OK, so it started as a nice little app, which leveraged Windows Workflow Foundation. So I created an initial workflow and then some custom activities. Later on I figured, that those activities needed some tuning, so I modified them.

That was the point where I realized that custom activities and workflow-definitions should not reside in the same assembly!! All my existing (and persisted) instances could not be deserialized anymore 🙁

OK – so what do we have refactoring for? So I moved all my custom activities to a newly create library-project. Just in time to figure, that this wouldn’t work either. Workflow Activities cannot reside inside just any c# project – it has to be a workflow-activity-library-project (what else would you expect!?).

After moving the activities once more it finally compiled … 🙂