[API Proposal]: Integrate Reactive's System.Linq.Async APIs into the Base Class Libraries

[wsugarman]

EDITED by @stephentoub 1/21/2025 adding API proposal:

Open questions:

• Naming. I've only used the Async suffix on sinks. Should it be used on others?
• Overloads. Only extensions on IAsyncEnumerable, all sync args or all async args. More combinations?
• Comparers. I've collapsed non-comparer / comparer overloads into a single one with optional comparer. Do we want to re-split back into dedicated overloads?
• CancellationToken. I've only included CancellationToken parameters in the sinks. Do we want them in other methods rather than relying on WithCancellation?
• Removals. I removed Min/Max non-generic overloads in favor of the generic ones. Do we still want them?
• Additions. I added ToAsyncEnumerable(IEnumerable) and ToObservable(IAsyncEnumerable). Anything else critical?
• Tweaks. OfType and Cast now have two generic parameters. Still worthwhile?
• IGrouping. I kept things directly equivalent with the synchronous LINQ methods, but there was feedback about possibly either using IList instead of IEnumerable or using IAsyncEnumerable instead of IEnumerable. Same for ILookup. What do we want to do here?
• SelectMany. What overloads do we want? I have Func<..., IEnumerable>, Func<..., ValueTask<IEnumerable>>, and Func<..., IAsyncEnumerable>. Do we want to only have the last one?

API:
System.Linq.AsyncEnumerable library shipping both in the netcoreapp shared framework and as a nuget package.

namespace System.Linq;

