Compiler error when referencing a NetStandard 2.0 library using ValueTask<T>

[yinyue200]

Steps to Reproduce

1. Build it

Repro project: https://github.com/yinyue200/taskextest/tree/v2

Expected Behavior

Build successful

Actual Behavior

error CS0012: The type 'ValueTask<>' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Threading.Tasks.Extensions, Version=4.2.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.

Version Information

Log File

VS bug #639363

[jonathanpeppers]

I'm not able to build this project with VS 2017 15.7.4 or current master (0b5ce8f).

[atsushieno]

Build environment information is essential to determine if it is expected or not.

I could build ClassLibrary1 if I correctly run dotnet restore or I opened it from monodevelop/VSMac that does support .NET Standard 2.0 library.

[jonathanpeppers]

I think there is an issue in using ValueTask<T> with Xamarin.Android, I need to spend more time looking into it.

[jonathanpeppers]

Ok, here is a better summary of the problem:

1. If you create a Xamarin.Android app, you can use ValueTask<T> without any NuGets
2. If you instead use ValueTask<T> in a NetStandard 2.0 project, it requires the System.Threading.Tasks.Extensions NuGet. Reference the NetStandard lib from a Xamarin.Android app: it doesn't work

[AdamDotNet]

In case this helps at all, here's some of my experience.
VS 2017 15.58 P3
Xamarin.Android SDK 8.4.0.1

When referencing a NuGet that has a dependency on System.Threading.Tasks.Extensions 4.5.1 and is netstandard2.0, such as Asp.Net Core, then I get the following build error:

Xamarin.Android.Common.targets(1831,2): error XA2002: Can not resolve reference: 'System.Threading.Tasks.Extensions', referenced by 'Microsoft.AspNetCore.Server.Kestrel.Core'.

The project.assets.json file shows an empty placeholder for it:

"System.Threading.Tasks.Extensions/4.5.1": {
    "type": "package",
    "compile": {
        "ref/MonoAndroid10/_._": {}
    },
    "runtime": {
        "lib/MonoAndroid10/_._": {}
    }
}

I noticed that System.Threading.Tasks.Extensions 4.5.0 does not have empty place holders for MonoAndroid, so it compiles and runs. (Well, Asp.Net Core breaks because FileSystemWatcher throws NotImplementedException, but that's another story I think the mono repo is tracking... mono/mono/issues/6966). The one con is that the compiler constantly warns about NuGet version downgrade.

NU1605: Detected package downgrade: System.Threading.Tasks.Extensions from 4.5.1 to 4.5.0. Reference the package directly from the project to select a different version. 
 KestrelTest2.Android -> Microsoft.AspNetCore.Server.Kestrel 2.1.1 -> Microsoft.AspNetCore.Server.Kestrel.Core 2.1.1 -> System.Threading.Tasks.Extensions (>= 4.5.1) 
 KestrelTest2.Android -> System.Threading.Tasks.Extensions (>= 4.5.0)

Does the milestone label d15-9 mean if this gets fixed, it won't be in Visual Studio 2017 15.8, but rather the next VS update, presumably 15.9?

[yinyue200]

@AdamDotNet ，4.5.0 has a another issue. see dotnet/corefx#29585

[jdluzen]

I think this is the same issue that I was having. My current workaround is to downgrade System.Threading.Tasks.Extensions to 4.4.0.