In this case there's a very simple reason. I'm one of the authors, for the newest version we did some restructuring and took advantage of the C# 6 Quality-of-Li

In this case there's a very simple reason. I'm one of the authors, for the newest version we did some restructuring and took advantage of the C# 6 Quality-of-Life improvements For example the auto-property initializers introduced in C# 6/VS 2015 saved us about 100 LoC (on a 600 line class), and in addition made some properties truly immutable instead of privately mutable. As for actually using the library, we provide .NET 4 and .NET 4.5 DLL's (only a 4.5 nuget package) so you can still use it in older VS versions, just not build it.