Express releases are limited, so you cannot mix and match projects from one language inside the IDE to another language.
To test this, I simply tried to incorporate the VC ++ 2010 project into the VC # 2010 solution, and the GUI would prevent this. This also does not work.
Please note that you have nothing to stop doing all the work in the free versions and cross-reference the resulting binaries and libraries. You simply cannot create a single solution from a mixed set of projects.
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