Add System.Linq.AsyncEnumerable

[stephentoub]

Closes #79782

For the implementation, I've favored clarity/simplicity over lots of optimizations we don't know to be valuable in this context. We can add more specialization in as we see a strong need. The equation for LINQ over IAsyncEnumerable is different for over IEnumerable, where a) we're typically dealing with asynchronous streams of data rather than in-memory, contiguous collections and b) efficient implementations of the interfaces are complicated.

For the tests, I've mainly used the equivalent LINQ functionality as an oracle.

cc: @eiriktsarpalis, @idg10, @HowardvanRooijen

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[Copilot]

Copilot reviewed 125 out of 140 changed files in this pull request and generated no comments.

Files not reviewed (15)

• src/installer/pkg/sfx/Microsoft.NETCore.App/PackageOverrides.txt: Language not supported
• src/libraries/NetCoreAppLibrary.props: Language not supported
• src/libraries/System.Linq.AsyncEnumerable/Directory.Build.props: Language not supported
• src/libraries/System.Linq.AsyncEnumerable/System.Linq.AsyncEnumerable.sln: Language not supported
• src/libraries/System.Linq.AsyncEnumerable/ref/System.Linq.AsyncEnumerable.csproj: Language not supported
• src/libraries/System.Linq.AsyncEnumerable/src/Resources/Strings.resx: Language not supported
• src/libraries/System.Linq.AsyncEnumerable/src/System.Linq.AsyncEnumerable.csproj: Language not supported
• src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/ContainsAsync.cs: Evaluated as low risk
• src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/Chunk.cs: Evaluated as low risk
• src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/Cast.cs: Evaluated as low risk
• src/libraries/System.Linq.AsyncEnumerable/ref/System.Linq.AsyncEnumerable.cs: Evaluated as low risk
• src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/AsyncEnumerable.cs: Evaluated as low risk
• src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/AverageAsync.cs: Evaluated as low risk
• src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/Append.cs: Evaluated as low risk
• src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/AggregateBy.cs: Evaluated as low risk

Comments suppressed due to low confidence (2)

src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/AnyAsync.cs:56

• The predicate parameter should be checked for null before being used.

Func<TSource, bool> predicate

src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/AnyAsync.cs:93

• The predicate parameter should be checked for null before being used.

Func<TSource, CancellationToken, ValueTask<bool>> predicate

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[eiriktsarpalis]

Thanks

[stephentoub]

@ViktorHofer, is this acceptable?

See https://aka.ms/dotnet/prebuilts for guidance on what pre-builts are and how to eliminate them.
Package IDs are:
System.ValueTuple.4.5.0
  Prebuilt usages are different from the baseline found at '/__w/1/s/eng/SourceBuildPrebuiltBaseline.xml'. If it's acceptable to update the baseline, copy the contents of the automatically generated baseline '/__w/1/s/artifacts/sb/prebuilt-report/generated-new-baseline.xml'.

[stephentoub]

@idg10, @HowardvanRooijen, as maintainers of the existing System.Linq.Async, you stated you want to deprecate that and push developers to use an implementation built into .NET. Before we merge this, I just want to triple check that's still your desired outcome 😄 Any feedback before this merges?

[HowardvanRooijen]

Very much so! It's been great to watch this PR & discussion.

[jkotas]

System.ValueTuple.4.5.0
prebuilt usages are different from the baseline

All other places in the repo reference the ValueTuple package for .NET Framework only:

runtime/src/libraries/System.Formats.Cbor/src/System.Formats.Cbor.csproj

Lines 60 to 62 in 7d75878

	<ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework'">
	<PackageReference Include="System.ValueTuple" Version="$(SystemValueTupleVersion)" />
	</ItemGroup>

. Do the same here?

[stephentoub]

What's being disposed here ? What the dispose code look like ? Does it try to .Wait ?

That's a compiler-generated dispose method for an async iterator. Here's what it looks like in sharplab.

