I was creating a test Web Forms application and added some existing code files.
I first added them to the app_code folder, but in there they could not find the OracleDataClient namespace. I found something that said to move the code file out of app_code and that fixed the Oracle problem.
Then when I would reference classes in those files the page code did not recognize the namespace.
After a while of searching I found someone who said to right click on the files and view properties.
The property they said to look at was Build Action. My file was set to Content and I saw to change it to Compile. That fixed it.