Since the earlier release of the CitizenFX C# templates, people have reported that certain built-in language types did not work, leading to vague errors such as '' is not a type supported by the compiler or such.
It took a while, but these should be fixed as of version 0.2.2 of the C# templates/SDK/framework packages, and these should now be usable for writing client code with compile-time checks for APIs available to the client-side subset framework.