cannot add a reference .NET

Question

I have a DLL which I would like to add as a reference to my project, but everytime I try to do it a dialog pops up telling me:

The reference could not be

Answer 1

after trying to analyse the issue, I discover the following

[1] sometimes you see the reference in the reference manager, but you will not find the dll files in the mentioned directory where the Reference manager is not able to actualize the references.

[2] check the permission of your dll file.