.Default)or similar, so that bothIComparer<T>andIEqualityComparer<T>can be implemented maybe.eiriktsarpalis commentedon Oct 29, 2021
Go for it. I would recommend creating a fresh issue using the API proposal template so we can start from scratch. Please also check if there are other related issues in the backlog so we can consider including them in the design.
eiriktsarpalis commentedon Oct 29, 2021
@jnm2 there are a few
IEqualityComparer<T>requests as well lurking in the backlog. Would it make sense to merge everything in one big Comparer/EqualityComparer factories proposal? For example there's #44796 and #19644.jnm2 commentedon Oct 29, 2021
@eiriktsarpalis I'm not sure, what's been done before that's similar?
eiriktsarpalis commentedon Nov 1, 2021
I would probably look at the static APIs in the
Comparer<T>andEqualityComparer<T>classes.dotnet-policy-service commentedon Oct 19, 2026
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