[API Proposal]: Adding ToSortedSet and ToSortedDictionary extension methods to the Enumerable class

[AlexRadch]

Background and motivation

All sets and dictionaries (HashSet, Dictionary, ILookup, ImmutableHashSet, ImmutableSortedSet, ImmutableDictionary, ImmutableSortedDictionary, FrozenSet, FrozenDictionary) have extension methods that convert enumerable to that set/dictionary except SortedSet and SortedDictionary.

I suggest adding ToSortedSet and ToSortedDictionary extension methods to the Enumerable class.

API Proposal

namespace System.Linq;

public static class Enumerable
{
    public static SortedSet<TSource> ToSortedSet<TSource> (this IEnumerable<TSource> source);
    public static SortedSet<TSource> ToSortedSet<TSource> (this IEnumerable<TSource> source,
        IEqualityComparer<TSource>? compare);

    public static SortedDictionary<TKey,TSource> ToSortedDictionary<TSource,TKey> (
        this IEnumerable<TSource> source, Func<TSource,TKey> keySelector);
    public static SortedDictionary<TKey,TSource> ToSortedDictionary<TSource,TKey> (
        this IEnumerable<TSource> source, Func<TSource,TKey> keySelector, IEqualityComparer<TKey>? compare);
    public static SortedDictionary<TKey,TElement> ToSortedDictionary<TSource,TKey,TElement> (
        this IEnumerable<TSource> source, Func<TSource,TKey> keySelector, Func<TSource,TElement> elementSelector);
    public static SortedDictionary<TKey,TElement> ToSortedDictionary<TSource,TKey,TElement> (
        this IEnumerable<TSource> source, Func<TSource,TKey> keySelector, Func<TSource,TElement> elementSelector,
        IEqualityComparer<TKey>? compare);
}

API Usage

The same as ToHashSet and ToDictionary.

Alternative Designs

No response

Risks

No response

[ghost]

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Issue Details

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### Background and motivation All sets and dictionaries (HashSet, Dictionary, ILookup, ImmutableHashSet, ImmutableSortedSet, ImmutableDictionary, ImmutableSortedDictionary, FrozenSet, FrozenDictionary) have extension methods that convert enumerable to that set/dictionary except `SortedSet` and `SortedDictionary`. I suggest adding ToSortedSet and ToSortedDictionary API extension methods to the Enumerable class. ### API Proposal ```csharp namespace System.Linq; public static class Enumerable { public static SortedSet ToSortedSet (this IEnumerable source); public static SortedSet ToSortedSet (this IEnumerable source, IEqualityComparer? compare); public static SortedDictionary ToSortedDictionary ( this IEnumerable source, Func keySelector); public static SortedDictionary ToSortedDictionary ( this IEnumerable source, Func keySelector, IEqualityComparer? compare); public static SortedDictionary ToSortedDictionary ( this IEnumerable source, Func keySelector, Func elementSelector); public static SortedDictionary ToSortedDictionary ( this IEnumerable source, Func keySelector, Func elementSelector, IEqualityComparer? compare); } ``` ### API Usage The same as `ToHashSet` and `ToDictionary`. ### Alternative Designs _No response_ ### Risks _No response_

Author:	AlexRadch
Assignees:	-
Labels:	`api-suggestion`, `area-System.Collections`
Milestone:	-

[eiriktsarpalis]

Note that the immutable/frozen extension methods are defined on the collection types themselves rather than Linq. ToHashSet/ToDictionary/ToLookup are exceptions as they serve as accelerator methods for common lookup patterns. ToSortedSet/ToSortedDictionary don't add much functionality-wise other than providing alternative set/dictionary implementations that are typically slower than their hashtable equivalents.

I don't see how these additions would be useful other than use cases specifically looking to construct the particular collection types. Similar to the immutable/frozen extensions, we might consider adding these to the collection types directly, but I'm not sure that it would be valuable for most users.