A Class in App_Code Folder Does Not Get Compiled

Today I’ve experienced one odd thing with Visual Studio 2008.
As we know, App_Code is not needed for a Web application project under VS2008 since class files can be put anywhere.
However, to keep it clean and easy to find, I decided to created one and create class files under the folder.
It appears that those created objects do not be available.
In the other words, they do not get compiled.
After messing around, I found that the “Build Action” property of those files have been set to “Content” by default.
Well, it should be “Compile” so that the files get compiled.
I’ve also tried to create another folder with the name of “cs”.
The created class files under “cs” have the “Build Action” property of “Compile”.
Therefore, the solution is either set the “Build Action” property to “Compile” or never use a folder with the name of “App_Code”.

Now the question is… why does the “Build Action” property of the files under “App_Code” is “Content”?