When an item is Excluded from Build, its source code is generated during the build process, but the MsBuild Template does not send the generated source code to the compiler. Project Item Templates and a few other Template Types may be excluded from the build process. This can be configured using their Excluded from Build context menu option in the Project Explorer.
When project items are Excluded from Build, this is indicated by the Project Explorer using Text color status indication. When Excluded from Build is turned on for an item, the icon for this option is highlighted in the item's context menu.
Re-including items in the build
An item can be re-included in the build by selecting the option once more from its context menu.
See also
Project items can also be Excluded from Generation, stopping source code generation for those items.