Using .NET 4.0 library with other dependencies from a .NET 4.5.1 project - .net-4.0

Question

I have a .NET project whose target framework is 4.5.1. This project depends on a NuGet library which only has a .NET 4.0 release and this library in turn depends on MVVMLight:
MyProject (4.5.1) --> Depends on MyLibrary (4.0) --> Depends on MVVMLight (4.0)
The MVVMLight NuGet also has a 4.5 version so when it's installed on MyProject it ends up referencing the 4.5 version (as opposed to MyLibrary which was compiled with MVVMLight for .NET 4.0).
This results in the following compilation error:
5>...\Adorners\CircleAdorner.cs(19,18,19,31): error CS0012: The type 'GalaSoft.MvvmLight.ObservableObject' is defined in an assembly that is not referenced. You must add a reference to assembly 'GalaSoft.MvvmLight, Version=4.4.32.18939, Culture=neutral, PublicKeyToken=null'.
The error is from a class CircleAdorner which inherits from a class defined in MyLibrary which in turn inherits from ObservableObject from MVVMLight.
Is this situation "legal"? Is it OK to reference a .NET 4.0 library from a .NET 4.5.1 project? From my understanding it should be OK. However, the catch here is that the .NET 4.0 project expects another dependency which is not satisfied here.
Also, I noticed that the DLL version of GalaSoft.MvvmLight for the .NET 4.5 is 4.4.32.39728 and not 4.4.32.18939. In the project I marked this reference with <SpecificVersion>False</SpecificVersion> but it didn't help.

Answer 1

Yes it is legal for a .NET 4.5.1 project to use a .NET 4.0 assembly. .NET 4.5 is an in place update to .NET 4.0 and is backward compatible.
From NuGet's point of view if the version of project's target framework is less than or equal to the assembly version in the NuGet package then they are considered compatible. So NuGet will allow you to add a NuGet package that targets .NET 4.0 into a project that targets .NET 4.5. What you cannot do is add a NuGet package that only targets .NET 4.5 into a project that targets .NET 4.0 since the assemblies in the NuGet package may use parts of the .NET framework that is not included with .NET 4.0.
When installing a package NuGet will pick the highest version of the .NET framework that the NuGet package contains that is compatible with your project.
With your GalaSoft.MvvmLight version mismatch you should be able to resolve the problem using one of two options:
Update MyLibrary to use the same version of MvvmLight that your project is using.
Add a binding redirect to your project's app.config for GalaSoft.MvvmLight so MyLibrary's reference to it is mapped to the later version. If you are using Visual Studio 2013 and writing a .NET 4.5.1 desktop application you can enable automatic binding redirects instead of updating your app.config.