Deployed aspx page can't access methods from same project

I'm using ASP.NET with VB, and .NET version 3.5. In the project, I've got some utility functions that I call from expressions in the aspx pages and also in code behind. This all works fine on my local machine, but when I deploy to the production server, I get this compilation error on each of those utility methods:

BC30456: 'XXX' is not a member of 'String'.

where XXX is an extension method defined on System.String. I'm baffled about this for a number of reasons:

1. Why is ASP.NET compiling anything at all, since I've precompiled the application and put everything in the bin directory.
2. ASP.NET knows the functions are there because they work in code behind. It's only when used in the aspx page that I have this problem. (e.g. if I do something like this: <%= "A string to XXX-ify".XXX())%>)
3. This method is public, which I verified with Reflector.
4. I imported the relevant namespaces in the web.config file, and I can see that these namespaces are being imported in the call th开发者_JAVA技巧at ASP.NET makes to the compiler.

What on earth is going on here? I really need access to these utility functions because, well, they're really useful.

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Check the .Net version of the site in IIS.

This may be set to a version that does not support extension methods.

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When I looked more closely at one example, the function in question was a Friend (internal in c#), and the page can't see them. So maybe I was hallucinating about the method being public. Still, it's something to look out for.

I guess I don't fully understand the compilation system in ASP.NET, but maybe the page is compiled into a separate assembly from the code modules, and so such functions must be public if you want it to work.