App_Code folder issues

The problem t开发者_JAVA技巧hat your classes are not compiled, You'll solve this issue simply by going to the properties of any class in the App_Code folder and change it's 'Build Action' property from "Content" to "Compile"

But how can I set the property?

Right-click on the item in the solution explorer, and select Properties. In the properties window, there is a Build Action option.