msbuildtasks | MSBuild Community Tasks Project is an open source project | Command Line Interface library

In your projects you have something like:

This is weird because every other target works. And NuGet.Config should be under BaseFolder and not MySolution. Any ideas why NuGet.Pack is looking under MySolution?

And so far, to do some operation on your soloution, you can create a new Nuget.config file under the solution folder(It is designed by nuget mechanism). It will apply to all projects in the solution subdirectory.

You can refer to this link for more info about nuget.config file.

Besides, you do not have to use NuGet.NuGetPack to change the external packages and it is too old. You can just add these nodes in the new Nuget.config file to change it:

When I go to properties of this DLL reference in Visual Studio Solution Explorer, it shows version 4.2.0.1 referenced from xxxxxx

First, I think you have a little misunderstanding about the DLL version and the nuget version.

version 4.5.3 is just the nuget package System.Threading.Tasks.Extensions's version number while version 4.2.0.1 is just the System.Threading.Tasks.Extensions.dll( exist in the nuget pacakge) number. They are not a concept at all. And you can see such dlls referenced in xxx.csproj file.

A version of the nuget package contains multiple competing DLLS for different project frameworks, which will be automatically installed into the corresponding project according to the nuget mechanism.

DLL hell - Could not load file or assembly System.Threading.Tasks.Extensions

As you said, you have a nuget package called System.Reactive version 4.3.2 that depends on System.Threading.Tasks.Extensions 4.5.3 in your project. You can see this:

And from your error log, it seems that the project referenced System.Threading.Tasks.Extensions.dll 4.2.0.0 which it does not exists in the nuget package System.Threading.Tasks.Extensions 4.5.3 and it should be System.Threading.Tasks.Extensions.dll 4.2.0.1. Not sure if you changed the version number or the reason for the project.

You can check these steps:

Solution

1) make sure that System.Threading.Tasks.Extensions version is 4.5.3 in packages.config file.

2) make sure that you change the dll version to 4.2.0.1 in xxx.csproj file

3) Before you reinstall these, please clean the nuget cache first, then run update-Package -reinstall under Tools-->Nuget Package Manager->Package Manager Console to reinstall the packages

4) create a new framework 4.7.2 project and then referenced these nuget packages to test whether it is caused by your project.