之所以发生这种情况，是因为，你已经移出了类文件它的位置..它是你的错误，因为你已经编译了你的应用程序，当它没有被移动..现在从App_Code构建你的ClassLibrary然后添加它的应用程序的引用..所有***：）

I am getting the following error when I put class files in subfolders of my App_Code folder:

errorCS0246: The type or namespace name ''MyClassName'' could not be found (are you missing a using directive or an assembly reference?)

This class is not in a namespace at all. Any ideas?
;)

It happened because, you''ve moved out class file from its place.. It is givin you error because you''ve compiled your application when it wasn''t moved.. Now build your ClassLibrary from App_Code and then add its reference to your application.. All the Best :)