Support File.ReadAllLinesAsync and friends on .NET Core 3 / netstandard2.1

[christianrondeau]

Async methods were implemented in #481 (comment) but I think the precompiler directives or somehow the nuget binding doesn't get them.

Sample csproj:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.1</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="System.IO.Abstractions" Version="6.0.21" />
  </ItemGroup>
</Project>

Sample code:

var lines = await new FileSystem().File.ReadAllLinesAsync(@"C:\file.txt")

I think the problem is simply that we're limited to NETCOREAPP2_0: https://github.com/fedoranimus/System.IO.Abstractions/blob/master/System.IO.Abstractions/FileBase.cs#L322 - you can probably instead use NETSTANDARD2_0 but I won't be able to test that right away. I won't be able to provide a PR as I'm off until the end of next week, but if no one took this issue when I'm back all provide one.

[fgreinacher]

Your sample csproj is targeting .NET Standard 2.1 and will therefore pull in the .NET Standard 2.0 version of System.IO.Abstractions.

Back in #481 we added support for the async overloads to a newly added .NET Core 2.0 target. To solve your issue we need to add an additional target for .NET Standard 2.1 in which we could light up the async overloads.

Edit: If you change your csproj to target .NET Core 3.0 it should work (because NuGet would then pick the .NET Core 2.0 target of S.IO.A)