Load another DLL failed

[ms44]

Hi , I've created a revit in C# and it works fine.
However when I add a dll as reference to this plugin , it failed and not even trigger debug break point when I ran it again .

It pop up :"Revit encounter System.IO.FileNotFoundException :Could not load file or assembly 'xxxxxxx".
I checked C:\ProgramData\Autodesk\Revit\Addins\2016\
and C:\Program Files\Autodesk\Revit 2016
both folder contain my dependency dll.

Any advice ?
Thanks a lot .