[API Proposal]: Make Lookup<TKey,TElement> publicly constructable

[ajcvickers]

Background and motivation

This would allow EF Core to implement ToLookUpAsync in the same way we implement ToDictionaryAsync without needing to go through an intermediate data structure.

See dotnet/efcore#26867

API Proposal

Make current internal Create methods public.

    public static Lookup<TKey, TElement> Create<TSource>(
      IEnumerable<TSource> source,
      Func<TSource, TKey> keySelector,
      Func<TSource, TElement> elementSelector,
      IEqualityComparer<TKey> comparer)
    {
    }

    public static Lookup<TKey, TElement> Create(
      IEnumerable<TElement> source,
      Func<TElement, TKey> keySelector,
      IEqualityComparer<TKey> comparer)
    {
    }

API Usage

var childLookup = await context.ChildEntities
    .Where(c => Enumerable.Contains(parentIds, c.ParentId))
    .ToLookupAsync(c => c.ParentId);

Alternative Designs

No response

Risks

No response

[ghost]

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Issue Details

---

Background and motivation

This would allow EF Core to implement ToLookUpAsync in the same way we implement ToDictionaryAsync without needing to go through an intermediate data structure.

See dotnet/efcore#26867

API Proposal

Make current internal Create methods public.

    public static Lookup<TKey, TElement> Create<TSource>(
      IEnumerable<TSource> source,
      Func<TSource, TKey> keySelector,
      Func<TSource, TElement> elementSelector,
      IEqualityComparer<TKey> comparer)
    {
    }

    public static Lookup<TKey, TElement> Create(
      IEnumerable<TElement> source,
      Func<TElement, TKey> keySelector,
      IEqualityComparer<TKey> comparer)
    {
    }

API Usage

var childLookup = await context.ChildEntities
    .Where(c => Enumerable.Contains(parentIds, c.ParentId))
    .ToLookupAsync(c => c.ParentId);

Alternative Designs

No response

Risks

No response

Author:	ajcvickers
Assignees:	-
Labels:	api-suggestion, area-System.Linq, untriaged
Milestone:	-

[ajcvickers]

/cc @smitpatel

[terrajobst]

Video

• Looks good as proposed
  • We want to avoid adding conflicts with RX/IX so we don't to create extension methods like ToLookupAsync()
  • At the same time we don't want to make Lookup<TKey, TElement> mutable
  • This means we'd need a single entry point that takes an IAsyncEnumerable<TSource> and produces a Lookup<TKey, TElement>
  • Easiest seems to be factory method directly on Lookup<TKey, TElement>
• Alternatively, EF could return a custom implementation of ILookup<TKey, TElement>, similar to PLinq
  • Sounds like we want to explore this first

namespace System.Linq
{
    public partial class Lookup<TKey, TElement>
    {
        public static Task<Lookup<TKey, TElement>> CreateAsync<TSource>(
            IAsyncEnumerable<TSource> source,
            Func<TSource, TKey> keySelector,
            Func<TSource, TElement> elementSelector,
            IEqualityComparer<TKey> comparer);
    }
}

[eiriktsarpalis]

Looks good as proposed

I missed the review, but this seems inconsistent with the points made below.

It seems to me that we can close this until dotnet/efcore#26867 can explore alternative avenues, what say you @ajcvickers?

[ghost]

This issue has been marked needs more info since it may be missing important information. Please refer to our contribution guidelines for tips on how to report issues effectively.

[terrajobst]

Yes, the conclusion was that @smitpatel will try the custom implementation of ILookup<,> first.

[eiriktsarpalis]

Closing in that case, can revisit if the need arises.