The class SharedBaseClass must be public:

Public Partial Class SharedBaseClass ... 

The class SharedBaseClass must be public and code file SharedBaseClass.vb must be located under App_Code folder.

Public Partial Class SharedBaseClass  Inherits System.Web.UI.Page .... 

The class SharedBaseClass must be public:

Public Shared Class SharedBaseClass ... 

The class SharedBaseClass must be public:

Public Partial Class SharedBaseClass ...