我删除了全局包缓存中的所有包，现在我的项目正在为标准库结构抛出大量CS0012：
error CS0012: The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.
error CS0012: The type 'IDisposable' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.
我运行过：
- 解决方案中的
dotnet restore和每个抛出错误的项目 - 解决方案中的
nuget restore和每个项目抛出错误
我的dotnet配置：
> dotnet --info
.NET Core SDK (reflecting any global.json):
Version: 3.0.100
Commit: 04339c3a26
Runtime Environment:
OS Name: Windows
OS Version: 10.0.19025
OS Platform: Windows
RID: win10-x64
Base Path: C:Program Filesdotnetsdk3.0.100
Host (useful for support):
Version: 3.0.0
Commit: 7d57652f33
.NET Core SDKs installed:
3.0.100 [C:Program Filesdotnetsdk]
.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.9 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.12 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.13 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.3 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.6 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.7 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.9 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.12 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.13 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.3 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.6 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.7 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.0.0 [C:Program FilesdotnetsharedMicrosoft.AspNetCore.App]
Microsoft.NETCore.App 2.0.9 [C:Program FilesdotnetsharedMicrosoft.NETCore.App]
Microsoft.NETCore.App 2.1.9 [C:Program FilesdotnetsharedMicrosoft.NETCore.App]
Microsoft.NETCore.App 2.1.12 [C:Program FilesdotnetsharedMicrosoft.NETCore.App]
Microsoft.NETCore.App 2.1.13 [C:Program FilesdotnetsharedMicrosoft.NETCore.App]
Microsoft.NETCore.App 2.2.3 [C:Program FilesdotnetsharedMicrosoft.NETCore.App]
Microsoft.NETCore.App 2.2.6 [C:Program FilesdotnetsharedMicrosoft.NETCore.App]
Microsoft.NETCore.App 2.2.7 [C:Program FilesdotnetsharedMicrosoft.NETCore.App]
Microsoft.NETCore.App 3.0.0 [C:Program FilesdotnetsharedMicrosoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.0.0 [C:Program FilesdotnetsharedMicrosoft.WindowsDesktop.App]
为什么删除全局包缓存会导致此问题，以及如何从中恢复？
编辑：
这是最后一个.csproj，它抛出了这些错误：
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>netstandard2.0</TargetFramework>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="GraphQL" Version="2.4.0" />
<PackageReference Include="GraphQL-Parser" Version="4.1.1" />
<PackageReference Include="dotNetRDF" Version="2.2.0" />
<PackageReference Include="Newtonsoft.Json" Version="12.0.2" />
</ItemGroup>
</Project>
这似乎是visualstudio 15.5之前版本(或者更确切地说是相关的msbuild(的已知问题
总结是，你要么需要升级到VS2017的最新版本，要么使用VS2019。
或者，如果你不想这样做，在所有失败的csproj文件中插入以下引用(或者只是你的Directory.Build.Props更安全(就可以了：
<ItemGroup>
<Reference Include="netstandard" />
</ItemGroup>
另一个建议的解决方法是使用目标框架属性，而不是单一版本。例如：
<TargetFrameworks>netstandard2.0</TargetFrameworks>