The visual studio tool bar provides a way to select which arduino circuit board each project will use

When a board is selected, all of the items in the "_core" filter are removed and replaced by the files from the newly selected boards arduino core

If the _core filter does not exist then it is created. In the example below, the board is about to switch from "atmega168" to "arduino mega"

In the current visual micro version the include and source files are automatically separated into "inc" and "src" filters (see below)