Sharing a struct or class among multiple user controls?

I'm building an application where custom modules may be developed and "dropped in" to the system, where they can be picked up and utilized. 
I'm building a forum module and have a user control to create a login/registration region. I plan on using a struct to store the user session data. However, i need this class structure to be shared between the forum module and the login/registration control. Because the functionality is to be contained in the module's folder, i cant add any assemblies to the app_code folder. 
How can i share a class or struct among two user controls? thanks.
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