ASP.NET Core referencing TPL NuGet package Cannot Find File or Assembly System.Threading - .net-4.0

Question

I am making a new ASP.NET Core 2.0 project, essentially a Web API. I have to reference several assemblies that are .NET 3.5 assemblies, so I am running this as a .NET Framework project, just using the .NET Core improvements where possible.
The referenced .NET framework 3.5 assembly references TaskParallelLibrary (1.0.2856). And I think that's where my issues are coming in. It's worth noting the Web API worked on its sample endpoints and all worked as expected.
When I run the application and try and use the referenced library, the call fails and I receive the following exception:
Could not load file or assembly 'System.Threading, Version=1.0.2856.102, Culture=neutral,
PublicKeyToken=31bf3856ad364e35'
or one of its dependencies. The system cannot find the file specified.
I have tried several things, including:
Adding an App.configx file to try and force it to target .NET framework 4's System.Threading, on the assumption that that will be available. Inspired by a page I can't find the link for:
Here's the content:
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<!-- this is only here for binding redirects -->
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="System.Threading" publicKeyToken="31bf3856ad364e35" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-4.0.0.0" newVersion="4.0.0.0" />
</dependentAssembly>
</assemblyBinding>
</runtime>
</configuration>
Directly adding the TaskTPLLibrary package at that version to the project. So far, it hasn't helped.
Adding the Microsoft.Bcl.Async library to my project. Again, nothing.
Trying to directly import System.Threading from "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETCore\v4.5\System.Threading.dll". I get an error stating "The reference is invalid or unsupported."
I've tried several other things, including re-cloning, cleaning, re-cleaning, swearing vociferously and coming back at it another day, all to no avail.
My reading list includes (on this computer):
https://github.com/dotnet/corefx/issues/26084
Could not load file or assembly System.Threading.Tasks, Version=2.5.19.0
ASP.NET Core 2.0 Site - FileIOException System.Runtime
And probably others that I can't remember
Some additional information as this issue continues:
Creating a .NET Framework ASP.NET MVC app using .NET Framework 4.6.2 with the same underlying library does work. It does it indirectly using an intermediary .NET Framework 3.5 one (just for convenience as much as anything else).
A .NET Framework 4.6.2 Console App that uses the library directly also runs into this problem. So it seems not to be ASP.NET Core itself. I'll change the tags accordingly.
Making the 4.6.2 console use the intermediary 3.5 library doesn't resolve the issue.
Any help in resolving this issue much appreciated.

Answer 1

As there's no reasonable answer forthcoming, I'm assuming that having a .NET 3.5 Framework library referenced that uses the TaskParallelLibrary will in essence mean that using any .NET above 4 will cause this error. Under unspecified conditions. Most of the time.
The only solution to this seems to be rewrite everything to multi-target .NET 3.5 and .NET Standard and have the Standard implementation use the real TPL and have the 3.5 one use the NuGet package.