Is Use of the Microsoftl.Bcl.AsyncInterfaces Library Actually Supported?

[jimn-purecars]

We are interested in using IAsyncEnumerable<> in our .NET Framework 4.8 codebase. To do this, I have installed the Microsft.Bcl.AsyncInterfaces nuget package, and changed the <LangVersion> to 8. In my test code, things work great!

But according to official MS docs, the default language version for Framework 4.8 is 7.3. In fact, all targets prior to .NET Core 2.1 default to 7.3. The doc also states:

Choosing a language version newer than the default can cause hard to diagnose compile-time and runtime errors

This library is not required from .NET Core 3.0 onwards, so apart from .NET Core 2.1 specifically, it would seem the only use case for this library is in a configuration that is warned against.

So is using e.g. .NET Framework 4.8 with a <LangVersion> of 8 actually a supported use case?

Is the use of this library in this configuration guaranteed to work properly? And is there any guarantee it will continue to work with e.g. new point-releases of .NET Framework 4.8.x?

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[jeffschwMSFT]

As you pointed out, .NET Framework's supported lang ver is 7.3 (https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version). This means that moving the lang version ahead may lead to undefined behavior.

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Issue Details

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We are interested in using IAsyncEnumerable<> in our .NET Framework 4.8 codebase. To do this, I have installed the Microsft.Bcl.AsyncInterfaces nuget package, and changed the <LangVersion> to 8. In my test code, things work great!

But according to official MS docs, the default language version for Framework 4.8 is 7.3. In fact, all targets prior to .NET Core 2.1 default to 7.3. The doc also states:

Choosing a language version newer than the default can cause hard to diagnose compile-time and runtime errors

This library is not required from .NET Core 3.0 onwards, so apart from .NET Core 2.1 specifically, it would seem the only use case for this library is in a configuration that is warned against.

So is using e.g. .NET Framework 4.8 with a <LangVersion> of 8 actually a supported use case?

Is the use of this library in this configuration guaranteed to work properly? And is there any guarantee it will continue to work with e.g. new point-releases of .NET Framework 4.8.x?

Author:	jimn-purecars
Assignees:	-
Labels:	question, area-Meta, untriaged
Milestone:	-

[teo-tsirpanis]

It's a bit complicated situation. First, given that Microsoft.Bcl.AsyncInterfaces still receives updates and targets .NET Framework, it should be supported fine. In fact, IAsyncEnumerable and IAsyncDisposable are just interfaces; there is nothing actual to support in them. They use ValueTask from the System.Tasks.Extensions package, which is supported on .NET Framework.

As for C# 8, it is not officially supported in earlier frameworks, but you can enable it at your own risk. I would do it if I were you and if the codebase is sufficiently tested, but if you don't want to do it, you can use IAsyncEnumerable from earlier language versions the manual way:

IAsyncEnumerable<int> xs = // …
// C# >= 8
await foreach (int x in xs)
{
    Console.WriteLine(x);
}

// C# < 8
IAsyncEnumerator<int> enumerator = xs.GetAsyncEnumerator();
try
{
    while (await enumerator.MoveNextAsync())
    {
        Console.WriteLine(enumerator.Current);
    }
}
finally
{
    await enumerator.DisposeAsync();
}