public static partial class AsyncEnumerable
{
    public static ValueTask<TSource> AggregateAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, TSource, CancellationToken, ValueTask<TSource>> func, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> AggregateAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, TSource, TSource> func, CancellationToken cancellationToken = default);
    public static ValueTask<TAccumulate> AggregateAsync<TSource, TAccumulate>(this IAsyncEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, CancellationToken, ValueTask<TAccumulate>> func, CancellationToken cancellationToken = default);
    public static ValueTask<TAccumulate> AggregateAsync<TSource, TAccumulate>(this IAsyncEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func, CancellationToken cancellationToken = default);
    public static ValueTask<TResult> AggregateAsync<TSource, TAccumulate, TResult>(this IAsyncEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, CancellationToken, ValueTask<TAccumulate>> func, Func<TAccumulate, CancellationToken, ValueTask<TResult>> resultSelector, CancellationToken cancellationToken = default);
    public static ValueTask<TResult> AggregateAsync<TSource, TAccumulate, TResult>(this IAsyncEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func, Func<TAccumulate, TResult> resultSelector, CancellationToken cancellationToken = default);
    public static IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> AggregateBy<TSource, TKey, TAccumulate>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TKey, CancellationToken, ValueTask<TAccumulate>> seedSelector, Func<TAccumulate, TSource, CancellationToken, ValueTask<TAccumulate>> func, IEqualityComparer<TKey>? keyComparer = null) where TKey : notnull;
    public static IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> AggregateBy<TSource, TKey, TAccumulate>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, TAccumulate seed, Func<TAccumulate, TSource, CancellationToken, ValueTask<TAccumulate>> func, IEqualityComparer<TKey>? keyComparer = null) where TKey : notnull;
    public static IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> AggregateBy<TSource, TKey, TAccumulate>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, TAccumulate> seedSelector, Func<TAccumulate, TSource, TAccumulate> func, IEqualityComparer<TKey>? keyComparer = null) where TKey : notnull;
    public static IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> AggregateBy<TSource, TKey, TAccumulate>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func, IEqualityComparer<TKey>? keyComparer = null) where TKey : notnull;
    public static ValueTask<bool> AllAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<bool> AllAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<bool> AnyAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<bool> AnyAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<bool> AnyAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static IAsyncEnumerable<TSource> Append<TSource>(this IAsyncEnumerable<TSource> source, TSource element);
    public static ValueTask<decimal> AverageAsync(this IAsyncEnumerable<decimal> source, CancellationToken cancellationToken = default);
    public static ValueTask<double> AverageAsync(this IAsyncEnumerable<double> source, CancellationToken cancellationToken = default);
    public static ValueTask<double> AverageAsync(this IAsyncEnumerable<int> source, CancellationToken cancellationToken = default);
    public static ValueTask<double> AverageAsync(this IAsyncEnumerable<long> source, CancellationToken cancellationToken = default);
    public static ValueTask<decimal?> AverageAsync(this IAsyncEnumerable<decimal?> source, CancellationToken cancellationToken = default);
    public static ValueTask<double?> AverageAsync(this IAsyncEnumerable<double?> source, CancellationToken cancellationToken = default);
    public static ValueTask<double?> AverageAsync(this IAsyncEnumerable<int?> source, CancellationToken cancellationToken = default);
    public static ValueTask<double?> AverageAsync(this IAsyncEnumerable<long?> source, CancellationToken cancellationToken = default);
    public static ValueTask<float?> AverageAsync(this IAsyncEnumerable<float?> source, CancellationToken cancellationToken = default);
    public static ValueTask<float> AverageAsync(this IAsyncEnumerable<float> source, CancellationToken cancellationToken = default);
    public static ValueTask<decimal> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, decimal> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, double> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, long> selector, CancellationToken cancellationToken = default);
    public static ValueTask<decimal?> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, decimal?> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double?> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, double?> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double?> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int?> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double?> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, long?> selector, CancellationToken cancellationToken = default);
    public static ValueTask<float?> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, float?> selector, CancellationToken cancellationToken = default);
    public static ValueTask<float> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, float> selector, CancellationToken cancellationToken = default);
    public static ValueTask<decimal> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<decimal>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<double>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<int>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<long>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<decimal?> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<decimal?>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double?> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<double?>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double?> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<int?>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double?> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<long?>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<float?> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<float?>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<float> AverageAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<float>> selector, CancellationToken cancellationToken = default);
    public static IAsyncEnumerable<TResult> Cast<TSource, TResult>(this IAsyncEnumerable<TSource> source);
    public static IAsyncEnumerable<TSource[]> Chunk<TSource>(this IAsyncEnumerable<TSource> source, int size);
