To get the relative path in the class library

Hello,
I have to access a relative path of the XSD file to validate a XML file. The XML file and the XSD file are part of the class library. How do I access the relative path in the class library?
In case of website, I would use server.mappath(path). Is there anything similar to get the virtual path to the corresponding physical directory on the server in the class library?
Thanks
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