Visual Studio Rebuilds unmodified projects

Open Tools - Options, select Projects and Solutions - Build and Run in tree, then set "MSBuild project build output verbosity" to Diagnostic. This will output the reason for building a project, i.e.

Project 'ReferencedProject' is not up to date. Project item 'c:\some.xml' has 'Copy to Output Directory' attribute set to 'Copy always'.

or

Project 'MyProject' is not up to date. Input file 'c:\ReferencedProject.dll' is modified after output file 'c:\MyProject.pdb'.

In this case the fix is to copy some.xml only if newer.

Pre and post build events can trigger build as well.

While I don't think this is a fix, it is a workaround that has worked for my situation...

I originally had about 5 projects out of 50 that contained a Resources section. These projects would always be rebuilt and thus anything that they depended on would also be rebuilt. One of those 5 projects was a "base" level library that 48 of the other projects referenced, thus 96% of my project would be rebuilt every time even if it didn't need it.

My workaround was to use dependency injection, interfaces, and a dedicated "Resources" project. Instead of having those 5 projects reference their own Resources object, I created an interface in each project that would supply the desired resources. Then, the classes that needed those resources would require that interface be passed in during their creation in the constructor (constructor injection).

I then created a separate "Resources" project that had an actual Resources section like normal. This project only contained the resources themselves, and a class for each interface that was needed to provide those resources via an interface. This project would reference every other project that had a resource dependency and implement the interface that the project needed.

Finally, in my "Top Level" project which nothing referenced (and where the exe was actually built and my composition root lives) I referenced the "Resources" project, wired up the DI, and away we went.

This means that only two projects (the "Resources" and the "Top Level") will be rebuilt every time, and if I do a partial build (Shift-F6) then they won't get rebuilt at all.

Again, not a great work around, but with 48 projects being built every time a build would take about 3 minutes, so I was losing 30 to 90 minutes a day with needless rebuilds. It took awhile to refactor, but I think it was a good investment.

Here is a simplified diagram. Note that the dependencies from Main.exe to Proj1 and Proj2 are not shown in order to reduce clutter.

With this design, I can do a build of Proj1 or Proj2 without triggering a full rebuild, since they don't have any dependencies on a Resources section. Only Main knows about the Resources implementation.