    public static IAsyncEnumerable<TSource> Concat<TSource>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second);
    public static ValueTask<bool> ContainsAsync<TSource>(this IAsyncEnumerable<TSource> source, TSource value, IEqualityComparer<TSource>? comparer = null, CancellationToken cancellationToken = default);
    public static ValueTask<int> CountAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<int> CountAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<int> CountAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static IAsyncEnumerable<KeyValuePair<TKey, int>> CountBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? keyComparer = null) where TKey : notnull;
    public static IAsyncEnumerable<KeyValuePair<TKey, int>> CountBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? keyComparer = null) where TKey : notnull;
    public static IAsyncEnumerable<TSource?> DefaultIfEmpty<TSource>(this IAsyncEnumerable<TSource> source);
    public static IAsyncEnumerable<TSource> DefaultIfEmpty<TSource>(this IAsyncEnumerable<TSource> source, TSource defaultValue);
    public static IAsyncEnumerable<TSource> DistinctBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TSource> DistinctBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TSource> Distinct<TSource>(this IAsyncEnumerable<TSource> source, IEqualityComparer<TSource>? comparer = null);
    public static ValueTask<TSource> ElementAtAsync<TSource>(this IAsyncEnumerable<TSource> source, System.Index index, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> ElementAtAsync<TSource>(this IAsyncEnumerable<TSource> source, int index, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> ElementAtOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, System.Index index, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> ElementAtOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, int index, CancellationToken cancellationToken = default);
    public static IAsyncEnumerable<TResult> Empty<TResult>();
    public static IAsyncEnumerable<TSource> ExceptBy<TSource, TKey>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TKey> second, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TSource> ExceptBy<TSource, TKey>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TKey> second, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TSource> Except<TSource>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second, IEqualityComparer<TSource>? comparer = null);
    public static ValueTask<TSource> FirstAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> FirstAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> FirstAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> FirstOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> FirstOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> FirstOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> FirstOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, TSource defaultValue, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> FirstOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> FirstOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, TSource defaultValue, CancellationToken cancellationToken = default);
    public static IAsyncEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TSource, CancellationToken, ValueTask<TElement>> elementSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TResult> GroupBy<TSource, TKey, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TKey, IEnumerable<TSource>, CancellationToken, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TResult> GroupBy<TSource, TKey, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, IEnumerable<TSource>, TResult> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TSource, CancellationToken, ValueTask<TElement>> elementSelector, Func<TKey, IEnumerable<TElement>, CancellationToken, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TResult> GroupBy<TSource, TKey, TElement, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, Func<TKey, IEnumerable<TElement>, TResult> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(this IAsyncEnumerable<TOuter> outer, IAsyncEnumerable<TInner> inner, Func<TOuter, CancellationToken, ValueTask<TKey>> outerKeySelector, Func<TInner, CancellationToken, ValueTask<TKey>> innerKeySelector, Func<TOuter, IEnumerable<TInner>, CancellationToken, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(this IAsyncEnumerable<TOuter> outer, IAsyncEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<(int Index, TSource Item)> Index<TSource>(this IAsyncEnumerable<TSource> source);
    public static IAsyncEnumerable<TSource> IntersectBy<TSource, TKey>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TKey> second, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TSource> IntersectBy<TSource, TKey>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TKey> second, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TSource> Intersect<TSource>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second, IEqualityComparer<TSource>? comparer = null);
    public static IAsyncEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this IAsyncEnumerable<TOuter> outer, IAsyncEnumerable<TInner> inner, Func<TOuter, CancellationToken, ValueTask<TKey>> outerKeySelector, Func<TInner, CancellationToken, ValueTask<TKey>> innerKeySelector, Func<TOuter, TInner, CancellationToken, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(this IAsyncEnumerable<TOuter> outer, IAsyncEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, TInner, TResult> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static ValueTask<TSource> LastAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> LastAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> LastAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> LastOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> LastOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> LastOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> LastOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, TSource defaultValue, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> LastOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> LastOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, TSource defaultValue, CancellationToken cancellationToken = default);
