ASP.NET site upgraded VS08 - VS10, and now compile-time errors do not show until I request the page at run-time

I have an ASP.NET website that worked fine using and debugging in VS2008. I went through the upgrade process opening the solution in VS2010. I can run the site, but as I make changes in the app_code folder classes, they don't seem to commpile and warn me of compile-time errors. As soon as I get to a point that calls the class,开发者_开发知识库 the errors show up. Sounds JIT I guess, but this isn't how it was working in 08. Is there an option that was changed in the upgrade process? This is a large project, I really don't want to break something and not find out until some obscure page is opened.

I feel like a bit of an idiot, after asking the question, I went to my Configuration Manager and found that the web project was un-checked in the build column. I still don't know why this would have changed from 08 to 10, but everything seems to be working now as it did on 08.