My feeling is that it is caused by your "target framework" setting in the project properties.
Instead of guessing about your project details I just created an extra sample that contains two VS projects (msi + CA assembly) and it targets .NET v4.0. The sample is also added to the codebase (<Wix# Samples>\DTF (ManagedCA)\Different Scenarios\ExternalAssembly\DTF (ManagedCA) .NET v4.0.7z).
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Instead of guessing about your project details I just created an extra sample that contains two VS projects (msi + CA assembly) and it targets .NET v4.0. The sample is also added to the codebase (<Wix# Samples>\DTF (ManagedCA)\Different Scenarios\ExternalAssembly\DTF (ManagedCA) .NET v4.0.7z).