    public static IAsyncEnumerable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(this IAsyncEnumerable<TOuter> outer, IAsyncEnumerable<TInner> inner, Func<TOuter, CancellationToken, ValueTask<TKey>> outerKeySelector, Func<TInner, CancellationToken, ValueTask<TKey>> innerKeySelector, Func<TOuter, TInner?, CancellationToken, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(this IAsyncEnumerable<TOuter> outer, IAsyncEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, TInner?, TResult> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static ValueTask<long> LongCountAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<long> LongCountAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<long> LongCountAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static ValueTask<decimal> MaxAsync(this IAsyncEnumerable<decimal> source, CancellationToken cancellationToken = default);
    public static ValueTask<double> MaxAsync(this IAsyncEnumerable<double> source, CancellationToken cancellationToken = default);
    public static ValueTask<int> MaxAsync(this IAsyncEnumerable<int> source, CancellationToken cancellationToken = default);
    public static ValueTask<long> MaxAsync(this IAsyncEnumerable<long> source, CancellationToken cancellationToken = default);
    public static ValueTask<decimal?> MaxAsync(this IAsyncEnumerable<decimal?> source, CancellationToken cancellationToken = default);
    public static ValueTask<double?> MaxAsync(this IAsyncEnumerable<double?> source, CancellationToken cancellationToken = default);
    public static ValueTask<int?> MaxAsync(this IAsyncEnumerable<int?> source, CancellationToken cancellationToken = default);
    public static ValueTask<long?> MaxAsync(this IAsyncEnumerable<long?> source, CancellationToken cancellationToken = default);
    public static ValueTask<float?> MaxAsync(this IAsyncEnumerable<float?> source, CancellationToken cancellationToken = default);
    public static ValueTask<float> MaxAsync(this IAsyncEnumerable<float> source, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> MaxAsync<TSource>(this IAsyncEnumerable<TSource> source, IComparer<TSource>? comparer, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> MaxAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static ValueTask<TResult?> MaxAsync<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TResult>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<TResult?> MaxAsync<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TResult> selector, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> MaxByAsync<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IComparer<TKey>? comparer = null, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> MaxByAsync<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IComparer<TKey>? comparer = null, CancellationToken cancellationToken = default);
    public static ValueTask<decimal> MinAsync(this IAsyncEnumerable<decimal> source, CancellationToken cancellationToken = default);
    public static ValueTask<double> MinAsync(this IAsyncEnumerable<double> source, CancellationToken cancellationToken = default);
    public static ValueTask<int> MinAsync(this IAsyncEnumerable<int> source, CancellationToken cancellationToken = default);
    public static ValueTask<long> MinAsync(this IAsyncEnumerable<long> source, CancellationToken cancellationToken = default);
    public static ValueTask<decimal?> MinAsync(this IAsyncEnumerable<decimal?> source, CancellationToken cancellationToken = default);
    public static ValueTask<double?> MinAsync(this IAsyncEnumerable<double?> source, CancellationToken cancellationToken = default);
    public static ValueTask<int?> MinAsync(this IAsyncEnumerable<int?> source, CancellationToken cancellationToken = default);
    public static ValueTask<long?> MinAsync(this IAsyncEnumerable<long?> source, CancellationToken cancellationToken = default);
    public static ValueTask<float?> MinAsync(this IAsyncEnumerable<float?> source, CancellationToken cancellationToken = default);
    public static ValueTask<float> MinAsync(this IAsyncEnumerable<float> source, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> MinAsync<TSource>(this IAsyncEnumerable<TSource> source, IComparer<TSource>? comparer, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> MinAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static ValueTask<TResult?> MinAsync<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TResult>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<TResult?> MinAsync<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TResult> selector, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> MinByAsync<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IComparer<TKey>? comparer = null, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> MinByAsync<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IComparer<TKey>? comparer = null, CancellationToken cancellationToken = default);
    public static IAsyncEnumerable<TResult> OfType<TSource, TResult>(this IAsyncEnumerable<TSource> source);
    public static IOrderedAsyncEnumerable<TSource> OrderByDescending<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IComparer<TKey>? comparer = null);
    public static IOrderedAsyncEnumerable<TSource> OrderByDescending<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IComparer<TKey>? comparer = null);
    public static IOrderedAsyncEnumerable<TSource> OrderBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IComparer<TKey>? comparer = null);
    public static IOrderedAsyncEnumerable<TSource> OrderBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IComparer<TKey>? comparer = null);
    public static IOrderedAsyncEnumerable<T> OrderDescending<T>(this IAsyncEnumerable<T> source, IComparer<T>? comparer = null);
    public static IOrderedAsyncEnumerable<T> Order<T>(this IAsyncEnumerable<T> source, IComparer<T>? comparer = null);
    public static IAsyncEnumerable<TSource> Prepend<TSource>(this IAsyncEnumerable<TSource> source, TSource element);
    public static IAsyncEnumerable<int> Range(int start, int count);
    public static IAsyncEnumerable<TResult> Repeat<TResult>(TResult element, int count);
    public static IAsyncEnumerable<TSource> Reverse<TSource>(this IAsyncEnumerable<TSource> source);
    public static IAsyncEnumerable<TResult> RightJoin<TOuter, TInner, TKey, TResult>(this IAsyncEnumerable<TOuter> outer, IAsyncEnumerable<TInner> inner, Func<TOuter, CancellationToken, ValueTask<TKey>> outerKeySelector, Func<TInner, CancellationToken, ValueTask<TKey>> innerKeySelector, Func<TOuter?, TInner, CancellationToken, ValueTask<TResult>> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TResult> RightJoin<TOuter, TInner, TKey, TResult>(this IAsyncEnumerable<TOuter> outer, IAsyncEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter?, TInner, TResult> resultSelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, IAsyncEnumerable<TResult>> selector);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, IAsyncEnumerable<TResult>> selector);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, IEnumerable<TResult>> selector);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, CancellationToken, ValueTask<IEnumerable<TResult>>> selector);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<IEnumerable<TResult>>> selector);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, IAsyncEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, CancellationToken, ValueTask<TResult>> resultSelector);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, IAsyncEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, CancellationToken, ValueTask<TResult>> resultSelector);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, CancellationToken, ValueTask<IEnumerable<TCollection>>> collectionSelector, Func<TSource, TCollection, CancellationToken, ValueTask<TResult>> resultSelector);
