WPF: The name does not exist in the namespace
One common solution to this known VS bug that you haven't specified as having tried is changing the build target platform.
If your current build target platform is x64, change to x86. If it's currently x86, change to x64.
Clean and Build solution for new target platform.
Change back to desired target platform and re-build.
One possible solution is by removing all the .dll files from Debug and Release folder.
Because sometimes when you refer to the project it refers to the pre-built dlls, even if you running your project in debug mode.
Remove all files from debug and release folder and build that project again, now add reference of this project to where you want. Worked perfectly fine on my end.