I have checked again, interesting fact that with visual studio 2012, I could only reference AF SDK 2.0 with .NET 3.5. If I reference to AF SDK 4.0 with .NET 4.5. It couldn't detect the library.
with visual studio 2015, I could reference AF SDK 4.0 with .NET 4.5.2.
I believe the problem that you have is just to reference to the AF SDK 4.0, not to the AF SDK 2.0.
Recent versions of the AFSDK target .NET 4.5.2. Visual Studio 2012 and Visual Studio shipped before .NET 4.5.2 did, so you have to install an add-on to use .NET 4.5.2 with those versions of Visual Studio:
Visual Studio 2015 and Visual Studio 2017 come with support for .NET 4.5.2 built in.