    public static IAsyncEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<IEnumerable<TCollection>>> collectionSelector, Func<TSource, TCollection, CancellationToken, ValueTask<TResult>> resultSelector);
    public static IAsyncEnumerable<TResult> Select<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, CancellationToken, ValueTask<TResult>> selector);
    public static IAsyncEnumerable<TResult> Select<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, TResult> selector);
    public static IAsyncEnumerable<TResult> Select<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TResult>> selector);
    public static IAsyncEnumerable<TResult> Select<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TResult> selector);
    public static ValueTask<bool> SequenceEqualAsync<TSource>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second, IEqualityComparer<TSource>? comparer = null, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> SingleAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> SingleAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> SingleAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> SingleOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> SingleOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> SingleOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> SingleOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, TSource defaultValue, CancellationToken cancellationToken = default);
    public static ValueTask<TSource?> SingleOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static ValueTask<TSource> SingleOrDefaultAsync<TSource>(this IAsyncEnumerable<TSource> source, TSource defaultValue, CancellationToken cancellationToken = default);
    public static IAsyncEnumerable<TSource> SkipLast<TSource>(this IAsyncEnumerable<TSource> source, int count);
    public static IAsyncEnumerable<TSource> SkipWhile<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate);
    public static IAsyncEnumerable<TSource> SkipWhile<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int, bool> predicate);
    public static IAsyncEnumerable<TSource> SkipWhile<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int, CancellationToken, ValueTask<bool>> predicate);
    public static IAsyncEnumerable<TSource> SkipWhile<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate);
    public static IAsyncEnumerable<TSource> Skip<TSource>(this IAsyncEnumerable<TSource> source, int count);
    public static ValueTask<decimal> SumAsync(this IAsyncEnumerable<decimal> source, CancellationToken cancellationToken = default);
    public static ValueTask<double> SumAsync(this IAsyncEnumerable<double> source, CancellationToken cancellationToken = default);
    public static ValueTask<int> SumAsync(this IAsyncEnumerable<int> source, CancellationToken cancellationToken = default);
    public static ValueTask<long> SumAsync(this IAsyncEnumerable<long> source, CancellationToken cancellationToken = default);
    public static ValueTask<decimal?> SumAsync(this IAsyncEnumerable<decimal?> source, CancellationToken cancellationToken = default);
    public static ValueTask<double?> SumAsync(this IAsyncEnumerable<double?> source, CancellationToken cancellationToken = default);
    public static ValueTask<int?> SumAsync(this IAsyncEnumerable<int?> source, CancellationToken cancellationToken = default);
    public static ValueTask<long?> SumAsync(this IAsyncEnumerable<long?> source, CancellationToken cancellationToken = default);
    public static ValueTask<float?> SumAsync(this IAsyncEnumerable<float?> source, CancellationToken cancellationToken = default);
    public static ValueTask<float> SumAsync(this IAsyncEnumerable<float> source, CancellationToken cancellationToken = default);
    public static ValueTask<decimal> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, decimal> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, double> selector, CancellationToken cancellationToken = default);
    public static ValueTask<int> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int> selector, CancellationToken cancellationToken = default);
    public static ValueTask<long> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, long> selector, CancellationToken cancellationToken = default);
    public static ValueTask<decimal?> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, decimal?> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double?> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, double?> selector, CancellationToken cancellationToken = default);
    public static ValueTask<int?> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int?> selector, CancellationToken cancellationToken = default);
    public static ValueTask<long?> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, long?> selector, CancellationToken cancellationToken = default);
    public static ValueTask<float?> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, float?> selector, CancellationToken cancellationToken = default);
    public static ValueTask<float> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, float> selector, CancellationToken cancellationToken = default);
    public static ValueTask<decimal> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<decimal>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<double>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<int> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<int>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<long> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<long>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<decimal?> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<decimal?>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<double?> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<double?>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<int?> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<int?>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<long?> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<long?>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<float?> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<float?>> selector, CancellationToken cancellationToken = default);
