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I want to create a library project which acts as a sort of wrapper/intermediary class and has all my variables/functions to be used in other classes
Why? Have you a specific requirement (e.g. you need a proxy) or do you see a general advantage in doing that?
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Is there any efficiency in creating an interface within this library to store variables or that can easily just be declared in the library class itself?
You know, there is no performance gain in using iterfaces. Interface has a very specific role in Object Oriented Programming, if you have any doubt then I suggest you studying a good OOP book (or, at least, read goodtutorials on the web).