This might shed some light, although the topic is quite old I think it’s still relevant:
In short - WWF uses different allocation schemes (context) than traditional .Net.
For most common ones (f.e. dates) Microsoft has supplied some supplementary packages (check your available packages for Workflow Manager Activities and Microsoft.Activities.Extensions). For the rest you’ll need a combination of InvokeMethod, InvokeCode (if you have a version that supports it) and/or custom activities.