8000 System.Linq.Async is being assimilated into .NET 10 but System.Linq.AsyncEnumerable doesn't seem to be on parity and breaks pre-.NET 10 code. · Issue #49162 · dotnet/sdk · GitHub
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System.Linq.Async is being assimilated into .NET 10 but System.Linq.AsyncEnumerable doesn't seem to be on parity and breaks pre-.NET 10 code. #49162

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Xen0byte opened this issue May 27, 2025 · 3 comments
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System.Linq.Async is being assimilated into .NET 10 but System.Linq.AsyncEnumerable doesn't seem to be on parity and breaks pre-.NET 10 code. #49162

Xen0byte opened this issue May 27, 2025 · 3 comments
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@Xen0byte
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Describe the bug

It seems like async lambdas are no longer supported in System.Linq.AsyncEnumerable in .NET 10 but at the same time the compiler will raise an ambiguous reference error if System.Linq.Async is installed.

To Reproduce

previously, with System.Linq.Async, something along the lines of the following was possible

int number = await new List<int>() { 1, 2, 3 }.ToAsyncEnumerable()
    .SingleAwaitAsync(async (value) => { await Task.Delay(500); return value.Equals(2); });

however, with System.Linq.AsyncEnumerable, the equivalent doesn't compile

int number = await new List<int>() { 1, 2, 3 }.ToAsyncEnumerable()
    .SingleAsync(async (value) => { await Task.Delay(500); return value.Equals(2); });

Exceptions (if any)

Cannot convert async lambda expression to delegate type 'Func<int, bool>'.
An async lambda expression may return void, Task or Task<T>, none of which are convertible to 'Func<int, bool>'.

Further technical details

  • Visual Studio Enterprise 2022 (64-bit) 17.14.2 Preview 1.0
  • .NET 10.0.100-preview.4.25258.110
@dotnet-policy-service dotnet-policy-service bot added the untriaged Request triage from a team member label May 27, 2025
@reflectronic
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reflectronic commented May 31, 2025 •
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You need to add a CancellationToken parameter to your lambda

Func<TSource,System.Threading.CancellationToken,System.Threading.Tasks.ValueTask<bool>> predicate

https://learn.microsoft.com/en-us/dotnet/api/system.linq.asyncenumerable.singleasync?view=net-10.0#system-linq-asyncenumerable-singleasync-1(system-collections-generic-iasyncenumerable((-0))-system-func((-0-system-threading-cancellationtoken-system-threading-tasks-valuetask((system-boolean))))-system-threading-cancellationtoken)

@Xen0byte
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I see! Many thanks @reflectronic, I'll give it a try as soon as I get the chance and then update this issue accordingly.

@Xen0byte
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Indeed, the following works just fine:

int number = await new List<int>() { 1, 2, 3 }.ToAsyncEnumerable()
    .SingleAsync(async (value, cancellationToken) => { await Task.Delay(500); return value.Equals(2); });

And then the cancellation token can be propagated to inner tasks, if needed:

... await Task.Delay(500, cancellationToken); ...

I should have inspected the overloads better.

@dotnet-policy-service dotnet-policy-service bot removed the untriaged Request triage from a team member label May 31, 2025
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