If you want to add a dll from GAC through selecting the file from

C:\windows\Asmebly directly , maybe you will get the “File is not found” error.

Because this file is not located there.

And you can not visit the folder or file using windows browser.

However how do you add reference dll from GAC?

I found a way to solve this problem.

 If you watch the folder structure of C:\windows\asembly in command environment,

You will found the real structure and the real path of GAC.

You can get the real path using this way, and type it in the add reference dialog with

the absolute full path to add GAC dll to your project references.
OK, it's done :)