    public static ValueTask<float> SumAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<float>> selector, CancellationToken cancellationToken = default);
    public static IAsyncEnumerable<TSource> TakeLast<TSource>(this IAsyncEnumerable<TSource> source, int count);
    public static IAsyncEnumerable<TSource> TakeWhile<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate);
    public static IAsyncEnumerable<TSource> TakeWhile<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int, bool> predicate);
    public static IAsyncEnumerable<TSource> TakeWhile<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int, CancellationToken, ValueTask<bool>> predicate);
    public static IAsyncEnumerable<TSource> TakeWhile<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate);
    public static IAsyncEnumerable<TSource> Take<TSource>(this IAsyncEnumerable<TSource> source, int count);
    public static IAsyncEnumerable<TSource> Take<TSource>(this IAsyncEnumerable<TSource> source, System.Range range);
    public static IOrderedAsyncEnumerable<TSource> ThenByDescending<TSource, TKey>(this IOrderedAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IComparer<TKey>? comparer = null);
    public static IOrderedAsyncEnumerable<TSource> ThenByDescending<TSource, TKey>(this IOrderedAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IComparer<TKey>? comparer = null);
    public static IOrderedAsyncEnumerable<TSource> ThenBy<TSource, TKey>(this IOrderedAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IComparer<TKey>? comparer = null);
    public static IOrderedAsyncEnumerable<TSource> ThenBy<TSource, TKey>(this IOrderedAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IComparer<TKey>? comparer = null);
    public static ValueTask<TSource[]> ToArrayAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static IAsyncEnumerable<TSource> ToAsyncEnumerable<TSource>(this IEnumerable<TSource> source);
    public static ValueTask<Dictionary<TKey, TValue>> ToDictionaryAsync<TKey, TValue>(this IAsyncEnumerable<KeyValuePair<TKey, TValue>> source, IEqualityComparer<TKey>? comparer = null, CancellationToken cancellationToken = default) where TKey : notnull;
    public static ValueTask<Dictionary<TKey, TValue>> ToDictionaryAsync<TKey, TValue>(this IAsyncEnumerable<(TKey Key, TValue Value)> source, IEqualityComparer<TKey>? comparer = null, CancellationToken cancellationToken = default) where TKey : notnull;
    public static ValueTask<Dictionary<TKey, TSource>> ToDictionaryAsync<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer = null, CancellationToken cancellationToken = default) where TKey : notnull;
    public static ValueTask<Dictionary<TKey, TSource>> ToDictionaryAsync<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer = null, CancellationToken cancellationToken = default) where TKey : notnull;
    public static ValueTask<Dictionary<TKey, TElement>> ToDictionaryAsync<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TSource, CancellationToken, ValueTask<TElement>> elementSelector, IEqualityComparer<TKey>? comparer = null, CancellationToken cancellationToken = default) where TKey : notnull;
    public static ValueTask<Dictionary<TKey, TElement>> ToDictionaryAsync<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey>? comparer = null, CancellationToken cancellationToken = default) where TKey : notnull;
    public static ValueTask<HashSet<TSource>> ToHashSetAsync<TSource>(this IAsyncEnumerable<TSource> source, IEqualityComparer<TSource>? comparer = null, CancellationToken cancellationToken = default);
    public static ValueTask<List<TSource>> ToListAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default);
    public static ValueTask<ILookup<TKey, TSource>> ToLookupAsync<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer = null, CancellationToken cancellationToken = default);
    public static ValueTask<ILookup<TKey, TSource>> ToLookupAsync<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer = null, CancellationToken cancellationToken = default);
    public static ValueTask<ILookup<TKey, TElement>> ToLookupAsync<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, Func<TSource, CancellationToken, ValueTask<TElement>> elementSelector, IEqualityComparer<TKey>? comparer = null, CancellationToken cancellationToken = default);
    public static ValueTask<ILookup<TKey, TElement>> ToLookupAsync<TSource, TKey, TElement>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey>? comparer = null, CancellationToken cancellationToken = default);
    public static IObservable<TSource> ToObservable<TSource>(this IAsyncEnumerable<TSource> source);
    public static IAsyncEnumerable<TSource> UnionBy<TSource, TKey>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TSource> UnionBy<TSource, TKey>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer = null);
    public static IAsyncEnumerable<TSource> Union<TSource>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second, IEqualityComparer<TSource>? comparer = null);
    public static IAsyncEnumerable<TSource> Where<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate);
    public static IAsyncEnumerable<TSource> Where<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int, bool> predicate);
    public static IAsyncEnumerable<TSource> Where<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int, CancellationToken, ValueTask<bool>> predicate);
    public static IAsyncEnumerable<TSource> Where<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate);
    public static IAsyncEnumerable<(TFirst First, TSecond Second)> Zip<TFirst, TSecond>(this IAsyncEnumerable<TFirst> first, IAsyncEnumerable<TSecond> second);
    public static IAsyncEnumerable<(TFirst First, TSecond Second, TThird Third)> Zip<TFirst, TSecond, TThird>(this IAsyncEnumerable<TFirst> first, IAsyncEnumerable<TSecond> second, IAsyncEnumerable<TThird> third);
    public static IAsyncEnumerable<TResult> Zip<TFirst, TSecond, TResult>(this IAsyncEnumerable<TFirst> first, IAsyncEnumerable<TSecond> second, Func<TFirst, TSecond, CancellationToken, ValueTask<TResult>> resultSelector);
    public static IAsyncEnumerable<TResult> Zip<TFirst, TSecond, TResult>(this IAsyncEnumerable<TFirst> first, IAsyncEnumerable<TSecond> second, Func<TFirst, TSecond, TResult> resultSelector);
}
public partial interface IOrderedAsyncEnumerable<out TElement> : IAsyncEnumerable<TElement>
{
    IOrderedAsyncEnumerable<TElement> CreateOrderedAsyncEnumerable<TKey>(Func<TElement, CancellationToken, ValueTask<TKey>> keySelector, IComparer<TKey>? comparer, bool descending);
    IOrderedAsyncEnumerable<TElement> CreateOrderedAsyncEnumerable<TKey>(Func<TElement, TKey> keySelector, IComparer<TKey>? comparer, bool descending);
}