            [DebuggerHidden]
            ValueTask IAsyncDisposable.DisposeAsync()
            {
                if (<>1__state >= -1)
                {
                    throw new NotSupportedException();
                }
                if (<>1__state == -2)
                {
                    return default(ValueTask);
                }
                <>w__disposeMode = true;
                <>v__promiseOfValueOrEnd.Reset();
                <<Prepend>g__Impl|0_0>d<TSource> stateMachine = this;
                <>t__builder.MoveNext(ref stateMachine);
                return new ValueTask(this, <>v__promiseOfValueOrEnd.Version);
            }

It does not do any synchronous blocking.

Or maybe it's some shape of SIMD that we don't have on WASM yet?

There's no use of SIMD in this library.

[stephentoub]

It does not do any synchronous blocking.

DisposeAsync doesn't, but I just pushed a commit that removes three occurrences of calls to an xunit Assert.Equal overload that was doing synchronous blocking while enumerating an async enumerable. As these three occurrences look to correspond to the same methods that were hitting these strange failures, I suspect this commit is going to fix the issue in this PR. If it does, we then need to figure out why the failure mode was resulting in erroneous accesses to the generated iterator.
cc: @jcouv just for visibility

[nietras]

@stephentoub question, why is for example TSource not annotated with allows ref struct when appropriate?, asking cause I have a library with ref struct as TSource.

[stephentoub]

@stephentoub question, why is for example TSource not annotated with allows ref struct when appropriate?, asking cause I have a library with ref struct as TSource.

Can you share the cases you see where it would be appropriate?

[nietras]

My open source csv library Sep, where row or column points to internal state that should not be stored on heap, to avoid programmer errors or similar.

It seems backwards to me to ask for use case, though, why should TSource not be allowed ref struct when no issue with it? Use cases will come, can't predict all.

[stephentoub]

I'm asking about which methods in this PR it would be relevant to. It wouldn't work for anything using an async iterator where the value would be yielded or otherwise captured, for example. Are you specifically asking about TSource rather than TResult and in cases where it wouldn't need to be stored, passed through a value task, etc?

[nietras]

Yes with regards to TSource, Count() is an example, Sum(), Select() presumably too. I don't have a complete list. I'd suggest this being added also to sync LINQ, where also issue, afaik.

[stephentoub]

Yes with regards to TSource, Count() is an example, Sum(), Select() presumably too. I don't have a complete list. I'd suggest this being added also to sync LINQ, where also issue, afaik.

Please feel free to open an API proposal for it.

Note that as the code is written, most of those won't actually compile, either. It'd require either rewriting to not use WithCancellation and friends, or updating those as well to also support ref structs.

[timcassell]

@stephentoub question, why is for example TSource not annotated with allows ref struct when appropriate?, asking cause I have a library with ref struct as TSource.

How? IAsyncEnumerable<T> interface does not allow ref struct. I imagine it's even more difficult to support that for IAsyncEnumerable<T> than it is to support it for IEnumerable<T>, which also does not.

[stephentoub]

@stephentoub question, why is for example TSource not annotated with allows ref struct when appropriate?, asking cause I have a library with ref struct as TSource.

How? IAsyncEnumerable<T> interface does not allow ref struct. I imagine it's even more difficult to support that for IAsyncEnumerable<T> than it is to support it for IEnumerable<T>, which also does not.

They both do in .NET 9.

[timcassell]

@stephentoub question, why is for example TSource not annotated with allows ref struct when appropriate?, asking cause I have a library with ref struct as TSource.

How? IAsyncEnumerable<T> interface does not allow ref struct. I imagine it's even more difficult to support that for IAsyncEnumerable<T> than it is to support it for IEnumerable<T>, which also does not.

They both do in .NET 9.

Ah, I didn't realize. It's not shown on the documentation. https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.iasyncenumerable-1?view=net-9.0

[nietras]

API proposal #111830