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Its really hard to find any information on IAsyncEnumerable, other than a few mentions in the 'What's New c# 8.0' articles. Trying to use it in Visual Studio 2019 with netstandard 2.0 and C# 8 enabled, it does recognize the class but i get a ton of errors on build:

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Theodor Zoulias
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maraaaaaaaa
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4 Answers4

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C# 8 supports these features. However, this wont work with .Net standard 2.0

IAsyncEnumerable Interface

Applies to

.NET Core 3.0 Preview 3

.NET Standard 2.1 Preview

You will have to get either one of the previews.

You can find more information on .Net Core 3 Preview here

TheGeneral
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    So just a heads up for anyone planning on trying this, unfortunately it wont work if you use `Net Core 3.0` along with `EntityframeworkCore`, because of the following error: `Error CS0433 The type 'IAsyncEnumerable' exists in both 'System.Interactive.Async, Version=3.2.0.0, Culture=neutral, PublicKeyToken=94bc3704cddfc263' and 'System.Runtime, Version=4.2.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'` – maraaaaaaaa Apr 24 '19 at 03:52
  • Just ran into the above comment myself. Was trying to try out IAsyncEnumerable with providing a InMemory database for data but nope! Dreams crushed. – Cubicle.Jockey Aug 08 '19 at 19:43
5

For .NET Standard 2.0 you should install Microsoft.Bcl.AsyncInterfaces

https://www.nuget.org/packages/Microsoft.Bcl.AsyncInterfaces/

Vlad
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  • Please don't just post some tool or library as an answer. At least demonstrate [how it solves the problem](https://meta.stackoverflow.com/a/251605) in the answer itself. – Yunnosch Sep 30 '20 at 07:13
  • Since the problem is about missing member IAsyncEnumerable .NET Standard 2.0 - this packet publishied by Microsoft for compatibility in .NET Standard 2.0. You don't need do anything acept install it. – Vlad Sep 30 '20 at 11:47
5

The answer of Vlad is the right one.

  1. Add LangVersion 8.0 to the desired Projects PropertGroup
  2. Add the Microsoft.Bcl.AsyncInterfaces as PackageReference

For example: MyDummyLib.csproj

<PropertyGroup>
    <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
    <LangVersion>8.0</LangVersion>
    <AssemblyName>MyDummyLib</AssemblyName>
    <RootNamespace>MyDummyLib</RootNamespace>
    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
</PropertyGroup>

<ItemGroup Condition="'$(TargetFramework)'=='netstandard2.0'">
    <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="1.1.1" />
</ItemGroup>
damjik
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0

Looks like you need to target .NET Standard 2.1 but it’s still only in preview.

https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.iasyncenumerable-1?view=netstandard-2.1

Casey Rojas
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