Mike,
The \app_code directory is a special directory that holds uncompiled classes.
At runtime, the ASP.NET runtime compiles the contents of this directory into
an assembly that is automatically referenced by the ASPX pages in the application.
If the code isn't placed in the app_code directory,
then the classes in your code aren't compiled into assemblies,
and you get error messages like the one you got.
Read this document called "ASP.NET 2.0 Internals" :
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnvs05/html/internals.asp
After you read it, you will understand better how the App_Code directory works,
and why placing code there is necessary if your classes are referenced in ASPX files.
Juan T. Llibre, ASP.NET MVP
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