Mono for Android has access to most of .NET 4.0 from the Mono framework. Unfortunately, when I try to add NuGet packages in Visual Studio that specifically target .NET 3.5 or beyond, with a /lib/net*/ folder structure, it tosses up an error.
You are trying to install this package into a project that targets 'MonoAndroid,Version=v2.1', but the package does not contain any assembly references that are compatible with that framework.
If a NuGet package doesn't have the .NET version breakdown, or one where you removed that subdirectory structure and published it to a NuGet source, it works fine. For example, NUnit just has a /lib/ folder with no further breakdown by .NET framework version and installed fine.
It appears NuGet could be checking compatibility by using the <TargetFrameworkVersion> project file XML element. If so, that is the problem as Mono for Android usurps that element to mean a "target Android framework version". For example, to target Android v2.2, it looks like this:
<TargetFrameworkVersion>v2.1</TargetFrameworkVersion>
In the project properties dialog, where the "[.NET] Target framework" drop-down would be is a drop-down for "Minimum Android to target" that affects this XML element.
If this is the problem NuGet is encountering, I won't be able to use unmodified NuGet for version-specific packages until targeting an Android app for a minimum of Ice Cream Sandwich (v4.0).
Side note
You can reference the DLLs from these packages via project reference. It definitely throws a warning at you, but you can ignore it fairly safely (assuming the assembly doesn't wander too far from the code implemented in Mono).
'Some.dll', or one of its dependencies, requires a later version of the .NET Framework than the one specified in the project...
