It is possible to build, rebuild or clean all (or some) of the projects in your solution.
SS - Build drop-down
The "build" option changes to "build solution" when a solution includes more than one project.
Build - Will compile only those project files and components that have changed since the most recent build
Rebuild - will "clean" the solution and then "build" the solution
Clean - will delete any intermediate and output files
If a project has errors then it will not clean
If the build folder contains any resources these will not be removed
If you have the "Copy to Output Directory" property set to the "Copy always" or "Copy if newer" then a resources subfolder is created in the build folder to contain these resources.
Build Action ?
Adding Existing Forms - only browse to the .cs file (ignore the designer.cs and .resx files)