---

Background and motivation

As IAsyncEnumerable<T> increases its presence in the BCLs and user libraries, and as new language constructs are added to support asynchronous streams, I think libraries like System.Linq.Async will become more crucial to their adoption by developers. Personally, as I work with our own Azure SDKs, I find myself quickly importing the package to easily aggregate paginated results using ToListAsync(CancellationToken) without a loop while Azure table queries can be easily aggregated locally using GroupBy.

The LINQ APIs have become incredibly popular in .NET, and I think the lack of LINQ support for asynchronous streams is becoming more apparent. I am especially worried that until recently the entire Reactive project itself was at risk of shutting down without active maintainers! The asynchronous LINQ APIs are far too important to leave outside of the BCLs where their support may disappear.

Why should the System.Linq namespace only include support for IEnumerable<T> when System.Collections.Generic includes IAsyncEnumerable<T>?

API Proposal

Functionally equivalent LINQ APIs (as detailed in this older MSDN documentation) used by the asynchronous equivalents of IEnumerable<T>, IOrderedEnumerable<TElement>, and IGrouping<TKey, TElement>. The current library exists here.

This may also be a good opportunity to re-evaluate these APIs if desired.

E.g.

namespace System.Linq;

public static partial class AsyncEnumerable
{
    public static IAsyncEnumerable<TResult> Select<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TResult> selector);
}

API Usage

using System.Collections.Generic;
using System.Linq;

IAsyncEnumerable<Element> elements = /* ... */;
List<string> ids = await elements
    .Where(x => x.IsValid)
    .Select(x => x.Id)
    .Take(10)
    .OrderBy(x => x.Id)
    .ToListAsync(cancellationToken);

Alternative Designs

Alternatively, these could be kept out-of-band.

Risks

While this may include a large number of APIs, they will be quite familiar to those who use LINQ today on their synchronous counterparts.

Tagging subscribers to this area: @dotnet/area-system-linq
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Background and motivation

As IAsyncEnumerable<T> increases its presence in the BCLs and user libraries, and as new language constructs are added to support asynchronous streams, I think libraries like System.Linq.Async will become more crucial to their adoption by developers. Personally, as I work with our own Azure SDKs, I find myself quickly importing the package to easily aggregate paginated results using ToListAsync(CancellationToken) without a loop while Azure table queries can be easily aggregated locally using GroupBy.

The LINQ APIs have become incredibly popular in .NET, and I think the lack of LINQ support for asynchronous streams is becoming more apparent. I am especially worried that until recently the entire Reactive project itself was at risk of shutting down without active maintainers! The asynchronous LINQ APIs are far too important to leave outside of the BCLs where their support may disappear.

Why should the System.Linq namespace only include support for IEnumerable<T> when System.Collections.Generic includes IAsyncEnumerable<T>?

API Proposal

Functionally equivalent LINQ APIs (as detailed in this older MSDN documentation) used by the asynchronous equivalents of IEnumerable<T>, IOrderedEnumerable<TElement>, and IGrouping<TKey, TElement>. The current library exists here.

This may also be a good opportunity to re-evaluate these APIs if desired.

E.g.

namespace System.Linq;

public static partial class AsyncEnumerable
{
    public static IAsyncEnumerable<TResult> Select<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TResult> selector);
}

API Usage

using System.Collections.Generic;
using System.Linq;

IAsyncEnumerable<Element> elements = /* ... */;
List<string> ids = await elements
    .Where(x => x.IsValid)
    .Select(x => x.Id)
    .Take(10)
    .OrderBy(x => x.Id)
    .ToListAsync(cancellationToken);

Alternative Designs

Alternatively, these could be kept out-of-band.

Risks

While this may include a large number of APIs, they will be quite familiar to those who use LINQ today on their synchronous counterparts.

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

[jkotas]

Duplicate of #31580

[wsugarman]

While this was indeed discussed previously, in the time since #31580 it has been discovered that there were no active maintainers for Reactive until volunteers came forward this October. And there haven't been any commits since April!

