access code in app_code from aspx (not aspx.cs)

Hello.  I put common code in the app_code directory.  I access classes in the code all the time from aspx.cs files by "using" the name space from the C# file in app_code and then referencing the class.  Now I want to access a class from an app_code file a aspx file.  How do I do this?  Thanks
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