Could not find type or namespace name ' XXXX ' (is missing a using directive or assembly reference?)

The namespace of the class in the App_Code folder in the project, the prompt could not find the type or namespace name "xxxx" (if a using directive or assembly reference is missing) when using XXXX for the added page, indicating that the build Action property of the class file to be referenced is not selected Compilation, VS does not recognize this namespace.

Modify: Right-click the class file-"Properties-" production operations-"Change the content" to "compile", and then rebuild, problem resolution.