I think it's worth another discussion of whether this API (or a subset) should be deemed "core" enough to be included in the BCLs.

[eiriktsarpalis]

While this was indeed discussed previously, in the time since #31580 it has been discovered that there were no active maintainers for Reactive until volunteers came forward this October. And there haven't been any commits since April!

I'm curious, why would moving the library to the shared framework solve the problem of maintenance? If anything, it would make the components more difficult to evolve, since it would prevent introducing breaking changes or versioning it independently of dotnet. There are clear benefits to keeping some components out of the shared framework.

[wsugarman]

I'm curious, why would moving the library to the shared framework solve the problem of maintenance? If anything, it would make the components more difficult to evolve, since it would prevent introducing breaking changes or versioning it independently of dotnet. There are clear benefits to keeping some components out of the shared framework.

I am not (yet) concerned about the speed of changes in the shared framework. Instead, I am more concerned with making sure someone is responsible for maintaining these (what I think are) critical APIs. While it seems that some dotnet foundation members have charitably helped the project in the past, it now appears that the project is entirely dependent on the kindness of unpaid volunteers.

It's wonderful that developers have come forward to help, and for many OSS projects this model is sufficient, but I wonder if these APIs are too important to leave outside the purview of Microsoft. What happens when the current developers move onto other projects? Will the community continue to be so lucky that new volunteers will step forward? It seems that the recent questions of ownership have only arisen due to enough GitHub issues.

Instead, I think the core System.Linq.Async APIs would benefit greatly from being included in the shared library for the following reasons:

1. A team of developers at Microsoft would be responsible for their ongoing development, maintenance, and quality, ensuring that they continue to grow as both .NET and async streams evolve
2. Developers can easily discover and use the async stream LINQ APIs without the use of an external package
   • Today, even performing a simple projection from an IAsyncEnumerable<T> requires a NuGet package, but why should that be the case if it can be done for IEnumerable<T>? I think we'd be hard pressed to explain why there are language constructs for IEnumerable<T> and IAsyncEnumerable<T> like foreach and await foreach, but the LINQ APIs are only included for IEnumerable<T>.
3. A whole new set of features could be possible, such as adding support for asynchronous LINQ queries in a future dotnet SDK!

[eiriktsarpalis]

I am not (yet) concerned about the speed of changes in the shared framework.

It's not an issue of speed, it is an issue of possibility. Most types of source and binary breaking changes are not allowed for shared framework components, ever.

A team of developers at Microsoft would be responsible for their ongoing development, maintenance, and quality, ensuring that they continue to grow as both .NET and async streams evolve

Bare minimum, it would guarantee security fixes but it's not necessarily the case that we'd be able to fund continued evolution of the library.

There's also the issue of code size, we've rejected API proposals in regular Linq in the past out of size concerns.

[wsugarman]

It's not an issue of speed, it is an issue of possibility. Most types of source and binary breaking changes are not allowed for shared framework components, ever.

@eiriktsarpalis I'll defer to you as one of the maintainers of the System.Linq namespace. Do you feel ever hindered by being included in the shared framework? Based on your other comments, has your team considered an out-of-band package (or perhaps an extension package) for new LINQ APIs to help manage the size?

My biggest concern here is that there is (what I believe to be) a critical library in System.Linq.Async. I believe async streams' usability depends on the presence of this 3rd party library, and without some sort of formal funding from the dotnet org/Microsoft its future remains too uncertain.

As argued in this issue, I think its most logical home -- ignoring the history of how the library came to be and perhaps the burdens of being in the shared library -- is within the BCLs; why should the treatment of the LINQ APIs differ between IEnumerable<T> and IAsyncEnumerable<T>? If it's just a matter of their development history, should that be reconciled? However, if it is believed that the cost of integrating this library into the BCL is too high, then I'm certainly open to alternatives for a more formal investment of engineering resources into this library. Although, it sounds like there should also be a separate discussion around how future LINQ APIs should be added given their burdensome size.

Perhaps most importantly, as the new owner of the library @HowardvanRooijen, would you prefer more direct investment by the dotnet team in the System.Linq.Async library? If so, would you have a preference as to how?

[HowardvanRooijen]

FYI first alpha release of System.Reactive.Async is available on NuGet: https://www.nuget.org/packages/System.Reactive.Async/6.0.0-alpha.3