ManualResetValueTaskSourceLogic`1 missing in System.Private.CoreLib

[fubar-coder]

I used .NET Core 3.0.0-preview-27122-01, win10-x64 (downloaded from dot.net) and tried to build this example application:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace TestCsConsole
{
    class Program
    {
        static async Task Main(string[] args)
        {
            await foreach (var num in GetNumbersAsync())
            {
                Console.WriteLine(num);
            }
        }

        private static async IAsyncEnumerable<int> GetNumbersAsync()
        {
            foreach (var num in Enumerable.Range(0, 10))
            {
                yield return num;
            }
        }
    }
}

The build fails with the following errors:

1>------ Neues Erstellen gestartet: Projekt: TestCsConsole, Konfiguration: Debug Any CPU ------
1>C:\Program Files\dotnet\sdk\3.0.100-preview-009812\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.RuntimeIdentifierInference.targets(142,5): message NETSDK1057: Sie arbeiten mit einer Vorschauversion des .NET Core SDK. Sie können die SDK-Version über eine Datei "global.json" im aktuellen Projekt definieren. Weitere Informationen finden Sie unter https://go.microsoft.com/fwlink/?linkid=869452.
1>Program.cs(19,9,25,10): error CS0656: Missing compiler required member 'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1..ctor'
1>Program.cs(19,9,25,10): error CS0656: Missing compiler required member 'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.GetResult'
1>Program.cs(19,9,25,10): error CS0656: Missing compiler required member 'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.GetStatus'
1>Program.cs(19,9,25,10): error CS0656: Missing compiler required member 'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.get_Version'
1>Program.cs(19,9,25,10): error CS0656: Missing compiler required member 'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.OnCompleted'
1>Program.cs(19,9,25,10): error CS0656: Missing compiler required member 'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.Reset'
1>Program.cs(19,9,25,10): error CS0656: Missing compiler required member 'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.SetException'
1>Program.cs(19,9,25,10): error CS0656: Missing compiler required member 'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.SetResult'
1>Program.cs(19,9,25,10): error CS0656: Missing compiler required member 'System.Runtime.CompilerServices.IStrongBox`1.get_Value'
1>Program.cs(19,9,25,10): error CS0656: Missing compiler required member 'System.Runtime.CompilerServices.IStrongBox`1.Value'
1>Die Erstellung des Projekts "TestCsConsole.csproj" ist abgeschlossen -- FEHLER.
========== Alles neu erstellen: 0 erfolgreich, 1 fehlerhaft, 0 übersprungen ==========

Project file:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.0</TargetFramework>
    <LangVersion>8.0</LangVersion>
    <NullableReferenceTypes>true</NullableReferenceTypes>
  </PropertyGroup>

</Project>

[benaadams]

Was renamed to ManualResetValueTaskSourceCore<TResult>

[benaadams]

Ah, looks like mismatch between C#8 and the runtime? /cc @stephentoub

[fubar-coder]

I was also unable to find ManualResetValueTaskSourceCore in the System.Private.CoreLib assembly.

[stephentoub]

It's not missing exactly, but like @benaadams said things are just out-of-sync between the compiler and library in Preview 1. The compiler is looking for the old design (ManualResetValueTaskSourceLogic<T> and IStrongBox<T>), while the libraries include the approved API surface area (ManualResetValueTaskSourceCore<T>), and we didn't have time to get the compiler updated. You just need to include a bit of boilerplate in your app, e.g.

namespace System.Threading.Tasks
{
    using System.Runtime.CompilerServices;
    using System.Threading.Tasks.Sources;

    internal struct ManualResetValueTaskSourceLogic<TResult>
    {
        private ManualResetValueTaskSourceCore<TResult> _core;
        public ManualResetValueTaskSourceLogic(IStrongBox<ManualResetValueTaskSourceLogic<TResult>> parent) : this() { }
        public short Version => _core.Version;
        public TResult GetResult(short token) => _core.GetResult(token);
        public ValueTaskSourceStatus GetStatus(short token) => _core.GetStatus(token);
        public void OnCompleted(Action<object> continuation, object state, short token, ValueTaskSourceOnCompletedFlags flags) => _core.OnCompleted(continuation, state, token, flags);
        public void Reset() => _core.Reset();
        public void SetResult(TResult result) => _core.SetResult(result);
        public void SetException(Exception error) => _core.SetException(error);
    }
}

namespace System.Runtime.CompilerServices
{
    internal interface IStrongBox<T> { ref T Value { get; } }
}

[update by jcouv:] Note that this boilerplate is no longer necessary in Preview 2 (ie. using preview2 versions of the compiler/VS and of Core).

[pkanavos]

The code compiles but enumerating Enumerable.Range(0, 10) ends after only 2 iterations. This doesn't happen with an array

[stephentoub]

The code compiles but enumerating Enumerable.Range(0, 10) ends after only 2 iterations. This doesn't happen with an array

This is a compiler bug: dotnet/roslyn#31268

[shawty]

This is also manifesting itself in the following way:

MSB6006 "csc.exe" exited with code -2146232797. AsyncStreams C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\MSBuild\15.0\Bin\Roslyn\Microsoft.CSharp.Core.targets 52

Just for reference.

[shawty]

Sorry, I meant to add that comment to "31268" in the post above.

[ericnewton76]

from the visual studio blog:

Now all the squiggles are gone, and the program is correct. But if you try compiling and running it, you get an embarassing number of errors. That’s because we messed up a bit, and didn’t get the previews of .NET Core 3.0 and Visual Studio 2019 perfectly aligned. Specifically, there’s an implementation type that async iterators leverage that’s different from what the compiler expects.

I just wanted to say kudos for acknowledging the mistake and pointing to a clear workaround for the time being.

[rossbuggins]

I'm seeing:
CS0656 Missing compiler required member 'System.Collections.Generic.IAsyncEnumerable`1.GetAsyncEnumerator'
on async IAsynEnumerable methods. Got VS2019 preview2 and core 3 preview.

[rossbuggins]

I'm seeing:
CS0656 Missing compiler required member 'System.Collections.Generic.IAsyncEnumerable`1.GetAsyncEnumerator'
on async IAsynEnumerable methods. Got VS2019 preview2 and core 3 preview.

using .net core 3 3.0.100-preview-010184 sorted this

[vip32]

Note that this boilerplate is no longer necessary in Preview 2 (ie. using preview2 versions of the compiler/VS and of Core