1. Create a subfolder of App_Code folder named VBCode, and another subfolder named CSCode.
2. Change web.config file as follows:
<compilation debug="false"> <codeSubDirectories> <add directoryName="VBCode" /> <add directoryName="CSCode" /> </codeSubDirectories> </compilation>
Reference:
http://msdn.microsoft.com/en-us/library/t990ks23(vs.80).aspx