MSBuild Code Generation in VS2015

A couple of weeks ago I was helping Jason Smith debug an issue with the build-time code generation in Xamarin.Forms. The build targets were based on my Build Time Code Generation in MSBuild post, but in recent Visual Studio 2015 versions the generated classes would occasionally disappear from IntelliSense.

Eventually we figured out that this happened after a very specific sequence of circumstances: when the project was cleaned, then closed and reopened.

I verified that the CoreCompile target is still run on each project when first opening a solution in Visual Studio. However, it wasn't getting run when the solution was closed and reopened unless the project file had changed, or the SUO files were deleted. All of this pointed to Visual Studio caching the Csc inputs in the solution options in order to improve solution load performance by eliminating the need to run CoreCompile.

This is a fine optimization, except for one minor detail. The cache was not getting flushed when the project was cleaned. This is a HUGE problem when the CoreCompile target creates input files for Csc, and the Clean targets correctly cleans them up. If the solution is closed and reopened after a clean, and no build has taken place since the clean, the input files will be missing!

For now, a reasonable workaround is simply not to incrementally clean generated files, and I've updated my original post with this. As long as you regenerate and collect the generated files correctly, skipping cleaning them shouldn't cause any problems except wasting a tiny bit of disk space.