问Microsoft.Bcl.Async软件包与现代csproj

EN

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

  <PropertyGroup>
    <TargetFrameworks>netcoreapp2.0;net40</TargetFrameworks>
  </PropertyGroup>

  <ItemGroup Condition=" '$(TargetFramework)' == 'net40' ">
    <PackageReference Include="Microsoft.Bcl.Async" Version="1.0.168" />
  </ItemGroup>

</Project>

using System;
using System.Threading.Tasks;

namespace ClassLibrary1
{
    public class Class1
    {
        public static async Task Test()
        {
#if NET40
            await TaskEx.Delay(2000);
#else
            await Task.Delay(2000);
#endif
            Console.WriteLine("Test done");
        }
    }
}

1>------ Build started: Project: ClassLibrary1, Configuration: Debug Any CPU ------
1>C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3268: The primary reference "C:\Users\Demon\.nuget\packages\microsoft.bcl.async\1.0.168\lib\net40\Microsoft.Threading.Tasks.Extensions.dll" could not be resolved because it has an indirect dependency on the framework assembly "System.Threading.Tasks, Version=1.5.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0". To resolve this problem, either remove the reference "C:\Users\Demon\.nuget\packages\microsoft.bcl.async\1.0.168\lib\net40\Microsoft.Threading.Tasks.Extensions.dll" or retarget your application to a framework version which contains "System.Threading.Tasks, Version=1.5.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
1>C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3268: The primary reference "C:\Users\Demon\.nuget\packages\microsoft.bcl.async\1.0.168\lib\net40\Microsoft.Threading.Tasks.dll" could not be resolved because it has an indirect dependency on the framework assembly "System.Threading.Tasks, Version=1.5.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0". To resolve this problem, either remove the reference "C:\Users\Demon\.nuget\packages\microsoft.bcl.async\1.0.168\lib\net40\Microsoft.Threading.Tasks.dll" or retarget your application to a framework version which contains "System.Threading.Tasks, Version=1.5.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
1>C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3268: The primary reference "C:\Users\Demon\.nuget\packages\microsoft.bcl.async\1.0.168\lib\net40\Microsoft.Threading.Tasks.Extensions.dll" could not be resolved because it has an indirect dependency on the framework assembly "System.Runtime, Version=1.5.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0". To resolve this problem, either remove the reference "C:\Users\Demon\.nuget\packages\microsoft.bcl.async\1.0.168\lib\net40\Microsoft.Threading.Tasks.Extensions.dll" or retarget your application to a framework version which contains "System.Runtime, Version=1.5.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
1>C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets(2106,5): warning MSB3268: The primary reference "C:\Users\Demon\.nuget\packages\microsoft.bcl.async\1.0.168\lib\net40\Microsoft.Threading.Tasks.dll" could not be resolved because it has an indirect dependency on the framework assembly "System.Runtime, Version=1.5.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0". To resolve this problem, either remove the reference "C:\Users\Demon\.nuget\packages\microsoft.bcl.async\1.0.168\lib\net40\Microsoft.Threading.Tasks.dll" or retarget your application to a framework version which contains "System.Runtime, Version=1.5.11.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
1>Class1.cs(11,19,11,25): error CS0103: The name 'TaskEx' does not exist in the current context
1>Done building project "ClassLibrary1.csproj" -- FAILED.
1>ClassLibrary1 -> C:\Users\Demon\Documents\Visual Studio 2017\Projects\ConsoleApp257\ClassLibrary1\bin\Debug\netcoreapp2.0\ClassLibrary1.dll
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========