Project vs. Web Site and App_Code
Creating an ASP.NET Project provides for a .sln file, but you cannot add an App_Code directory--at least not one recognized by the IDE for purposes of compilation (this is confirmed throughout the internet).
Creating a Web Site does not provide for a .sln file, but you can add an App_Code directory.
In the downloaded TBH code, there is both a .sln file and an App_Code directory which the IDE apparently recognizes, because there's no problem with the Build.
It's my impression that the author started out with a Web Site rather than a Project (the Publish Web Site dialog box is different between the two).
If all this is true, how can it be that there is both a .sln file and an App_Code directory?