[API Proposal]: Add IEnumerable<T>.ToReadOnlyList() and ToReadOnlyCollection() extension methods

[dombrovsky]

Background and motivation

For more straightforward creation of read-only collections from IEnumerable and IAsyncEnumerable.
So it is possible instead of doing Array.AsReadOnly(source.ToArray()) do source.ToReadOnlyCollection().

API Proposal

namespace System.Linq;

public static class Enumerable
{
    public static IReadOnlyCollection<TSource> ToReadOnlyCollection<TSource>(this IEnumerable<TSource> source);

    public static IReadOnlyList<TSource> ToReadOnlyList<TSource>(this IEnumerable<TSource> source);
}

public static class AsyncEnumerable
{
      public static ValueTask<IReadOnlyCollection<TSource>> ToReadOnlyCollectionAsync<TSource>(
            this IAsyncEnumerable<TSource> source,
            CancellationToken cancellationToken = default(CancellationToken));

      public static ValueTask<IReadOnlyList<TSource>> ToReadOnlyListAsync<TSource>(
            this IAsyncEnumerable<TSource> source,
            CancellationToken cancellationToken = default(CancellationToken));
}

API Usage

IReadOnlyCollection<int> collection = Enumerable.Range(0, 10).ToReadOnlyCollection();

Alternative Designs

Use ToImmutableArray() from System.Collections.Immutable.
In this case would be nice to have one for IAsyncEnumerable:

public static ValueTask<ImmutableArray<TSource>> ToImmutableArrayAsync<TSource>(
            this IAsyncEnumerable<TSource> source,
            CancellationToken cancellationToken = default(CancellationToken));

Risks

No response

Tagging subscribers to this area: @dotnet/area-system-linq
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Issue Details

---

Background and motivation

For more straightforward creation of read-only collections from IEnumerable and IAsyncEnumerable.
So it is possible instead of doing Array.AsReadOnly(source.ToArray()) do source.ToReadOnlyCollection().

API Proposal

namespace System.Linq;

public static class Enumerable
{
    public static IReadOnlyCollection<TSource> ToReadOnlyCollection<TSource>(this IEnumerable<TSource> source);

    public static IReadOnlyList<TSource> ToReadOnlyList<TSource>(this IEnumerable<TSource> source);
}

public static class AsyncEnumerable
{
      public static ValueTask<IReadOnlyCollection<TSource>> ToReadOnlyCollectionAsync<TSource>(
            this IAsyncEnumerable<TSource> source,
            CancellationToken cancellationToken = default(CancellationToken));

      public static ValueTask<IReadOnlyList<TSource>> ToReadOnlyListAsync<TSource>(
            this IAsyncEnumerable<TSource> source,
            CancellationToken cancellationToken = default(CancellationToken));
}

API Usage

IReadOnlyCollection<int> collection = Enumerable.Range(0, 10).ToReadOnlyCollection();

Alternative Designs

Use ToImmutableArray() from System.Collections.Immutable.
In this case would be nice to have one for IAsyncEnumerable:

public static ValueTask<ImmutableArray<TSource>> ToImmutableArrayAsync<TSource>(
            this IAsyncEnumerable<TSource> source,
            CancellationToken cancellationToken = default(CancellationToken));

Risks

No response

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

[eiriktsarpalis]

Following the rule of accepting the most generic type and returning the most specific type, what do these additions offer over using ToArray() or ToList()? If the requirement is to return something that is not user-editable (even with a downcast), then why not just do ToArray().AsReadOnly();?

Regarding the IAsyncEnumerable<T> extensions, we've historically resisted adding a System.Linq.Async component (e.g. a ToListAsync proposal was recently rejected).

This issue has been marked needs-author-action and may be missing some important information.

[dombrovsky]

Thanks for quick reply @eiriktsarpalis. I missed AsReadOnly extension method has been added in .net7.

Being able to write source.ToArray().AsReadOnly() resolves the problem this proposal tried to address: to not have to write Array.AsReadOnly(source.ToArray()), which was not an extension method: https://learn.microsoft.com/en-us/dotnet/api/system.array.asreadonly?view=net-7.0.
Also I must admit AsReadOnly(IList) extension method is a better design than proposed ToReadOnlyCollection(IEnumerable)/ToReadOnlyList(IEnumerable).
I believe this issue can be closed.

[dombrovsky]

Does same motivation as for System.Linq.Async apply to System.Collections.Immutable library?
Would it make sense to add IAsyncEnumerable extensions to System.Collections.Immutable?

[eiriktsarpalis]

I don't think so, they are separate namespaces.