Why does NuGet download seemingly unnecessary dependencies? [duplicate]

Question

When installing System.Collections.Immutable NuGet downloads assemblies like System.Runtime, even though I already had that assembly installed with .NET 4.6.1.

Also, the directories under lib (like packages\System.Runtime.4.0.0\lib\net45) contain no dll files, just empty files called _._ .

Why is this happening? Why is this necessary? What am I missing here?

NuGet log:

Install-Package System.Collections.Immutable -Version 1.1.37
Attempting to gather dependency information for package 'System.Collections.Immutable.1.1.37' with respect to project 'ConsoleApplication1', targeting '.NETFramework,Version=v4.6.1'
Attempting to resolve dependencies for package 'System.Collections.Immutable.1.1.37' with DependencyBehavior 'Lowest'
Resolving actions to install package 'System.Collections.Immutable.1.1.37'
Resolved actions to install package 'System.Collections.Immutable.1.1.37'
  GET https://www.nuget.org/api/v2/package/System.Collections/4.0.0
Installing System.Collections 4.0.0.
Adding package 'System.Collections.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Collections.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Collections.4.0.0' to 'packages.config'
Successfully installed 'System.Collections 4.0.0' to ConsoleApplication1
  GET https://www.nuget.org/api/v2/package/System.Diagnostics.Debug/4.0.0
Installing System.Diagnostics.Debug 4.0.0.
Adding package 'System.Diagnostics.Debug.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Diagnostics.Debug.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Diagnostics.Debug.4.0.0' to 'packages.config'
Successfully installed 'System.Diagnostics.Debug 4.0.0' to ConsoleApplication1
  GET https://www.nuget.org/api/v2/package/System.Globalization/4.0.0
Installing System.Globalization 4.0.0.
Adding package 'System.Globalization.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Globalization.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Globalization.4.0.0' to 'packages.config'
Successfully installed 'System.Globalization 4.0.0' to ConsoleApplication1
  GET https://www.nuget.org/api/v2/package/System.Linq/4.0.0
Installing System.Linq 4.0.0.
Adding package 'System.Linq.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Linq.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Linq.4.0.0' to 'packages.config'
Successfully installed 'System.Linq 4.0.0' to ConsoleApplication1
  GET https://www.nuget.org/api/v2/package/System.Resources.ResourceManager/4.0.0
Installing System.Resources.ResourceManager 4.0.0.
Adding package 'System.Resources.ResourceManager.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Resources.ResourceManager.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Resources.ResourceManager.4.0.0' to 'packages.config'
Successfully installed 'System.Resources.ResourceManager 4.0.0' to ConsoleApplication1
  GET https://www.nuget.org/api/v2/package/System.Runtime/4.0.0
Installing System.Runtime 4.0.0.
Adding package 'System.Runtime.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Runtime.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Runtime.4.0.0' to 'packages.config'
Successfully installed 'System.Runtime 4.0.0' to ConsoleApplication1
  GET https://www.nuget.org/api/v2/package/System.Runtime.Extensions/4.0.0
Installing System.Runtime.Extensions 4.0.0.
Adding package 'System.Runtime.Extensions.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Runtime.Extensions.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Runtime.Extensions.4.0.0' to 'packages.config'
Successfully installed 'System.Runtime.Extensions 4.0.0' to ConsoleApplication1
  GET https://www.nuget.org/api/v2/package/System.Threading/4.0.0
Installing System.Threading 4.0.0.
Adding package 'System.Threading.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Threading.4.0.0' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Threading.4.0.0' to 'packages.config'
Successfully installed 'System.Threading 4.0.0' to ConsoleApplication1
  GET https://www.nuget.org/api/v2/package/System.Collections.Immutable/1.1.37
Installing System.Collections.Immutable 1.1.37.
Adding package 'System.Collections.Immutable.1.1.37' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Collections.Immutable.1.1.37' to folder 'd:\workspace\ConsoleApplication1\packages'
Added package 'System.Collections.Immutable.1.1.37' to 'packages.config'
Successfully installed 'System.Collections.Immutable 1.1.37' to ConsoleApplication1

Answer 1

I think this is because of the changes that are coming with dotnet core / cli. It's maintaining the dependency chain, which is much more important in donet core because you don't have a fully installed framework. That explains why lib\net45 folders are empty, there is nothing to actually include because they are part of the BCL installed on your machine.