add a variable to access across the solution c#

this might be a simple question but i could not find a way out through this , wanted some help

I have 3 common libraries in one project and a testing project( Module wise here we are testing hardware so modules differ from time to time but interaction libraries are same ) i wanted to add a variable which can be used across the libraries and this testing project.

Example :

i have

Testing Project -> Library1-> library2 -> library3 -> Hardware

response will be sent from hardware

one of the requirement that i have got is to define common variable in solution and use it across the libraries and testing project

i have tried to define app config but it is limited to one library and cannot be used in the other library