How to use WinRTLeagcy.dll

When i want to use WinRtleagcy.dll. I push the dll file into project's Assets.But only plugins can find reference.
My working env like this :VS2015 + Unity 5.3.2f1. Anthoer problem is when i build winstore project. Some types like ArrayList ,Hashtable ,are both in system.Collection and WinRtLeagcy. I create a library from VS.Later to add ref with WinRTLeagcy.But vs tells me " A reference to ..../WinRTLeagcy.dll"could not be added. Crying.
Excuse me for any mistakes since my not-so-good English.Thanks.