For me it turned out to be a nuget source credentials problem. I had recently changed my password for accessing a nexus server and visual studio was still using the old password when trying to access a nuget on that server through the windows credential manager. To fix it, I had to delete the entry for those outdated credentials in the credential manager and after, when I did a nuget restore, it prompted me for a password letting me enter the new password, which got saved in the credential manager again. You can access the credential manager from the cmd line using CmdKey.exe.

我没有在任何地方发现微软建议的删除“packages”文件夹的方法。

来源:解决包恢复错误

You may encounter build errors due to missing files, with a message saying to use NuGet restore to download them. However, running a restore might say, "All packages are already installed and there is nothing to restore." In this case, delete the packages folder (when using packages.config) or the obj/project.assets.json file (when using PackageReference) and run restore again. If the error still persists, use nuget locals all -clear or dotnet nuget locals all --clear from the command line to clear the global-packages and cache folders as described on Managing the global packages and cache folders.

选择工具> NuGet包管理器>包管理器控制台

然后运行:

dotnet restore <project or solution name>

在Azure DevOps中，当我在构建管道中设置“Visual Studio Build”任务来构建我的解决方案中的单个项目，而不是整个解决方案时，我在这个错误上浪费了几个小时。

这样做意味着DevOps要么不构建任何(或者可能是一些，我不确定是哪个)您构建的目标项目所引用的项目，因此这些项目不会生成它们的project.json.asset文件，这就导致了这个问题。

我的解决方案是从使用VS Build任务切换到MSBuild任务。为单个项目使用MSBuild任务可以正确地构建您正在构建的项目引用的任何项目，并消除此错误。

如果只是恢复NuGet包不工作，确保在工具->选项-> NuGet包管理器->通用下的包恢复，“允许NuGet下载丢失的包”是选中的。

然后恢复NuGet包再次或只是重建后删除obj和bin文件夹。

在我的案例中，我遇到了可用包源的问题。我已经将本地nuget存储库文件夹移动到一个新的路径，但我没有在nuget可用包源中更新它。当我纠正了路径问题，更新它在可用的包源之后，一切(nuget restor等)都工作正常。